Efficient photonic outcoupling of colour centers in the solid is a key requirement for quantum technology applications. Typical approaches improve the collection efficiency by shaping the surface of the host material to a solid-immersion lens (SIL) or fabricating nanopillars while more advanced approaches are based on high-Q optical resonators to increase the fluorescence from color centers by exploiting Purcell enhancement. While the latter structures offer a much larger photon countrate enhancement, they typically require tight operation conditions to keep the emitters on resonance with the structure during the measurement.

Our work shows a way of enhancing the photon outcoupling from V2 centers in 4H-silicon carbide based on a planar, low-Q Fabry-Pérot resonator, directly fabricated in the host material. The broadband resonance of the cavity is used to enhance fluorescence in the phonon sideband such that no tight resonance condition has to be fulfilled (i.e. no tuning of the structure or the emitters.

Our key findings are an average enhancement of 9.2 for emitters integrated into the structure, which is significantly (~ 3 times) higher compared to emitters integrated in a SIL. Further, we show resonant absorption linewidths below 80 MHz for the integrated V2 centers which is only slightly higher compared to linewidths in SILs or in bulk material.

Reference:
Körber, J., Heiler, J et al. Fluorescence Enhancement of Single V2 Centers in a 4H-SiC Cavity Antenna. Nano Letters 2024 24(30).

Fault-tolerant operations on logical qubits are crucial for achieving large-scale quantum computing amidst noise. Existing methods for fault-tolerant quantum computations often result in high qubit counts and intricate circuits, limiting practical implementation. In this work, we introduce techniques that substantially reduce the overhead for fault tolerance by eliminating redundant error correction components and developing efficient logical operations. These innovations enable the design of an optimized fault-tolerant circuit for the one-bit addition algorithm, requiring only ten physical qubits and 36 noisy operations. When implemented on a trapped-ion quantum computer, our circuit demonstrates a failure probability nearly an order of magnitude lower than that of physical operations on unencoded qubits. A key innovation of this work is the fault-tolerant protocol's leverage of variable qubit connectivity, exhibiting relatively high pseudo-thresholds for specific quantum computing tasks. Consequently, this work has significant implications for realizing fault-tolerant quantum computation on various platforms with high qubit connectivity, such as neutral atoms and solid-state defect centers.

The integration of colour centres into nanophotonic structures enables a high photonic efficiciency, one of the key requirements for quantum technologies. However, nanostructuring and the involved fabrication processes can decrease the spectral stability of integrated colour centres. In our recent work, we systematically investigate these parasitic effects on silicon vacancy colour centres in 4H-silicon carbide by fabricating membranes with thicknesses in the range of 3 µm to 0.25 µm starting from bulk material with integrated silicon vacancies.

Our first key result shows that the spectral position of the zero-phonon-line (ZPL) emission spreads more with a decreasing membrane thickness and bulk ZPL positions can be partially recovered by a post-fabrication annealing.  For the second key result, we study V2 center properties under photoluminescence-excitation (PLE) measurements.  As expected, we find that the PLE linewidths and spectral jumping increases with a decrease of the membrane thickness. We conclude a membrane thickness of ~ 0.6 µm seems to be a good sweet spot where most of the emitters show bulk-like PLE properties.

Reference:
Heiler, J., Körber, J., Hesselmeier, E. et al. Spectral stability of V2 centres in sub-micron 4H-SiC membranes. npj Quantum Mater. 9, 34 (2024).

Quantum state readout is a key requirement for a successful qubit platform, because it enables precise measurements and active-feedback algorithms.

In this work, we demonstrate a high-fidelity quantum state readout of a V2 center nuclear spin based on a repetitive readout technique by exploiting the long spin lifetime of a strongly coupled nuclear spin.

We demonstrate up to 99.5% readout fidelity and 99% for state preparation. Using this effective readout, we initialize a nuclear spin by measurement and demonstrate its Rabi and Ramsey nutation, which even shows coupling to a second nuclear spin. Finally, we use the nuclear spin as a long-lived memory for quantum sensing application of weakly coupled  nuclear spins, revealing a diatomic spin bath. 

References:
High-Fidelity Optical Readout of a Nuclear-Spin Qubit in Silicon Carbide, Erik Hesselmeier, Pierre Kuna, et al. Phys. Rev. Lett. 132, 180804

The research field of spintronics based on antiferromagnetic (AFM) materials paths the way for a new generation of spintronic devices, which, for example, can generate and manipulate spin-polarized currents.

Recently, theory predicted a new type of spin splitting in selected antiferromagnets. This splitting cannot be explained by Zeeman or Rashba-Dresselhaus interaction. Instead, it is induced by long-range magnetic order even when spin-orbit coupling is absent and leads to a giant and momentum-dependent spin polarization of energy bands in the antiferromagnet. However, a spectroscopic evidence of such spin splitting is still missing.

In this study, our collaborators demonstrate the existence of this AFM-induced spin splitting in the noncoplanar antiferromagnet MnTe2, utilizing spin-resolved and angle-resolved photoemission spectroscopy (SARPES) measurements and theoretical analysis. They uncover a quadratic spin-splitting effect with a plaid pattern in the spin texture. This effect is induced by magnetic exchange and is therefore distinct from the Zeeman or Rashba-Dresselhaus scenarios.

Our contribution to this work is the investigation of the AFM domain structure of MnTe2 with our new low temperature scanning probe magnetometer based on the nitrogen vacancy center in diamond. We confirm the AFM nature of the material and show over a larger scan area of 50 µm the absents of AFM domain walls. This result supports the argumentation of our collaborators, as it confirms that the AFM domains are larger than the incident beam used for the SARPES measurements.

References:
Observation of plaid-like spin splitting in a noncoplanar antiferromagnet; Yu-Peng Zhu, Xiaobing Chen, Xiang-Rui Liu, Yuntian Liu, Pengfei Liu, Heming Zha, Gexing Qu, Caiyun Hong, Jiayu Li, Zhicheng Jiang, Xiao-Ming Ma, Yu-Jie Hao, Ming-Yuan Zhu, Wenjing Liu, Meng Zeng, Sreehari Jayaram, Malik Lenger, Jianyang Ding, Shu Mo, Kiyohisa Tanaka, Masashi Arita, Zhengtai Liu, Mao Ye, Dawei Shen, Jörg Wrachtrup, Yaobo Huang, Rui-Hua He, Shan Qiao, Qihang Liu & Chang Liu; Nature volume 626, pages 523–528 (2024)

Three-dimensional semiconductor chip architectures promise high-density memories and vastly increased computational speeds. Currently, however, self-heating and leakage currents severely limit the overall performance.

Revealing these issues in situ through current density mapping is crucial, yet non-destructive imaging techniques have been limited to the two-dimensional analysis only.

In this study, we use ensembles of nitrogen-vacancy centers in diamond as nanoscale quantum sensors to probe all three vectorial components of magnetic fields associated with electric currents. We apply this technique to non-invasive imaging of three-dimensional currents in modern multi-layer integrated circuits. We also demonstrate a reliable detection of operational and non-operational chips.

Besides this technical application, further improvements of our technique can provide new insight into the local conductance of materials in condensed matter physics.

References:
Marwa Garsi, Rainer Stöhr, Andrej Denisenko, Farida Shagieva, Nils Trautmann, Ulrich Vogl, Badou Sene, Florian Kaiser, Andrea Zappe, Rolf Reuter, and Jörg Wrachtrup; Phys. Rev. Applied 21, 014055

Highly efficient measurements are among the main pillars which modern quantum technology stands. A projective readout can allow for single shot experiments and to initialise the system by measurements. However in reality the intrinsic decay of the system perturbs the state during the readout.

The analysis of initialisation fidelity performed by such measurement is usually done either empirically or using some assumptions.

In recent paper we provide an analytical model which provides accurate estimation of post selection fidelity and verify it experimentally using three most common photon counting readouts, such as charge state, low temperature resonant excitation readout and nuclear spin assisted readout.

References:
Majid Zahedian, Max Keller, Minsik Kwon, Javid Javadzade, Jonas Meinel, Vadim Vorobyov and Jörg Wrachtrup, Published 19 December 2023, Quantum Science and Technology, Volume 9, Number 1

Charge state instabilities have been a bottleneck for the implementation of solid-state spin systems and pose a major challenge to the development of spin-based quantum technologies. In this publication we investigate the stabilization of negatively charged nitrogen-vacancy (NV⁻) centers in phosphorus-doped diamond at liquid helium temperatures. Photoionization of phosphorous donors in conjunction with charge diffusion at the nanoscale enhances NV⁰ to NV⁻ conversion and stabilizes the NV⁻ charge state without the need for an additional repump laser. The phosphorus-assisted stabilization is explored and confirmed both with experiments and our theoretical model. Stable photoluminescence-excitation spectra are obtained for NV⁻ centers created during the growth. The fluorescence is continuously recorded under resonant excitation to real-time monitor the charge state and the ionization and recombination rates are extracted from time traces. We find a linear laser power dependence of the recombination rate as opposed to the conventional quadratic dependence, which is attributed to the photo-ionization of phosphorus atoms.

Publication:
Dopant-assisted stabilization of negatively charged single nitrogen-vacancy centers in phosphorus-doped diamond at low temperatures; Jianpei Geng, Tetyana Shalomayeva, Mariia Gryzlova, Amlan Mukherjee, Santo Santonocito, Dzhavid Dzhavadzade, Durga Bhaktavatsala Rao Dasari, Hiromitsu Kato, Rainer Stöhr, Andrej Denisenko, Norikazu Mizuochi & Jörg Wrachtrup; npj Quantum Information volume 9, Article number: 110 (2023)

To use batteries as large-scale energy storage systems it is necessary to measure and understand their degradation in-situ and in-operando. As a battery’s degradation is often the result of molecular processes inside the electrolyte, a sensing platform which allows to measure the ions with a high spatial resolution is needed. Using NV centers we were able to deduce the electric field distribution and propose a protocol, which allows to characterize the battery’s state of charge and to obtain insight into the transport processes happening inside the electrolyte, like for instance diffusion.

References:
M. Hollendonner, S. Sharma, S. K. Parthasarathy, D. B. R. Dasari, A. Finkler, S. V. Kusminskiy and R. Nagy, New J. Phys. 25 (2023) 093008

We show how the sinusoidally modulated, always rotating, and tailored (SMART) protocol, an extension of the dressed-qubit concept, can be implemented for continuous protection to offer Clifford gate fidelities compatible with fault-tolerant schemes, whilst prolonging the coherence time of a single NV− qubit at room temperature. We show an improvement in the average Clifford gate fidelity from 0.940±0.005 for the bare qubit to 0.993±0.002 for the SMART qubit, with the nitrogen nuclear spin in a random orientation. We further show a ≳30 times improvement in the qubit T∗2 coherence times compared to the bare qubit. This work paves the way for a practical implementation of a quantum computer that requires robust qubits that are protected against their noisy environment.

High-pressure experiments are crucial in modern interdisciplinary research fields such as engineering quantum materials, yet local probing techniques remain restricted due to the tight confinement of the pressure chamber in certain pressure devices. Recently, the negatively charged nitrogen-vacancy (N-$V$) center has emerged as a robust and versatile quantum sensor in pressurized environments. There are two popular ways to implement N-$V$ sensing in a diamond anvil cell (DAC), which is a conventional workhorse in the high-pressure community: create implanted N-$V$ centers (IN-$Vs$) at the diamond anvil tip or immerse N-$V$-enriched nanodiamonds (NDs) in the pressure medium. Nonetheless, there are limited studies on comparing the local stress environments experienced by these sensor types as well as their performances as pressure gauges. In this work, by probing the N-$V$ energy levels with the optically detected magnetic resonance (ODMR) method, we experimentally reveal a dramatic difference in the partially reconstructed stress tensors of IN-$Vs$ and NDs incorporated in the same DAC. Our measurement results agree with computational simulations, concluding that IN-$Vs$ perceive a more nonhydrostatic environment dominated by a uniaxial stress along the DAC axis. This provides insights on the suitable choice of N-$V$ sensors for specific purposes and the stress distribution in a DAC. We further propose some possible methods, such as using NDs and diamond nanopillars, to extend the maximum working pressure of quantum sensing based on ODMR spectroscopy, since the maximum working pressure could be restricted by nonhydrostaticity of the pressure environment. Moreover, we explore more sensing applications of the N-$V$ center by studying how pressure modifies different aspects of the N-$V$ system. We perform a PL study using both IN-$Vs$ and NDs to determine the pressure dependence of the zero-phonon line, which helps developing an all-optical pressure sensing protocol with the N-$V$ center. We also characterize the spin-lattice relaxation ($T_1$) time of IN-$Vs$ under pressure to lay a foundation for robust pulsed measurements with N-$V$ centers in pressurized environments.

References:
Spectroscopic Study of N-V Sensors in Diamond-Based High-Pressure Devices
Kin On Ho, Man Yin Leung, Wenyan Wang, Jianyu Xie, King Yau Yip, Jiahao Wu, Swee K. Goh, Andrej Denisenko, Jörg Wrachtrup, and Sen Yang; Phys. Rev. Applied 19, 044091

The work in the publication

Transition from quantum to classical dynamics in weak measurements and reconstruction of quantum correlation
Vadim V. Vorobyov, Jonas Meinel, Hitoshi Sumiya, Shinobu Onoda, Junichi Isoya, Oleg Gulinsky, and Jörg Wrachtrup in the Physical Review A 107, 042212, published in April 2023

summarizes the recent experimental and theoretical progress in understanding quantum measurements with spin systems such as NV and aims at an analytical description of the underlying dynamics.

In a nutshell, it accurately describes how quantum sensors work and extract information about a quantum object that causes a back-action, using two spins as an example. We used the NV center in diamond with a single weakly coupled spin 13C located at a magic angle.

Possible outcomes of this work are ways to improve color center quantum hardware through continuous monitoring and quantum feedback, as well as next generation quantum sensing algorithms.

Extracting useful signals is key to both classical and quantum technologies. Conventional noise filtering methods rely on different patterns of signal and noise in frequency or time domains, thus limiting their scope of application, especially in quantum sensing. Here, we propose a signal-nature-based (not signal-pattern-based) approach which singles out a quantum signal from its classical noise background by employing the intrinsic quantum nature of the system. We design a novel protocol to extract the quantum correlation signal and use it to single out the signal of a remote nuclear spin from its overwhelming classical noise backgrounds, which is impossible to be accomplished by conventional filter methods. Our Letter demonstrates the quantum or classical nature as a new degree of freedom in quantum sensing. The further generalization of this quantum nature-based method opens a new direction in quantum research.

References:
Yang Shen, Ping Wang, Chun Tung Cheung, Jörg Wrachtrup, Ren-Bao Liu and Sen Yang
Phys. Rev. Lett. 130, 070802 – Published 17 February 2023

PhD candidate Timo Steidl from the 3rd Physics Institute has happily received a new custom-design closed-cycle optical cryostat system from attocube systems AG, our collaboration partner in the InQuRe project (Horizon 2020, Grant Agreement no. 731473 and 101017733).

The cryostat reaches a sample temperature of 4.0 K with <15 mK stability, while providing an extremely large sample chamber (130mm diameter, 150mm height) and low vibration levels. Together with attocube engineers, we have pushed the boundaries of system integration. Our cryostat now features 9 piezo-positioners (open- and closed-loop operation), 2 optical fibre feedthroughs (upgradable to 6), 3 microwave feedthroughs (upgradable to 6), as well as 6 pairs of twisted cables for low-power control.

All optical components (windows, objective) have been optimized for the wavelength range 900 – 1200 nm to facilitate our work with silicon vacancy colour centres (VSi) in silicon carbide (SiC).

Using this cryostat system, Timo Steidl will pursue his work towards increasing the scalability of VSi centres in SiC, e.g., as required for distributed quantum computing architectures. Some of the experiments that we will implement in this system are:

• Control and spectral stability improvement of VSi centres in SiC p-i-n diodes.
• Wavelength tuning of individual VSi centres with strain and electric fields.
• Photonic interference experiments based on multiple VSi centres in different fibre coupled SiC waveguides.

We are very optimistic that this system will allow us to make the necessary steps towards distributed quantum computing based on SiC colour centres.

Single color centers in solid have emerged as promising physical platforms for quantum information science. Creating these centers with excellent quantum properties is a key foundation for further technological developments. In particular, the microscopic understanding of the spin-bath environments is the key to engineer color centers for quantum control. In this work, we propose and demonstrate a distinct high-temperature annealing (HTA) approach for creating high-quality nitrogen vacancy (N-V) centers in implantation-free diamonds. Simultaneously using the created N-V centers as probes for their local environment we verify that no damage is microscopically induced by the HTA. Nearly all single N-V centers created in ultralow-nitrogen-concentration membranes possess stable and Fourier-transform-limited optical spectra. Furthermore, HTA strongly reduces noise sources naturally grown in ensemble samples, and leads to more than threefold improvements of decoherence time and sensitivity. We also verify that the vacancy activation and defect reformation, especially H3 and P1 centers, can explain the reconfiguration between spin baths and color centers. This distinct approach will become a powerful tool in vacancy-based quantum technology.

References:
King Cho Wong, San Lam Ng, Kin On Ho, Yang Shen, Jiahao Wu, Kwing To Lai, Man Yin Leung, Wai Kuen Leung, Durga Bhaktavatsala Rao Dasari, Andrej Denisenko, Jörg Wrachtrup, and Sen Yang
Phys. Rev. Applied 18, 024044 – Published 16 August 2022

The quantum Zeno and anti-Zeno paradigms have thus far addressed the evolution control of a quantum system coupled to an immutable bath via non-selective measurements performed at appropriate intervals. We fundamentally modify these paradigms by introducing, theoretically and experimentally, the concept of controlling the bath state via selective measurements of the system (a qubit). We show that at intervals corresponding to the anti-Zeno regime of the system-bath exchange, a sequence of measurements has strongly correlated outcomes. These correlations can dramatically enhance the bath-state purity and yield a low-entropy steady state of the bath. The purified bath state persists long after the measurements are completed. Such purification enables the exploitation of spin baths as long-lived quantum memories or as quantum-enhanced sensors. The experiment involved a repeatedly probed defect center dephased by a nuclear spin bath in a diamond at low-temperature.

References:
Anti-Zeno purification of spin baths by quantum probe measurements; Durga Bhaktavatsala Rao Dasari, Sen Yang, Arnab Chakrabarti, Amit Finkler, Gershon Kurizki & Jörg Wrachtrup; Nature Communications volume 13, Article number: 7527 (2022) 

Quantum sensors are known for their high sensitivity in sensing applications. However, this sensitivity often comes with severe restrictions on other parameters which are also important. Examples are that in measurements of arbitrary signals, limitation in linear dynamic range could introduce distortions in magnitude and phase of the signal. High frequency resolution is another important feature for reconstructing unknown signals. In this publication, we demonstrate a distortion-free quantum sensing protocol that combines a quantum phase-sensitive detection with heterodyne readout. We present theoretical and experimental investigations using nitrogen-vacancy centers in diamond, showing the capability of reconstructing audio frequency signals with an extended linear dynamic range and high frequency resolution. Melody and speech based signals are used for demonstrating the features. The methods could broaden the horizon for quantum sensors towards applications, e.g. telecommunication in challenging environment, where low-distortion measurements are required at multiple frequency bands within a limited volume.

References:
Quantum-assisted distortion-free audio signal sensing, Chen Zhang, Durga Dasari, Matthias Widmann, Jonas Meinel, Vadim Vorobyov, Polina Kapitanova, Elizaveta Nenasheva, Kazuo Nakamura, Hitoshi Sumiya, Shinobu Onoda, Junichi Isoya & Jörg Wrachtrup, Nature Communications volume 13, Article number: 4637 (2022)
 

Towards high performance quantum technologies the understanding of qubit decoherence and correlations of fluctuations is paramount. Especially, higher order correlations give insights in the dynamics of quantum many-body systems and stochastic noise processes, as they suppress Gaussian noise while remaining sensitivity to non-linear signals. We extract high order correlations from a series of weak measurements of the target nuclear spin system. The quantumness and non-linearity of our sensor spin drives the target nuclear spin dynamics creating quantum and classical fourth order correlations. Experimentally, we measure high order correlations of a single nuclear spin and radio frequency magnetic fields with the nitrogen vacancy center in diamond. Quantum non-linear spectroscopy allows us to distinguish the 13C nuclear spin precession signal from the radio frequency signal via finger prints features in the high order spectra. Further the nuclear spin signal can originate from many nuclear spins N, which we can determine qualitatively and quantitatively through high order correlations. Quantum correlations scale with N while classical scale with N2, expressed as (1 – 1/N ), analogous to a g(2) photon correlation measurement. This work constitutes an initial step toward the application of higher-order correlations to quantum sensing. It may be used in future to examine the quantum foundation (by, e.g., higher-order Leggett-Garg inequality) and to study quantum many-body physics.

References:
Quantum nonlinear spectroscopy of single nuclear spins, Jonas Meinel, Vadim Vorobyov, Ping Wang, Boris Yavkin, Mathias Pfender, Hitoshi Sumiya, Shinobu Onoda, Junichi Isoya, Ren-Bao Liu & J. Wrachtrup, Nature Communications volume 13, Article number: 5318 (2022) 

The performance of qubit-based technologies can be strongly limited by environmental sources of noise and disorder that cause decoherence. Qubits used in quantum sensing are usually very close to the host surface to enhance their coupling to external targets. This leaves them vulnerable to the effects of the surrounding noisy electron spin bath near the surface, which is very challenging to eliminate. Here we developed an efficient method to engineer the immediate electrostatic environment of nitrogen vacancy centre qubits located several nanometres beneath the diamond surface. We adopt a ‘pull-and-push’ strategy for near-surface charge manipulation using the strong local electric field of an atomic force microscope tip. Our technique is particularly effective for extremely shallow nitrogen vacancy centres, increasing their spin echo time by up to 20 fold. This corresponds to an 80-fold enhancement in the potential sensitivity for detecting individual external proton spins. Our work not only represents a step towards overcoming a fundamental restriction to applications of shallow nitrogen vacancy centres for quantum sensing but may also provide a general route for enhancing the coherence of solid-state qubits.

References:
Coherence enhancement of solid-state qubits by local manipulation of the electron spin bath; Wentian Zheng, Ke Bian, Xiakun Chen, Yang Shen, Shichen Zhang, Rainer Stöhr, Andrej Denisenko, Jörg Wrachtrup, Sen Yang & Ying Jiang; Nature Physics (2022)

We investigate, theoretically and experimentally, the thermodynamic performance of a minimal three-qubit heat-bath algorithmic cooling refrigerator. We analytically compute the coefficient of performance, the cooling power, and the polarization of the target qubit for an arbitrary number of cycles, taking realistic experimental imperfections into account. We determine their fundamental upper bounds in the ideal reversible limit and show that these values may be experimentally approached using a system of three qubits in a nitrogen-vacancy center in diamond.

References:
Thermodynamics of a Minimal Algorithmic Cooling Refrigerator
Rodolfo R. Soldati, Durga B. R. Dasari, Jörg Wrachtrup, and Eric Lutz, Phys. Rev. Lett. 129, 030601 (2022)

Color centers in silicon carbide are emerging candidates for distributed spin-based quantum applications due to the scalability of host materials and the demonstration of integration into nanophotonic resonators. Recently, silicon vacancy centers in silicon carbide have been identified as a promising system with excellent spin and optical properties.

Here, we fully study the spin-optical dynamics of the single silicon vacancy center at hexagonal lattice sites, namely V1, in 4H-polytype silicon carbide. By utilizing resonant and above-resonant sublifetime pulsed excitation, we determine spin-dependent excited-state lifetimes and intersystem-crossing rates. Our new approach to inferring the intersystem-crossing rates is based on all-optical pulsed initialization and readout scheme, and is applicable to spin-active color centers with similar dynamics models. In addition, the optical transition dipole strength and the quantum efficiency of V1 defect are evaluated based on coherent optical Rabi measurement and local-field calibration employing electric field simulation. With our model, we can now fully explain spin-state polarization dynamics and determine optimal pulse sequences for quantum sensing protocols.

Additionally, we outline optimal pathways towards improving the photoemission dynamics in resonant enhancement structures, such as required in quantum networks based on optically connected silicon vacancy centers in silicon carbide.

References:
Spin-optical dynamics and quantum efficiency of a single V1 center in silicon carbide; Naoya Morioka, Di Liu, Öney O. Soykal, Izel Gediz, Charles Babin, Rainer Stöhr, Takeshi Ohshima, Nguyen Tien Son, Jawad Ul-Hassan, Florian Kaiser, and Jörg Wrachtrup, Physical Review Applied 17, 054005 (2022)

Electro-optical control of on-chip photonic devices is an essential tool for efficient integrated photonics. Lithium niobate on insulator (LNOI) is an emerging platform for on-chip photonics due to its large electro-optic coefficient and high nonlinearity. Integrating quantum emitters into LNOI would extend their versatility in classic photonics to quantum computing and communication. Here, we incorporate rare-earth ion (REI) quantum emitters into electro-optical tunable lithium niobite (LN) thin films and demonstrate control of LN microcavities coupled to REIs over a frequency range of 160 GHz with 5 µs switching speed. Dynamic control of the cavities enables modulation of the Purcell enhancement of REIs with short time constants. Using Purcell enhancement, we show evidence of detecting single Yb³⁺ions in LN cavities. Coupling quantum emitters in fast tunable photonic devices is an efficient method to shape the waveform of the emitter. It also offers a platform to encode quantum information in the integration of a spectral–temporal–spatial domain to achieve high levels of channel multiplexing, as well as an approach to generate deterministic single-photon sources.

References:
Tunable microcavities coupled to rare-earth quantum emitters; Kangwei Xia, Fiammetta Sardi, Colin Sauerzapf, Thomas Kornher, Hans-Werner Becker, Zsolt Kis, Laszlo Kovacs, Denis Dertli, Jonas Foglszinger, Roman Kolesov, and Jörg Wrachtrup,  Optica Vol. 9, Issue 4, pp. 445-450 (2022) 

Impurity-vacancy complexes in diamond are an attractive family of spin defects since NV ^–, SiV ^–, GeV ^–, and SnV ^– have emerged as promising platforms for quantum applications. Although boron is most easily incorporated into diamond, a boron-vacancy complex in the negative charge state (BV ^–) has eluded experimental observation. This center was theoretically predicted as another promising spin qubit. In this work, we experimentally observed an electron paramagnetic resonance (EPR) spectrum identified as BV ^– in synthetic diamonds via a Fermi-level tuning. Fingerprints of BV ^– such as the spin multiplicity f S = 1, C_3v symmetry, and the zero-field splitting (D = 2913 MHz), in addition to ¹⁰B and ¹¹B hyperfine (HF) interactions, have been confirmed. Moreover, optically pumped spin polarization has been observed with 3.0–3.6 eV excitation. However, unlike the NV ^– center, the photoluminescence as well as optically detected magnetic resonance from BV ^– have not been confirmed even at low temperatures.We speculate that the Jahn-Teller instability in the triplet excited states of the NV ^– and BV ^– centers results in different optical properties.

References:
Negatively charged boron vacancy center in diamond
Phys. Rev. B 105, 165201 – Published 4 April 2022 – T. Umeda, K. Watanabe, H. Hara, H. Sumiya, S. Onoda, A. Uedono, I. Chuprina, P. Siyushev, F. Jelezko, J. Wrachtrup, and J. Isoya

Nuclei surrounding single electron spins are valuable resources for quantum technology. For application in this area, one is often interested in weakly coupled nuclei with coupling strength on the order of a few 10–100 kHz. In this paper, we compare methods to address single nuclear spins with this type of hyperfine coupling to a single electron spin. To achieve the required spectral resolution, we specifically focus on two methods, namely dynamical decoupling and correlation spectroscopy. We demonstrate spectroscopy of two single nuclear spins and present a method to derive components of their hyperfine coupling tensor from those measurements.

References:
Addressing Single Nuclear Spins Quantum Memories by a Central Electron Spin; V. Vorobyov, J. Javadzade, M. Joliffe, F. Kaiser & J. Wrachtrup, Applied Magnetic Resonance (2022)

Moiré superlattices of twisted nonmagnetic two-dimensional (2D) materials are highly controllable platforms for the engineering of exotic correlated and topological states. Here, we report emerging magnetic textures in small-angle twisted 2D magnet chromium triiodide (CrI₃). Using single-spin quantum magnetometry, we directly visualized nanoscale magnetic domains and periodic patterns, a signature of moiré magnetism, and measured domain size and magnetization. In twisted bilayer CrI₃, we observed the coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) domains with disorder-like spatial patterns. In twisted double-trilayer CrI₃, AFM and FM domains with periodic patterns appear, which is in good agreement with the calculated spatial magnetic structures that arise from the local stacking-dependent interlayer exchange interactions in CrI₃ moiré superlattices. Our results highlight magnetic moiré superlattices as a platform for exploring nanomagnetism.

References:
Tiancheng Song, Qi-Chao Sun, Eric Anderson, Chong Wang, Jimin Qian, Takashi Taniguchi, Kenji Watanabe, Michael A McGuire, Rainer Stöhr, Di Xiao, Ting Cao, Jörg Wrachtrup, Xiaodong Xu, Science 374, 1140-1144 (2021)

A promising route towards larger quantum computers is to orchestrate multiple task-optimised smaller systems. To dynamically connect and entangle any two systems, photonic interference emerges as a powerful method, due to its compatibility with on-chip devices and long-distance propagation in quantum networks.

In our recent work, published in Nature Materials, we showed that silicon vacancy (VSi) colour centres in silicon carbide (SiC) can reach these goals.

Our work followed a two-step approach. First, VSi centres in SiC are particularly robust quantum systems to start with. Second, we fabricated nanophotonic waveguides around VSi centres using gentle processing methods that keep the host material essentially free of damage.

With these techniques, we showed for the first time that waveguide-integrated colour centres can maintain, at the same time, life-time limited optical absorption lines, long spin coherence times (Hahn echo, 1.4 ms), as well as coherent access to nuclear spin qubits with fidelities exceeding 90%.

Our next steps target high-efficiency light collection via direct waveguide-to-fibre coupling, as well as maximising electrical control to independently tune multiple VSi centres on a complex photonic chip.

References:
Fabrication and nanophotonic waveguide integration of silicon carbide colour centres with preserved spin-optical coherence; Charles Babin, Rainer Stöhr, Naoya Morioka, Tobias Linkewitz, Timo Steidl, Raphael Wörnle, Di Liu, Erik Hesselmeier, Vadim Vorobyov, Andrej Denisenko, Mario Hentschel, Christian Gobert, Patrick Berwian, Georgy V. Astakhov, Wolfgang Knolle, Sridhar Majety, Pranta Saha, Marina Radulaski, Nguyen Tien Son, Jawad Ul-Hassan, Florian Kaiser & Jörg Wrachtrup; Nature Materials (2021)

Point defects in crystals provide important building blocks for quantum applications. Since we optically address these defect qubits, having an efficient optical interface is a highly important aspect. However, conventional confocal fluorescence microscopy of high-refractive-index crystals suffers from limited photon collection efficiency and spatial resolution. Here, we demonstrate high-resolution, high-contrast imaging of defects in diamonds using microsphere-assisted confocal microscopy. A microsphere provides an excellent optical interface for point defects with a magnified virtual image that increases the spatial resolution up to lambda/5, as well as the optical signal-to-noise ratio by four times. These features enable individual optical addressing of single photons and single spins of multiple defects that are spatially unresolved in conventional confocal microscopy, with improved signal contrast. Combined with optical tweezers, this system also demonstrates the possibility of positioning or scanning the microspheres. The approach does not require any complicated fabrication or additional optical systems, but uses simple, off-the-shelf micro-optics. From these distinctive advantages of microspheres, our approach provides an efficient way to image and address closely spaced defects with much better resolution and sensitivity.

References:
High-Resolution, High-Contrast Optical Interface for Defect Qubits, Jong Sung Moon, Haneul Lee, Jin Hee Lee, Woong Bae Jeon, Dowon Lee, Junghyun Lee, Seoyoung Paik, Sang-Wook Han, Rolf Reuter, Andrej Denisenko, Jörg Wrachtrup, Sang-Yun Lee, and Je-Hyung Kim, ACS Photonics 2021, 8, 9, 2642–2649, August 19, 2021

The quantum Fourier transformation (QFT) is a key building block for a whole wealth of quantum algorithms. Despite its proven efficiency, only a few proof-of-principle demonstrations have been reported. Here we utilize QFT to enhance the performance of a quantum sensor. We implement the QFT algorithm in a hybrid quantum register consisting of a nitrogen-vacancy (NV) center electron spin and three nuclear spins. The QFT runs on the nuclear spins and serves to process the sensor - i.e., the NV electron spin signal. Specifically, we show the application of QFT for correlation spectroscopy, where the long correlation time benefits the use of the QFT in gaining maximum precision and dynamic range at the same time. We further point out the ability for demultiplexing the nuclear magnetic resonance (NMR) signals using QFT and demonstrate precision scaling with the number of used qubits. Our results mark the application of a complex quantum algorithm in sensing which is of particular interest for high dynamic range quantum sensing and nanoscale NMR spectroscopy experiments.

References:
Quantum Fourier transform for nanoscale quantum sensing; Vadim Vorobyov, Sebastian Zaiser, Nikolas Abt, Jonas Meinel, Durga Dasari, Philipp Neumann & Jörg Wrachtrup; npj Quantum Information volume 7, Article number: 124 (2021)

The achievable bounds of cooling quantum systems, and the possibility to violate them is not well-explored experimentally. For example, among the common methods to enhance spin polarization (cooling), one utilizes the low temperature and high-magnetic field condition or employs a resonant exchange with highly polarized spins. The achievable polarization, in such cases, is bounded either by Boltzmann distribution or by energy conservation. Heat-bath algorithmic cooling  schemes (HBAC), on the other hand, have shown the possibility to surpass the physical limit set by the energy conservation and achieve a higher saturation limit in spin cooling. Despite, the huge theoretical progress, and few principle demonstrations, neither the existence of the limit nor its application in cooling quantum systems towards the maximum achievable limit have been experimentally verified. Here, we show the experimental saturation of the HBAC limit for single nuclear spins, beyond any available polarization in solid-state spin system, the Nitrogen-Vacancy centers in diamond. We benchmark the performance of our experiment over a range of variable reset polarizations (bath temperatures), and discuss the role of quantum coherence in HBAC.

References:
Cyclic cooling of quantum systems at the saturation limit; Sebastian Zaiser, Chun Tung Cheung, Sen Yang, Durga Bhaktavatsala Rao Dasari, Sadegh Raeisi & Jörg Wrachtrup; npj Quantum Information volume 7, Article number: 92 (2021) 

Scattering experiments have revolutionized our understanding of nature. Examples include the discovery of the nucleus [R. G. Newton, Scattering Theory of Waves and Particles (1982)], crystallography [U. Pietsch, V. Holý, T. Baumback, High-Resolution X-Ray Scattering (2004)], and the discovery of the double-helix structure of DNA [J. D. Watson, F. H. C. Crick, Nature 171, 737–738]. Scattering techniques differ by the type of particles used, the interaction these particles have with target materials, and the range of wavelengths used. Here, we demonstrate a two-dimensional table-top scattering platform for exploring magnetic properties of materials on mesoscopic length scales. Long-lived, coherent magnonic excitations are generated in a thin film of yttrium iron garnet and scattered off a magnetic target deposited on its surface. The scattered waves are then recorded using a scanning nitrogen vacancy center magnetometer that allows subwavelength imaging and operation under conditions ranging from cryogenic to ambient environment. While most scattering platforms measure only the intensity of the scattered waves, our imaging method allows for spatial determination of both amplitude and phase of the scattered waves, thereby allowing for a systematic reconstruction of the target scattering potential. Our experimental results are consistent with theoretical predictions for such a geometry and reveal several unusual features of the magnetic response of the target, including suppression near the target edges and a gradient in the direction perpendicular to the direction of surface wave propagation. Our results establish magnon scattering experiments as a platform for studying correlated many-body systems.

References:
A magnon scattering platform, Tony X. Zhou, Joris J. Carmiggelt, Lisa M. Gächter, Ilya Esterlis, Dries Sels, Rainer J. Stöhr, Chunhui Du, Daniel Fernandez, Joaquin F. Rodriguez-Nieva, Felix Büttner, Eugene Demler, and Amir Yacoby,
PNAS June 22, 2021 118 (25) e2019473118

A plethora of single-photon emitters have been identified in the atomic layers of two-dimensional van der Waals materials. Here, we report on a set of isolated optical emitters embedded in hexagonal boron nitride that exhibit optically detected magnetic resonance. The defect spins show an isotropic ge-factor of ~2 and zero-field splitting below 10 MHz. The photokinetics of one type of defect is compatible with ground-state electron-spin paramagnetism. The narrow and inhomogeneously broadened magnetic resonance spectrum differs significantly from the known spectra of in-plane defects. We determined a hyperfine coupling of ~10 MHz. Its angular dependence indicates an unpaired, out-of-plane delocalized π-orbital electron, probably originating from substitutional impurity atoms.  We extracted spin–lattice relaxation times T1 of 13–17 μs with estimated spin coherence times T2 of less than 1 μs. Our results provide further insight into the structure, composition and dynamics of single optically active spin defects in hexagonal boron nitride.

References:
Single-spin resonance in a van der Waals embedded paramagnetic defect; Nathan Chejanovsky, Amlan Mukherjee, Jianpei Geng, Yu-Chen Chen, Youngwook Kim, Andrej Denisenko, Amit Finkler, Takashi Taniguchi, Kenji Watanabe, Durga Bhaktavatsala Rao Dasari, Philipp Auburger, Adam Gali, Jurgen H. Smet & Jörg Wrachtrup, Nature Materials (2021), 06 May 2021

Nitrogen-vacancy (NV) centers in diamond can be used as quantum sensors to image the magnetic field with nanoscale resolution. However, nanoscale electric-field mapping has not been achieved so far because of the relatively weak coupling strength between NV and electric field. Here, using individual shallow NVs, we quantitatively image electric field contours from a sharp tip of a qPlus-based atomic force microscope (AFM), and achieve a spatial resolution of ~10 nm. Through such local electric fields, we demonstrated electric control of NV’s charge state with sub-5 nm precision. This work represents the first step towards nanoscale scanning electrometry based on a single quantum sensor and may open up the possibility of quantitatively mapping local charge, electric polarization, and dielectric response in a broad spectrum of functional materials at nanoscale.

References:
Nanoscale electric-field imaging based on a quantum sensor and its charge-state control under ambient condition
Ke Bian, Wentian Zheng, Xianzhe Zeng, Xiakun Chen, Rainer Stöhr, Andrej Denisenko, Sen Yang, Jörg Wrachtrup & Ying Jiang; Nature Communications volume 12, Article number: 2457 (2021)

The emergence of atomically thin van der Waals magnets provides a new platform for the studies of two-dimensional magnetism and its applications. However, the widely used measurement methods in recent studies cannot provide quantitative information of the magnetization nor achieve nanoscale spatial resolution. These capabilities are essential to explore the rich properties of magnetic domains and spin textures. Here, we employ cryogenic scanning magnetometry using a single-electron spin of a nitrogen-vacancy center in a diamond probe to unambiguously prove the existence of magnetic domains and study their dynamics in atomically thin CrBr₃. By controlling the magnetic domain evolution as a function of magnetic field, we find that the pinning effect is a dominant coercivity mechanism and determine the magnetization of a CrBr₃ bilayer to be about 26 Bohr magnetons per square nanometer. The high spatial resolution of this technique enables imaging of magnetic domains and allows to locate the sites of defects that pin the domain walls and nucleate the reverse domains. Our work highlights scanning nitrogen-vacancy center magnetometry as a quantitative probe to explore nanoscale features in two-dimensional magnets.

References:
Magnetic domains and domain wall pinning in atomically thin CrBr3 revealed by nanoscale imaging; Qi-Chao Sun, Tiancheng Song, Eric Anderson, Andreas Brunner, Johannes Förster, Tetyana Shalomayeva, Takashi Taniguchi, Kenji Watanabe, Joachim Gräfe, Rainer Stöhr, Xiaodong Xu & Jörg Wrachtrup; Nature Communications volume 12, Article number: 1989 (2021)

Microscopic studies on thin film superconductors play an important role for probing non-equilibrium phase transitions and revealing dynamics at the nanoscale. However, magnetic sensors with nanometer scale spatial and picosecond temporal resolution are essential for exploring these. Here, we present an all-optical, microwave-free method that utilizes the negatively charged nitrogen-vacancy (NV) center in diamond as a non-invasive quantum sensor and enables the spatial detection of the Meissner state in a superconducting thin film. We place an NV implanted diamond membrane on a 20 nm thick superconducting La_2-xSr_xCuO₄ (LSCO) thin film with Tc of 34 K. The strong B-field dependence of the NV photoluminescence allows us to investigate the Meissner screening in LSCO under an externally applied magnetic field of 4.2 mT in a non-resonant manner. The magnetic field profile along the LSCO thin film can be reproduced using Brandt’s analytical model, revealing a critical current density j_c of 1.4 x 10⁸ A/cm². Our work can be potentially extended further with a combination of optical pump probe spectroscopy for the local detection of time-resolved dynamical phenomena in nanomagnetic materials.

References:
All-optical and microwave-free detection of Meissner screening using nitrogen-vacancy centers in diamond featured
Journal of Applied Physics 129, 024306 (2021); https://doi.org/10.1063/5.0037414, D. Paone, D. Pinto, G. Kim, L. Feng, M.-J. Kim, R. Stöhr, A. Singha, S. Kaiser, G. Logvenov, B. Keimer, J. Wrachtrup, and K. Kern

Atomic spins for quantum technologies need to be individually addressed and positioned with nanoscale precision. C60 fullerene cages offer a robust packaging for atomic spins, while allowing in-situ physical positioning at the nanoscale. However, achieving single-spin level readout and control of endofullerenes has so far remained elusive. In this work, we demonstrate electron paramagnetic resonance on an encapsulated nitrogen spin (14N@C60) within a C60 matrix using a single near-surface nitrogen vacancy (NV) center in diamond at 4.7 K. Exploiting the strong magnetic dipolar interaction between the NV and endofullerene electronic spins, we demonstrate radio-frequency pulse controlled Rabi oscillations and measure spin-echos on an encapsulated spin. Modeling the results using second-order perturbation theory reveals an enhanced hyperfine interaction and zero-field splitting, possibly caused by surface adsorption on diamond. These results demonstrate the first step towards controlling single endofullerenes, and possibly building large-scale endofullerene quantum machines, which can be scaled using standard positioning or self-assembly methods.

References:
Readout and control of an endofullerene electronic spin, Dinesh Pinto, Domenico Paone, Bastian Kern, Tim Dierker, René Wieczorek, Aparajita Singha, Durga Dasari, Amit Finkler, Wolfgang Harneit, Jörg Wrachtrup & Klaus Kern, Nature Communications 11,  6405 (2020).

Silicon carbide (SiC) is an appealing material for quantum technologies and nanophotonics due to its optical transparency, and low nuclear spin bath. One species of promising quantum emitters found in the 4H-SiC polytype is the silicon vacancy defects V_Si. It exhibits a long spin coherence time at room temperature and accessible coherence control of single spins. However, the high refractive index of the material leads to strong refraction and total internal reflection at the SiC-air interface, which results in reduction of the collected defects photoluminescence.

A possibility to circumvent this effect is the fabrication of shallow solid immersion lenses of a few micrometers in size on a SiC wafer, resulting in refractionless transmission of light through the SIL surface. A similar approach has already been proven to be advantageous for enhancing the fluorescence yield of nitrogen-vacancy (NV) centers in diamond or for studies in the spin properties of rare earth ions. Typically, SILs in diamond are fabricated by focused ion beam (FIB) milling. This approach is intrinsically time-consuming and unscalable. In addition, it leads to contamination of the surface of the SIL with gallium and damages the crystalline structure. Studies dealing with quantum-dots describe the scalable fabrication of lenses with a low NA of 0.4 and discussing the potential of high NA lenses with respect to collection efficiency. Also, previous work on scalable photonics for SiC dealt with nanopillars tailored to 0.65 NA for signal collection, potentially with lensed fiber arrays. With increasing NA of lensed fibers in experiments and theoretical proposals, a high NA collection of light in a scalable fashion comes into reach.

The experiment introduces a scalable, method of producing SILs capable to work with high NA objective by imprinting hemispherical droplets of a reflown photoresist onto the surface of SiC by plasma etching. The performance of the resulting SILs is evaluated by comparing the fluorescence collected from single V_Si centers under the SIL with centers under a pristine SiC surface. The fluorescence collection efficiency of single quantum emitters under the SILs shows 3.4 times enhancement confirmed by confocal microscopy of individual V₂.

References
Appl. Phys. Lett. 117, 022105 (2020); https://doi.org/10.1063/5.0011366

Paramagnetic ions in solid state crystals form the basis for many advanced technologies such as lasers, masers, frequency standards, and quantum-enhanced sensors. One of the most-studied examples is the ${\mathrm{Cr}}^{3+}$ ion in sapphire (${\mathrm{Al}}_2{O}_3$), also known as ruby, which has been intensely studied in the 1950s and 1960s. However, despite decades of research on ruby, some of its fundamental optical and spin properties have not yet been characterized at ultralow temperatures. In this paper, we present optical measurements on a ruby crystal in a dilution refrigerator at ultralow temperatures down to 20 mK. Analyzing the relative populations of its $4A{}_2$ ground-state spin levels, we extract a lattice temperature of $143\pm 7\mathrm{mK}$ under continuous laser excitation. We perform spin-lattice relaxation $T_1$ measurements in excellent agreement with the direct, one-phonon model. Furthermore, we perform optically detected magnetic resonance measurements showing magnetically driven transitions between the ground-state spin levels for various magnetic fields. Our measurements characterize some of ruby's low-temperature spin properties, and lay the foundations for more advanced spin control experiments.

References
Spin thermometry and spin relaxation of optically detected Cr3+ ions in ruby Al2O3, Vikas K. Sewani, Rainer J. Stöhr, Roman Kolesov, Hyma H. Vallabhapurapu, Tobias Simmet, Andrea Morello, and Arne Laucht Phys. Rev. B 102, 104114 – Published 25 September 2020

In current long-distance communications, classical information can be eavesdropped without notice. On the other hand, quantum communications can provide provable privacy and could make use of entanglement swapping via quantum repeaters to mitigate transmission losses.

To reach this goal, considerable efforts have been spent over the last few decades toward developing quantum repeaters, notably in the solid-state. Multiple candidate optical spin qubits in the solid state, including quantum dots, rare-earth ions, and color centers in diamond and silicon carbide (SiC), have been developed.

In this perspective article, we summarize the latest advances in developing optically active spin qubits in SiC, and outline future research directions. Thanks to established semiconductor fabrication and processing techniques, we show that the SiC platform is highly promising to realize the targeted applications in quantum repeater networks.

References:
Appl. Phys. Lett. 116, 190501 (2020); https://doi.org/10.1063/5.0004454

The occurrence of any physical process is restricted by the constraints imposed by the laws of thermodynamics on the energy and entropy exchange involved. A prominent class of processes where thermodynamic constraints are crucial involve polarization of nuclear spin baths that are at the heart of magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR), quantum information processing. Polarizing a spin bath, is the key to enhancing the sensitivity of these tools, leading to new analytical capabilities and improved medical diagnostics. In recent years, significant effort has been invested in identifying the far-reaching consequences of quantum modifications to classical thermodynamics for such processes. Here we focus on the adverse role of quantum correlations (entanglement) in the spin bath that can impede its cooling in many realistic scenarios. We propose to remove this impediment by modified cooling schemes, incorporating probe-induced disentanglement or, equivalently, alternating non-commuting probe–bath interactions to suppress the buildup of quantum correlations in the bath. The resulting bath polarization is thereby exponentially enhanced. The underlying quantum thermodynamic principles have far-reaching implications for a broad range of quantum technological applications.

References:
D. Durga Bhaktavatsala Rao Dasari, et. al. - Spin-bath polarization via disentanglement, New Journal of Physics (2020)

Machine learning (ML) has become an attractive tool for solving various problems in different fields of physics, including the quantum domain. Here, we show how classical reinforcement learning (RL) could be used as a tool for quantum state engineering (QSE). We employ a measurement based control for QSE where the action sequences are determined by the choice of the measurement basis and the reward through the fidelity of obtaining the target state. Our analysis clearly displays a learning feature in QSE, for example in preparing arbitrary two-qubit entangled states and delivers successful action sequences that generalise previously found human solutions from exact quantum dynamics. We provide a systematic algorithmic approach for using RL for quantum protocols that deal with a non-trivial continuous state space, and discuss the scaling of these approaches for the preparation of larger entangled (cluster) states.

References:
Jelena Mackeprang, Durga B. Rao Dasari & Jörg Wrachtrup
Quantum Machine Intelligence 2, 1–14(2020)

Quantum systems combining indistinguishable photon generation and spin-based quantum information processing are essential for remote quantum applications and networking. In our recent work, we use silicon vacancy centres in silicon carbide to demonstrate controlled emission of indistinguishable and distinguishable photons via coherent spin manipulation. Although using strong off-resonant excitation, the system’s exceptional spectral stability allows us to measure two-photon interference contrasts close to 90% in Hong-Ou-Mandel type experiments. We also exploit the system’s intimate spin-photon relation to spin-control the colour and indistinguishability of consecutively emitted photons. The resulting two-photon quantum beating at a frequency of 966 MHz is shown in the figure. These results prove that spin-photon entanglement generation and photonic cluster state production are achievable with the system.

Further, our temperature dependent studies provide a deep insight into the system’s spin-phonon-photon physics, underlining the potential of the industrially compatible silicon carbide platform.

Our future efforts will focus on realising spin-photon entanglement and utilising hyperfine coupling to nuclear spin quantum registers for single-shot spin readout and other high-level network-relevant quantum information processing protocols.

Reference: Naoya Morioka et al., Nature Communications 11, 2516 (2020)

The measurement of a single nuclear spin in a noisy spin environment opens up new possibilities for quantum technologies.

Rare-earth related electron spins in crystalline hosts are unique material systems, as they can potentially provide a direct interface between telecom band photons and long-lived spin quantum bits. Specifically, their optically accessible electron spins in solids interacting with nuclear spins in their environment are valuable quantum memory resources. Detection of nearby individual nuclear spins, so far exclusively shown for few dilute nuclear spin bath host systems such as the nitrogen-vacancy center in diamond or the silicon vacancy in silicon carbide, remained an open challenge for rare earths in their host materials, which typically exhibit dense nuclear spin baths. Here, we present the electron spin spectroscopy of single ${\mathrm{Ce}}^{3+}$ ions in a yttrium orthosilicate host, featuring a coherence time of $T_2=124\mu s$. This coherent interaction time is sufficiently long to isolate proximal $89Y$ nuclear spins from the nuclear spin bath of $89Y$. Furthermore, it allows for the detection of a single nearby $29\mathrm{Si}$ nuclear spin, native to the host material with $\sim 5\%$ abundance. This study opens the door to quantum memory applications in rare-earth ion related systems based on coupled environmental nuclear spins, potentially useful for quantum error correction schemes.

This research is published in Physical Review Letters.

Review by Marric Stephens based in Bristol, UK "Sensing Single Spins in Dense Spin Baths" in Physics.

The Anton- und Klara-Röser-Stiftung awarded on the February 7th Dr. Roland Nagy with the dissertation Price.  The price giving ceremony took place at the graduate celebration of the Electronics and Information Technologies department of the University of Stuttgart. The title of Dr. Roland Nagy’s thesis is „Silicon Vacancy Defects in 4H-Silicon Carbide Semiconductor for Quantum Applications “.

Nitrogen-vacancy (NV) centers in diamond are promising quantum sensors for their long spin coherence time under ambient conditions. However, their spin resonances are relatively insensitive to non-magnetic parameters such as temperature. A magnetic-nanoparticle-nanodiamond hybrid thermometer, where the temperature change is converted to the magnetic field variation near the Curie temperature, was demonstrated to have enhanced temperature sensitivity (11 mK Hz^-1/2) [Phys. Rev. X 8, 011042 (2018)], but the sensitivity was limited by the large spectral broadening of ensemble spins in nanodiamonds. To overcome this limitation, we showed an improved design of a hybrid nanothermometer using a single NV center in a diamond nanopillar coupled with a single magnetic nanoparticle of copper-nickel alloy, and demonstrated a temperature sensitivity of 76 µK Hz^-1/2. This hybrid design enabled detection of 2 millikelvins temperature changes with temporal resolution of 5 milliseconds. The ultra-sensitive nanothermometer offers a new tool to investigate thermal processes in nanoscale systems

References:
Ultra-sensitive hybrid diamond nanothermometer
Liu, W. Leong, K. Xia, et. al., arXiv. 1912. 12097 (2019).

Coupled micro- and nanomechanical oscillators are of fundamental and technical interest for emerging quantum technologies. Upon interfacing with long-lived solid-state spins, the coherent manipulation of the quantum hybrid system becomes possible even at ambient conditions. Although the ability of these systems to act as a quantum bus inducing long-range spin–spin interactions has been known, the possibility to coherently couple electron/nuclear spins to the common modes of multiple oscillators and map their mechanical motion to spin-polarization has not been experimentally demonstrated. We here report experiments on interfacing spins to the common modes of a coupled cantilever system and show their correlation by translating ultralow forces induced by radiation from one oscillator to a distant spin. Further, we analyze the coherent spin–spin coupling induced by the common modes and estimate the entanglement generation among distant spins.

Quantum technology relies on proper hardware, enabling coherent quantum state control as well as efficient quantum state readout. In this regard, wide-bandgap semiconductors are an emerging material platform with scalable wafer fabrication methods, hosting several promising spin-active point defects. Conventional readout protocols for defect spins rely on fluorescence detection and are limited by a low photon collection efficiency. Here, we demonstrate a photo-electrical detection technique for electron spins of silicon vacancy ensembles in the 4H polytype of silicon carbide (SiC). Further, we show coherent spin state control, proving that this electrical readout technique enables detection of coherent spin motion. Our readout works at ambient conditions, while other electrical readout approaches are often limited to low temperatures or high magnetic fields. Considering the excellent maturity of SiC electronics with the outstanding coherence properties of SiC defects, the approach presented here holds promises for scalability of future SiC quantum devices.

References
Coherent electrical readout of defect spins in silicon carbide by photo-ionization at ambient conditions; Matthias Niethammer, Matthias Widmann, Torsten Rendler, Naoya Morioka, Yu-Chen Chen, Rainer Stöhr, Jawad Ul Hassan, Shinobu Onoda, Takeshi Ohshima, Sang-Yun Lee, Amlan Mukherjee, Junichi Isoya, Nguyen Tien Son & Jörg Wrachtrup; Nature Communications volume 10, Article number: 5569 (2019), December 5th, 2019

References
M. Widmann, M. Niethammer et al., Nano Letters 2019, 19, 10, 7173-7180 September 18, 2019

A new technique inserts single-ion impurities into a crystal reliably and with a precision of tens of nanometers. Solid-state materials that contain specific impurities, such as nitrogen-vacancy centers in diamond, make promising platforms for future quantum information technologies. The quantum states of the impurities can be tailored to perform quantum logic operations, and these states can then be read out via the impurities’ fluorescence. However, implanting single-ion impurities inside crystals consistently and precisely has proved challenging. Current methods involve accelerating the ions to high energies, which allows detection of the implantation events but sacrifices spatial precision. A new implantation technique takes a step towards solving this problem. A team from Stuttgart University and Johannes Gutenberg University Mainz demonstrated that they could embed ions in a crystal at low energy to form patterns reliably and with nanometer-scale precision. The researchers insert praseodymium ions into crystals of yttrium aluminum garnet (YAG), a synthetic crystal commonly used as a lasing medium. To do this, they trap and cool praseodymium ions using lasers and electric fields. Then they accelerate the ions one at a time toward the crystal, producing an evenly spaced array 6 nm below the crystal’s surface. After annealing the crystal at 1200°C, the researchers use confocal microscopy to confirm that the impurities successfully form fluorescing color centers inside the crystal. They found that they could control the placement of the impurities to 30-nm precision and that up to 50% of the impurities fluoresced – a success rate comparable to techniques that require ion energies 3 orders of magnitude higher. In the future, the researchers say, this technique could be widely adapted to implant other types of impurities into a variety of solid-state systems.

References:
Deterministic Single-Ion Implantation of Rare-Earth Ions for Nanometer-Resolution Color-Center Generation, Karin Groot-Berning, Thomas Kornher, Georg Jacob, Felix Stopp, Samuel T. Dawkins, Roman Kolesov, Jörg Wrachtrup, Kilian Singer, and Ferdinand Schmidt-Kaler, Phys. Rev. Lett. 123, 106802 – Published 4 September 2019

References
D. D. Bhaktavatsala Rao, Sen Yang, Stefan Jesenski, Emre Tekin, Florian Kaiser, and Jörg Wrachtrup, Phys. Rev. A 100, 022307 (2019)  – Published 8. August 2019

Electron spins in solids constitute formidable quantum sensors. Individual defect centers in diamond were used to detect individual nuclear spins with nanometer scale resolution, and ensemble magnetometers rival SQUID and vapor cell magnetometers when taking into account room temperature operation and size. NV center spins can also detect electric field vectors, despite their weak coupling to electric fields. Here, we employ ensembles of NV center spins to measure macroscopic ac electric vector fields with high precision. We utilize low strain, 12C enriched diamond to achieve maximum sensitivity and tailor the spin Hamiltonian via proper magnetic field adjustment to map out the ac electric field strength and polarization and arrive at refined electric field coupling constants. For high precision measurements we combine classical lock-in detection with aspects from quantum phase estimation for effective suppression of technical noise. Eventually, this enables t^-1/2 uncertainty scaling of the electric field strength over extended averaging periods, enabling us to measure electric fields down to 10^-7 V/μm for an AC electric field with a frequency of 2 kHz and an amplitude of 0.012 V/µm.

References: Robust and accurate electric field sensing with solid state spin ensembles; Nano Lett.2019, July 26, 2019, https://doi.org/10.1021/acs.nanolett.9b00900

Scalable quantum networks require stable quantum platforms with quantum processing capabilities and a reliable spin-optical interface. The search for ideal systems is still ongoing, as today’s available platforms suffer from too large electron phonon interaction and/or spectral diffusion.

The 3^rd physics institute demonstrated recently that the negatively charged silicon vacancy centre in silicon carbide is immune to both drawbacks. Thanks to its ⁴A₂ symmetry in ground and excited states, optical resonances are stable with near Fourier transform limited linewidths. In combination with millisecond long spin coherence times originating from the high purity crystal, we developed methods for high fidelity optically-assisted spin initialization and coherent control. Crucially, we showed coherent coupling to single nuclear spins with ~1 kHz resolution, which is sufficient to implement quantum memories and quantum error correction.

The summary of our findings makes the silicon vacancy in silicon carbide a prime candidate for realising memory-assisted quantum network applications using semiconductor-based spin-to-photon interfaces and coherently coupled nuclear spins.

References: Nagy et al., Nature Communications 10, 1954 (2019)

Single photon emitters in silicon carbide (SiC) are attracting attention as quantum photonic systems (Awschalom et al. Nat. Photonics 2018, 12, 516−527; Atatüre et al. Nat. Rev. Mater. 2018, 3, 38–51). However, to achieve scalable devices, it is essential to generate single photon emitters at desired locations on demand. Here we report the controlled creation of single silicon vacancy (V_Si) centers in 4H-SiC using laser writing without any postannealing process. Due to the aberration correction in the writing apparatus and the nonannealing process, we generate single V_Si centers with yields up to 30%, located within about 80 nm of the desired position in the transverse plane. We also investigated the photophysics of the laser writing V_Si centers and concluded that there are about 16 photons involved in the laser writing V_Si center process. Our results represent a powerful tool in the fabrication of single V_Si centers in SiC for quantum technologies and provide further insights into laser writing defects in dielectric materials.

References:
Laser Writing of Scalable Single Color Centers in Silicon Carbide, Yu-Chen Chen, Patrick S. Salter, Matthias Niethammer, Matthias Widmann, Florian Kaiser, Roland Nagy, Naoya Morioka, Charles Babin, Jürgen Erlekampf, Patrick Berwian, Martin J. Booth, and Jörg Wrachtrup, Nano Letters, Article ASAP, 10.1021/acs.nanolett.8b05070 Publication Date (Web): March 18, 2019

Six young scientists took the over 400 spectators in the sold out Theaterhaus in ten-minutes lectures on a journey into the wondrous world of quanta. Not an easy task for the slammers. Even experts in the field describe quantum theory as incomprehensible or, like Max Born, as "hopeless mess".

Love always wins in the end. At least that's how the people in the hall saw it and chose Charles Babin, Bretone with a fresh French accent and a doctoral student at the 3rd Institute of Physics, as the winner of the evening. "Love between photons is deeper than love between people", Babin explains to us the amorous relationships between his photon friends Leon, Manon and Anton and, by the way, quantum phenomena such as entanglement, measurement and teleportation.

References: 5th Institute of Physics

Nuclear magnetic resonance (NMR) of single spins have recently been detected by quantum sensors. However, the spectral resolution has been limited by the sensor’s relaxation to a few kHz at room temperature. This can be improved by using quantum memories, at the expense of sensitivity. In contrast, classical signals can be measured with exceptional spectral resolution by using continuous measurement techniques, without compromising sensitivity. When applied to single-spin NMR, it is critical to overcome the impact of back action inherent of quantum measurement. Here we report sequential weak measurements on a single 13C nuclear spin. The back-action causes the spin to undergo a quantum dynamics phase transition from coherent trapping to coherent oscillation. Single-spin NMR at room-temperature with a spectral resolution of 3.8 Hz is achieved. These results enable the use of measurement-correlation schemes for the detection of very weakly coupled single spins.

References
High-resolution spectroscopy of single nuclear spins via sequential weak measurements, Matthias Pfender, Ping Wang, Hitoshi Sumiya, et al., Nature Communications, Volume 10, Article number: 594 (2019)

The Macaluso–Corbino effect describes the optical rotation of light in the spectral proximity to an atomic resonance. One use of this effect is narrowband optical filtering. So-called Faraday filters utilize the difference of the two components of the refractive indices, which are split by the Zeeman effect in a longitudinal magnetic field. This allows for a net rotation of a linearly polarized input beam within the medium. Placing it between crossed polarizers therefore only allows light near resonance to pass. Since any resonant spectrum implies anomalous dispersion on resonance, these filters are often characterized as being based on this anomalous dispersion. This new publication analyses to what extent the anomalous dispersion and the anomalous rotation are relevant for Faraday filters. Considering the sign of the anomalous rotation introduces a strict criterion if the filter is operated in the line center or in the spectral wing of an atomic resonance.

References
How anomalous is my Faraday filter? Optics Letters, Vol. 43, Issue 21, pp. 5295-5298 (2018), Ilja Gerhardt

Spins of impurities in solids provide a unique architecture to realize quantum technologies. A quantum register of electron and nearby nuclear spins in the lattice encompasses high-fidelity state manipulation and readout, long-lived quantum memory, and long-distance transmission of quantum states by optical transitions that coherently connect spins and photons. These features, combined with solid-state device engineering, establish impurity spins as promising resources for quantum networks, information processing and sensing. Focusing on optical methods for the access and connectivity of single spins, we review recent progress in impurity systems such as colour centres in diamond and silicon carbide, rare-earth ions in solids and donors in silicon. We project a possible path to chip-scale quantum technologies through sustained advances in nanofabrication, quantum control and materials engineering. The figure shows diverse modalities and applications of quantum sensing with impurity spins.

References
Quantum technologies with optically interfaced solid-state spins; David D. Awschalom, Ronald Hanson, Jörg Wrachtrup & Brian B. Zhou; Nature Photonics, Volume 12, pages 516–527 (2018)

References
Supported Lipid Bilayers on Fluorescent Nanodiamonds: A Structurally Defined and Versatile Coating for Bioapplications, Jan Vavra, Ivan Rehor, Torsten Rendler, Mona Jani, Jan Bednar, Michael M. Baksh, Andrea Zappe, Joerg Wrachtrup and Petr Cigler https://doi.org/10.1002/adfm.201803406

The magnetic field of a conventional hard disc drive (HDD) writer combines a high amplitude (over 200 mT) with a large gradient (up to 1 mT/nm). The usage in HDDs requires a gigahertz bandwidth that allows switching times within nanoseconds. These are much faster than characteristic timescales such as Rabi and Larmor periods, spin-spin couplings, optical transitions and spin relaxation times. The unique combination of the three features makes an HDD writer valuable for the coherent manipulation of individual spins in nanoscale networks.

In our recent experiments we exploited these properties of an HDD writer to manipulate single as well as multiple spins. With the gigahertz bandwidth we were able to drive spin transitions via non-adiabatic fast passages as well as to optically read-out the spin state in strong misaligned magnetic fields by rapidly applying control fields. Based on these techniques we used the HDD’s gradient to selectively address the spins of two parallel NVs by resonant microwave radiation with a fidelity of 99%. The individual control of the spin states allowed us to do deterministic emitter switch microscopy (DESM) to optically determine the parallel NVs’ relative distance with nanoscale resolution.

In addition, we characterized the writers’ noise level as a limiting factor for the interaction range of a coupled network. With gradients in the order of mT/nm individual spins with a few nanometer distance can be individually addressed. Therefore, HDD writers are suitable for applications in quantum information processing or sensing arrays.

Reference:
Nanoscale Spin Manipulation with Pulsed Magnetic Gradient Fields from a Hard Disc Drive Writer, Sven Bodenstedt, Ingmar Jakobi, Julia Michl, Ilja Gerhardt, Philipp Neumann, and Jörg Wrachtrup
https://dx.doi.org/10.1021/acs.nanolett.8b01387 (2018)

Quantum mechanics implies that a single photon can be in the superposition of two distant spatial modes and enable nonlocal interferences. The most vivid example is the two-photon coalescence on a $50:50$ beam splitter, known as Hong-Ou-Mandel interference. In the past decade, this experiment has been used to characterize the suitability of different single-photon sources for linear optical quantum gates. This characterization alone cannot guarantee the suitability of the photons in a scalable quantum network. As for a deeper insight, we perform a number of nonclassical interference measurements of single photons emitted by a single organic molecule that are optimized by an atomic Faraday filter. Our measurements reveal near unity visibility of the quantum interference, and a one-port correlation measurement proves the ideal Fourier limited nature of our single-photon source. A delayed choice quantum eraser allows us to observe a constructive interference between the photons, and a Hong-Ou-Mandel peak is formed additionally to the commonly observed dip. These experiments comprehensively characterize the involved photons for their use in a future quantum Internet, and they attest to the fully efficient interaction of the molecular photons with a next subsequent quantum node. They can be adapted to other emitters and will allow us to gain insights to their applicability for quantum information processing. We introduce a quality number that describes the photon’s properties for their use in a quantum network; this states that effectively 97% of the utilized molecular photons can be used in a scalable quantum optical system and interact with other quantum nodes. The experiments are based on a hybridization of solid state quantum optics, atomic systems, and all-optical quantum information processing.

References:
Mohammad Rezai, Jörg Wrachtrup, and Ilja Gerhardt, "Coherence Properties of Molecular Single Photons for Quantum Networks", Phys. Rev. X 8, 031026 – Published 26 July 2018

Single-photon emitting devices have been identified as an important building block for applications in quantum information and quantum communication. They allow us to transduce and collect quantum information over a long distance via photons as so-called flying qubits. In addition, substrates like silicon carbide provide an excellent material platform for electronic devices. In this work, we combine these two features and show that one can drive single photon emitters within a silicon carbide p-i-n-diode. To achieve this, we specifically designed a lateral oriented diode. We find a variety of new color centers emitting non-classical lights in the visible and near-infrared range. One type of emitter can be electrically excited, demonstrating that silicon carbide can act as an ideal platform for electrically controllable single photon sources.

References:
Matthias Widmann, Matthias Niethammer, Takahiro Makino, Torsten Rendler, Stefan Lasse, Takeshi Ohshima, Jawad Ul Hassan, Nguyen Tien Son, Sang-Yun Lee, and Jörg Wrachtrup, "Bright single photon sources in lateral silicon carbide light emitting diodes", 07 June 2018, Applied Physics Letters

A central goal in quantum optics and quantum information science is the development of quantum networks to generate entanglement between distributed quantum memories. Experimental progress relies on the quality and efficiency of the light-matter quantum interface connecting the quantum states of photons to internal states of quantum emitters. Quantum emitters in solids, which have properties resembling those of atoms and ions, offer an opportunity for realizing light-matter quantum interfaces in scalable and compact hardware. These quantum emitters require a material platform that enables stable spin and optical properties, as well as a robust manufacturing of quantum photonic circuits. Because no emitter system is yet perfect and different applications may require different properties, several light-matter quantum interfaces are being developed in various platforms.

This Review highlights the progress in three leading material platforms: diamond, silicon carbide and atomically thin semiconductors.

References:
Material platforms for spin-based photonic quantum technologies, Mete Atatüre, Dirk Englund, Nick Vamivakas, Sang-Yun Lee  and  Joerg Wrachtrup, Nature Reviews Materials volume 3, pages38–51, Published: MAY 2018 , DOI: 10.1038/s41578-018-0008-9

References:
F. Shagieva, S. Zaiser, P. Neumann , D. B. R. Dasari, R. Stöhr, A. Denisenko, R. Reuter, C. A. Meriles, and J. Wrachtrup, Nano Letters, Article ASAP, DOI: 10.1021/acs.nanolett.8b00925, Publication Date (Web): May 2, 2018

References
Quantum properties of dichroic silicon vacancies in silicon carbide, Roland Nagy, Matthias Widmann, Matthias Niethammer, Durga B. R. Dasari, Ilja Gerhardt, Öney O. Soykal, Marina Radulaski, Takeshi Ohshima, Jelena Vuckovic, Nguyen Tien Son, Ivan G. Ivanov, Sophia E. Economou, Cristian Bonato, Sang-Yun Lee, and Jörg Wrachtrup, Phys. Rev. Applied, 2018

References:
Unbiased All-Optical Random-Number Generator, Tobias Steinle, Johannes N. Greiner, Jörg Wrachtrup, Harald Giessen, and Ilja Gerhardt, Phys. Rev. X 7, 041050 – Published 30 November 2017

References:
Quantum Light in Curved Low Dimensional Hexagonal Boron Nitride Systems, Nathan Chejanovsky, Youngwook Kim, Andrea Zappe, Benjamin Stuhlhofer, Takashi Taniguchi, Kenji Watanabe, Durga Dasari, Amit Finkler, Jurgen H. Smet & Jörg Wrachtrup, Scientific Reports 7, Article number: 14758 (2017) doi:10.1038/s41598-017-15398-2

References:
Nonvolatile nuclear spin memory enables sensor-unlimited nanoscale spectroscopy of small spin clusters, M. Pfender, N. Aslam, H. Sumiya, S. Onoda, P. Neumann, J. Isoya, C. A. Meriles & J. Wrachtrup, Nature Communications 8, 834 (2017).

Switching on and off the coupling of proximal nuclear spins to the outside world

In recent years, solid-state spin systems have emerged as promising candidates for quantum information processing and sensing applications. Prominent examples are the nitrogen-vacancy (NV) center in diamond, phosphorous dopants in silicon (Si:P), rare-earth ions in solids and V _Si -centers in silicon-carbide. Typically, nuclear spins serve as memories whereas the electron spins of these candidate systems grant access to the memories in the first place. For example, the NV center electron spin state determines the photoluminescence response. In all these systems, however, the electron spin is not only a gift but also a burden. Its fast relaxation decreases the quantum memory’s lifetime. The Si:P system has demonstrated that its nuclear spins can yield exceedingly long quantum memory lifetimes by eliminating the electron spin of the dopant. For NV centers, however, a proper charge state for storage of quantum information has not been identified yet. In our recent publication, we have identified and characterized the positively-charged NV center as an electron-spin-less and optically inactive state by utilizing the nuclear spin qubit as a probe. We have controlled the electronic charge and spin utilizing nanometer scale gate electrodes. As a result, we have improved the quantum memory’s lifetime by a factor of four. Surprisingly, the new charge state allows switching the optical response of single NV spin systems facilitating full individual addressability even in dense arrays with mutual distances below the optical diffraction limit. Our results, open up new and promising possibilities for scaling up magnetic-dipole-coupled, NV center quantum registers in diamond. Furthermore, improved quantum storage times and optical switching improve applications in quantum communication, as for example in quantum repeaters.

Figure: Two photoluminescence (PL) microscopy images of the same NV center in different charge states (NV ^-, NV ⁺). The polarity (+,-) of adjacent electrodes allows setting the charge state. While NV ^- exhibits an electron and a nuclear spin (blue and purple arrow), NV ⁺only possesses a nuclear spin, which is therefore very isolated from many noise sources.

References:
M. Pfender, N. Aslam, P. Simon, D. Antonov, G. Thiering, S. Burk, F. Fávaro de Oliveira, A. Denisenko, H. Fedder, J. Meijer, J. A. Garrido, A. Gali, T. Teraji, J. Isoya, M. W. Doherty, A. Alkauskas, A. Gallo, A. Grüneis, P. Neumann, and J. Wrachtrup, Protecting a diamond quantum memory by charge state control, Nano Lett. (2017), doi: 10.1021/acs.nanolett.7b01796

References:
Molecular quantum spin network controlled by a single qubit, Lukas Schlipf, Thomas Oeckinghaus, Kebiao Xu, Durga Bhaktavatsala Rao Dasari, Andrea Zappe, Felipe Fávaro de Oliveira, Bastian Kern, Mykhailo Azarkh, Malte Drescher, Markus Ternes, Klaus Kern, Jörg Wrachtrup, Amit Finkler, Science Advances 2017

Atomic-size spin defects in solids are unique quantum systems. Most applications require nanometer positioning accuracy, which is typically achieved by low-energy ion implantation. A drawback of this technique is the significant residual lattice damage, which degrades the performance of spins in quantum applications. Here we show that the charge state of implantation-induced defects drastically influences the formation of lattice damage during implantation and thermal annealing. Charging of vacancies at, for example, nitrogen implantation sites suppresses the formation of vacancy complexes, resulting in tenfold-improved spin coherence times and twofold-improved formation yield of nitrogen-vacancy centers in diamond. This is achieved by confining implantation defects into the space-charge layer of free carriers (holes) generated within a p + -i junction structures with overgrown layer of diamond with high concentration of boron acceptors. Combined with numerical calculations, these results bring new insights into the dynamic mechanisms of self-diffusion and defect formation at single implantation sites. In the future, the concept of defect charging could be implemented into various solid-state systems for improved engineering of quantum devices.

References:
F. F. de Oliveira, et al. Nat. Commun. 8, 15409 doi: 10.1038/ncomms15409 (2017)

Nuclear magnetic resonance (NMR) is a valuable technique that has found application in hospitals in the form of magnetic resonance imaging (MRI). Furthermore NMR spectroscopy has been used in laboratories to identify almost all known molecules. The main limiting factor is its low sensitivity requiring millimeter-sized samples. This has been overcome by using the nitrogen-vacancy (NV) center in diamond as a NMR probe and decreased the required sample volume by many orders of magnitude down to the nanometer-scale [1]. Previous NV nanoscale NMR studies however showed poor spectral resolution making it unfeasible for spectroscopy. In our recent NV nanoscale NMR work [2], we demonstrate an improvement of the spectral resolution by a factor of 10,000 and resolve chemical shift effects in proton and fluorine containing molecules.

High quality optical thin film materials form a basis for on-chip photonic micro- and nanodevices, where several photonic elements form an optical circuit. Their realization generally requires the thin film to have a higher refractive index than the substrate material. Here, we demonstrate a method of depositing amorphous TiO ₂ films doped with 25% Si on various substrates, a way of shaping these films into photonic elements, such as optical waveguides and resonators, and finally, the performance of these elements.

Dynamical polarization of nuclear spin ensembles is of central importance for magnetic resonance studies, precision sensing and for applications in quantum information theory. Here we propose a scheme to generate long-lived singlet pairs in an unpolarized nuclear spin ensemble which is dipolar coupled to the electron spins of a Nitrogen Vacancy center in diamond. The quantum mechanical back-action induced by frequent spin-selective readout of the NV centers allows the nuclear spins to pair up into maximally entangled singlet pairs. Counterintuitively, the robustness of the pair formation to dephasing noise improves with increasing size of the spin ensemble. We also show how the paired nuclear spin state allows for enhanced sensing capabilities of NV centers in diamond.

References
Purification of an unpolarized spin ensemble into entangled singlet pairs
Johannes N. Greiner, Durga Bhaktavatsala Rao Dasari & Jörg Wrachtrup
Scientific Reports 7, Article number: 529 (2017), doi:10.1038/s41598-017-00603-z

Dissipative entanglement of solid-state spins in diamond

Generating robust entanglement among solid-state spins is key for applications in quantum information processing and precision sensing. Here we show a dissipative approach to generate such entanglement among the hyperfine coupled electron nuclear spins using the rapid optical decay of electronic excited states. The combined dark state interference effects of the optical and microwave driving fields in the presence of spontaneous emission from the short-lived excited state leads to a dissipative formation of an entangled steady state. We show that the dissipative entanglement is generated for any initial state conditions of the spins and is resilient to external field fluctuations. We analyze the scheme for both continuous and pulsed driving fields in the presence of realistic noise sources.

References
Dissipative entanglement of solid-state spins in diamond
Durga Bhaktavatsala Rao Dasari, Sen Yang & Jörg Wrachtrup
PHYSICAL REVIEW A, 95, 022310 (2017) DOI:10.1103/PhysRevA.95.022310

Silicon carbide is a promising platform for single photon sources [1], quantum bits (qubits), and nanoscale sensors [2] based on individual color centers. Toward this goal, we develop a scalable array of nanopillars incorporating single silicon vacancy centers in 4H-SiC, readily available for efficient interfacing with free-space objective and lensed-fibers.

Magnetic sensing and imaging instruments are important tools in biological and material sciences. There is an increasing demand for attaining higher sensitivity and spatial resolution, with implementations using a single qubit offering potential improvements in both directions. In this article we describe a scanning magnetometer based on the nitrogen-vacancy center in diamond as the sensor. By means of a quantum-assisted readout scheme together with advances in photon collection efficiency, our device exhibits an enhancement in signal to noise ratio of close to an order of magnitude compared to the standard fluorescence readout of the nitrogen-vacancy center. This is demonstrated by comparing non-assisted and assisted methods in a T1 relaxation time measurement.

Reference:
Nuclear quantum-assisted magnetometer
Review of Scientific Instruments, Volume 88, Issue 1, 10.1063/1.4973449
Thomas Häberle, Thomas Oeckinghaus, Dominik Schmid-Lorch, Matthias Pfender, Felipe Fávaro de Oliveira, Seyed Ali Momenzadeh, Amit Finkler and Jörg Wrachtrup

Newly discovered van der Waals materials like MoS2, WSe2, hexagonal boron nitride (h-BN) and recently C2N have sparked intensive research to unveil the quantum behavior associated with their 2D structure. Of great interest are 2D materials that host single quantum emitters. h-BN, with a band-gap of –5.95 eV, has been shown to host single quantum emitters which are stable at room temperature in the UV and visible spectral range. Here we investigate correlations between h-BN structural features and emitter location from bulk down to the mono-layer at room temperature. We demonstrate that chemical etching and ion irradiation can generate emitters in h-BN. We analyze the emitters’ spectral features and show that they are dominated by the interaction of their electronic transition with a single Raman active mode of h-BN. Photo-dynamics analysis reveals diverse rates between the electronic states of the emitter. The emitters show excellent photo stability even under ambient conditions and in monolayers. Comparing the excitation polarization between different emitters unveils a connection between defect orientation and the h-BN hexagonal structure. The sharp spectral features, color diversity, room-temperature stability, long lived meta-stable states, ease of fabrication, proximity of the emitters to the environment, outstanding chemical stability and biocompatibility of h-BN provide a completely new class of systems that can be used for sensing and quantum photonics applications.

Reference
Structural Attributes and Photodynamics of Visible Spectrum Quantum Emitters in Hexagonal Boron Nitride
Nathan Chejanovsky, Mohammad Rezai, Federico Paolucci, Youngwook Kim, Torsten Rendler, Wafa Rouabeh, Felipe Fávaro de Oliveira, Patrick Herlinger, Andrej Denisenko, Sen Yang, Ilja Gerhardt, Amit Finkler, Jurgen H. Smet and Jörg Wrachtrup
Link (extern): http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.6b03268

In a collaboration with Seagate Technology PLC, we have developed methods to characterize the magnetic fields of hard-disk writers using an atomic scale magnetometer. The results mark a significant increase in spatial resolution compared to established sensors and may enable further miniaturization of recording heads and ultimately increase storage capacities of hard disk drives (HDD).

HDDs store data as permanent magnetization on sectors as small as 20 nm on the recording medium of the hard disk. In order to encode data as magnetization bits the hard disk writer, a nanoscopic electromagnet, has to produce magnetic fields that only affect a single bit while it flies by on the rotating disk. Accordingly, its magnetic field is designed to be strong, fast and especially local. These features are beyond the capabilities of established magnetometers leaving the industry devoid of a suitable research and development tool to further miniaturize their products.

Heralded Control of Mechanical Motion by Single Spins

We propose a method to achieve a high degree of control of nanomechanical oscillators by coupling their mechanical motion to single spins. Manipulating the spin alone and measuring its quantum state heralds the cooling or squeezing of the oscillator even for weak spin-oscillator couplings. We analytically show that the asymptotic behavior of the oscillator is determined by a spin-induced thermal filter function whose overlap with the initial thermal distribution of the oscillator determines its cooling, heating, or squeezing. Counterintuitively, the rate of cooling dependence on the instantaneous thermal occupancy of the oscillator renders robust cooling or squeezing even for high initial temperatures and damping rates. We further estimate how the proposed scheme can be used to control the motion of a thin diamond cantilever by coupling it to its defect centers at low temperature.

Thin Circular Diamond Membrane with Embedded Nitrogen-Vacancy Centers for Hybrid Spin-Mechanical Quantum Systems

In quantum sensing, precision is typically limited by the maximum time interval over which phase can be accumulated. Memories have been used to enhance this time interval beyond the coherence lifetime and thus gain precision. Here, we demonstrate that by using a quantum memory also an increased sensitivity can be achieved. To this end, we use entanglement in a hybrid spin system comprising a sensing and a memory qubit associated with a single nitrogen-vacancy center in diamond (see upper figure part). With the memory we retain the full quantum state even after coherence decay of the sensor, which enables coherent interaction with distinct weakly coupled nuclear spin qubits (see lower figure part). To this end we do tomography of the memory’s quantum phase as we increase the interaction time τ with a sample qubit (here: ¹³C spin, a: cos φ, b: sin φ ). The phase is only accumulated when the sample spin is resonantly driven during the storage time T _c. The spectral selectivity is determined by 1/T _c which is much narrower than what is possible with the sensor alone. Furthermore, we benchmark the performance of our hybrid quantum system against use of the sensing qubit alone by gradually increasing the entanglement of sensor and memory. Finally, we further apply this quantum sensor-memory pair for high-resolution NMR spectroscopy of single ¹³C nuclear spins.

The results of our work are published in:
S. Zaiser, T. Rendler, I. Jakobi, T. Wolf, S.-Y. Lee, S. Wagner, V. Bergholm, T. Schulte-Herbrüggen, P. Neumann, and J. Wrachtrup, Enhancing quantum sensing sensitivity by a quantum memory. Nature Communications 7, 12279 (2016).

A tool for accurate and quantitative extraction of resonant vibrational responses

A novel approach based on the wavelet prism decomposition analysis is proposed to correct experimental artefacts in a coherent anti-Stokes Raman scattering (CARS) spectrum. In collaboration with Erik Vartiainen and co-workers of the Lappeenranta University in Finland, Gregor Hehl and Andreas Volkmer demonstrate that this method allows estimating and eliminating a slowly varying modulation error function in the measured normalized CARS spectrum and yields a corrected CARS line-shape. The main advantage of the approach is that any corrections of the spectral phase and amplitude are avoided in the retrieved vibrational line-shape spectrum, thus significantly simplifying the quantitative reconstruction of the sample’s Raman response from a normalized CARS spectrum in the presence of experimental artefacts. Moreover, unlike in conventional error-correction schemes, the method obviates the need for assumptions about the modulation error distribution and the chemical composition of the specimens under study. The method is quantitatively validated on normalized CARS spectra recorded for equimolar aqueous solutions of D-fructose, D-glucose, and their disaccharide combination sucrose. Besides being essential for the realization of quantitative multiplex CARS imaging applications, this method also offers a tool for accurate and quantitative reconstruction of complex material responses probed by other nonlinear optical spectroscopies, such as for the correction of experimental errors in measured third-harmonic generation, sum-frequency generation, and degenerate four-wave mixing power spectra.

Reference:
Y. Kan, L. Lensu, G. Hehl, A. Volkmer*, and E.M. Vartiainen*, Opt. Express 24 (2016) 11905-11916.
Link (extern): doi: 10.1364/OE.24.011905

Long distance quantum communication requires photons and quantum nodes that comprise qubits for interaction with light, good memory capabilities and processing qubits for storage and manipulation of photons. Owing to the unavoidable photon losses, robust quantum communication over lossy transmission channels requires quantum repeater networks. A necessary and highly demanding requirement for these networks is the existence of quantum memories with long coherence times to reliably store the incident photon states. Here, we demonstrate a high fidelity (~98%) coherent transfer of a photon polarisation state to a single solid state nuclear spin that has a coherence time of over 10 seconds. The storage process is achieved by coherently transferring the polarisation state of a photon to an entangled electron-nuclear spin state of a nitrogen vacancy (NV) centre in diamond. This nuclear spin based optical quantum memory demonstrated here paves the way towards an absorption based quantum repeater network.

Reference:
Nature Photonics | Letter
High-fidelity transfer and storage of photon states in a single nuclear spin
Sen Yang, Ya Wang, D. D. Bhaktavatsala Rao, Thai Hien Tran, Ali S. Momenzadeh, M. Markham, D. J. Twitchen, Ping Wang, Wen Yang, Rainer Stöhr, Philipp Neumann, Hideo Kosaka & Jörg Wrachtrup
Nature Photonics (2016) doi:10.1038/nphoton.2016.103 Published online: 06 June 2016

PhD Student Kangwei Xia Receives Prestigious 2015 Chinese Government Award for Outstanding Self-Financed Students Abroad

The award is set-up to honor overseas non-Chinese-government funded Chinese PhD students with outstanding academic accomplishments across all disciplines. This year, 35 PhD students studying in Germany received this award. Kangwei Xia is the 12th.award winner studying at University of Stuttgart since 2003. The prize amount for this award is $6,000, sponsored by the Chinese Ministry of Education.

Kangwei Xia is a 4th year PhD student in our institute. His research topic is spectroscopy of single rare earth solid-state qubits, including optical detection, coherent manipulation and optical readout different species single rare earth ions in crystals.

Link (external): 2015 Chinese Government Award for Extra-Outstanding Self-Financed Students

Toward Optimized Surface δ – Profiles of Nitrogen-Vacancy Centers Activated by Helium Irradiation in Diamond

Coherently coupled pairs or multimers of nitrogen-vacancy defect centers (NV) in diamond have many promising applications especially in quantum information processing. Scalable registers are essential to the progress of the field. Ion implantation is the only technique known to produce NV pairs close enough to be coupled with dipolar interaction. However, while there are many approaches to increase the resolution of ion implantation, no method is able to reliably produce working registers. Together with the density of implanted nitrogen the density of unwanted defects (e.g. divacancies) related to the implantation is also increased, resulting in inferior qubits. Here we present an estimation what nanomask implantation parameters (energy, fluence and aperture size) are most likely to produce interacting qubits. We apply our findings to an easily reproducible implantation technique, the e-beam lithography in PMMA, to create coupled NV pairs with a reasonable probability.

Appl. Phys. Lett. 108, 053108 (2016); http://dx.doi.org/10.1063/1.4941403

Rare-earth (RE) ion doped crystals are well known due to their key application in laser technology. Latest research, however, suggests their usage in the field of quantum telecommunication as quantum memories or even for information processing. Their generation in a well chosen and engineered environment can act as a prerequisite for versatile implementation of RE ions in experiments and might enable a range of novel integrated photonic devices for quantum applications.

Controlled doping into desired locations of optical crystals was achieved by ion implantation. We have investigated the production yield of two RE species, namely cerium and praseodymium. As a measure, the collected fluorescence intensity from both implanted samples and single centers was used. With a tailored annealing procedure for cerium, a yield up to 53% was estimated. Praseodymium yield amounts up to 91%. Such high implantation yield indicates a feasibility of creation of nanopatterned rare-earth doping and suggests strong potential of RE species for on-chip photonic devices.

via an individual paramagnetic centre

... on shallow nitrogen-vacancy centers in diamond

Near-surface nitrogen-vacancy (NV) centers in diamond have been successfully employed as atomic-sized magnetic field sensors for external spins over the last years. A key challenge is still to develop a method to bring NV centers at nanometer proximity to the diamond surface while preserving their optical and spin properties. To that aim we present a method of controlled diamond etching with nanometric precision using an oxygen inductively coupled plasma process. Importantly, no traces of plasma-induced damages to the etched surface could be detected by X-ray photoelectron spectroscopy and confocal photoluminescence microscopy techniques. In addition, by profiling the depth of NV centers created by 5.0 keV of nitrogen implantation energy, no plasma-induced quenching in their fluorescence could be observed. Moreover, the developed etching process allowed even the channeling tail in their depth distribution to be resolved. Furthermore, treating a ¹² C isotopically purified diamond revealed a threefold increase in T ₂ times for NV centers with <4 nm of depth (measured by nuclear magnetic resonance signal from protons at the diamond surface) in comparison to the initial oxygen-terminated surface.

of Superparamagnetic Magnetite Nanoparticles Using a Single Qubit

To study the magnetic dynamics of superparamagnetic nanoparticles, we use scanning probe relaxometry and dephasing of the nitrogen vacancy (NV) center in diamond, characterizing the spin noise of a single 10 nm magnetite particle. Additionally, we show the anisotropy of the NV sensitivity’s dependence on the applied decoherence measurement method. By comparing the change in relaxation (T1) and dephasing (T2) time in the NV center when scanning a nanoparticle over it, we are able to extract the nanoparticle’s diameter and distance from the NV center using an Ornstein–Uhlenbeck model for the nanoparticle’s fluctuations. This scanning probe technique can be used in the future to characterize different spin label substitutes for both medical applications and basic magnetic nanoparticle behavior.

Reference: Relaxometry and Dephasing Imaging of Superparamagnetic Magnetite Nanoparticles Using a Single Qubit, Dominik Schmid-Lorch, Thomas Häberle, Friedemann Reinhard, Andrea Zappe, Michael Slota, Lapo Bogani, Amit Finkler, and Jörg Wrachtrup, Nano Lett., Article ASAP, DOI: 10.1021/acs.nanolett.5b00679, Publication Date (Web): July 28, 2015

Single electron and nuclear spin magnetic resonance experiments in the context of quantum information processing and nanoscale metrology can benefit strongly from high magnetic fields.

... of Nucleic Acids and Tryptophan for Distinction of Normal Human Skin Cells and Tumorigenic Keratinocytes

Tumor diagnostic imaging is commonly based on staining of biopsies, which requires tissue excision, fixation, and staining with exogenous marker molecules. Here, P. Piredda and A. Volkmer demonstrate label-free tumor imaging using confocal spontaneous Raman scattering microspectroscopy, which exploits the intrinsic vibrational contrast of endogenous biomolecular species. In a collaboration with M. Berning and P. Boukamp of the DKFZ in Heidelberg, we present a chemically specific and quantitative approach to monitoring normal human skin cells as well as the human HaCaT in vitro skin carcinogenesis model and the tumor-derived MET in vivo skin cancer progression model. Mapping the amplitudes of two spectrally well isolated Raman bands allowed for direct visualization of the distributions representative of tryptophan-rich proteins and nucleic acids with subcellular spatial resolution. Using these Raman markers, it was feasible to discriminate between normal human epidermal keratinocytes and dermal fibroblasts, and to confine all tumorigenic cells from both. First evidence for the successful application of the proposed intracellular nucleic acid and tryptophan Raman signatures for skin cancer diagnosis was further demonstrated in an organotypic cutaneous squamous cell carcinomas model, allowing for the identification of tumor cells and their surrounding stroma in the tissue context.

Reference: P. Piredda, M. Berning, P. Boukamp, and A. Volkmer, Subcellular Raman Microspectroscopy Imaging of Nucleic Acids and Tryptophan for Distinction of Normal Human Skin Cells and Tumorigenic Keratinocytes, Anal. Chem. 87 (2015), advance online, DOI: 10.1021/acs.analchem.5b01009

Link (extern): Publication

Universität Stuttgart, 6. März 2015. Diamant-Sensor macht einzelne Proteine in natürlicher Umgebung sichtbar
Proteine sind die zentralen Bausteine des Lebens. Zellen beispielsweise sind aus einer Vielzahl verschiedener Proteinen zusammengesetzt. Ihr Zusammenspiel bestimmt praktisch alle wichtigen biologischen Funktionen und entscheidet über Gesundheit beziehungsweise Krankheit von Organismen. Proteine selbst bestehen aus vielen Tausend Atomen, deren genaue Anordnung die Funktionsweise des Proteins bestimmt. Entsprechend begehrt sind Verfahren, die es erlauben, die genaue Form und Dynamik von Proteinen in ihrer natürlichen Umgebung zu untersuchen. Ein internationales Forscherteam der Universität Stuttgart und der Chinesischen Akademie der Wissenschaften ist dabei nun einen entscheidenden Schritt vorangekommen. Darüber berichtet die Zeitschrift Science in Ihrer Ausgabe vom 6. März 2015.

Der Gruppe um Prof. Jörg Wrachtrup vom 3. Physikalischen Institut der Universität Stuttgart und Prof. Jiangfeng Du ist es gelungen, mit einem neuen Verfahren einzelne Proteine in natürlicher Umgebung sichtbar zu machen. Dazu haben sie einen neuen Sensor benutzt, der auf einzelnen Defekten in Diamant beruht. Die Defekte können Magnetfelder in ihrer Umgebung nachweisen und sind zum Beispiel empfindlich genug, um das Feld einzelner Elektronen detektieren zu können. Dies haben die Forscher ausgenutzt, um die Proteine sichtbar zu machen.

Vollständiger Artikel

Weitere Infos – MPG

Originalpublikation:
Single-protein Spin Resonance Spectroscopy under Ambient Conditions
Fazhan Shi,Qi Zhang, Pengfei Wang, Hongbin Sun, Jiarong Wang, Xing Rong, Ming Chen, Chenyong Ju, Friedemann Reinhard, Hongwei Chen, Jörg Wrachtrup, Junfeng Wang, Jiangfeng Du
Link (extern): http://www.sciencemag.org/content/347/6226/1135

Paola Cappellaro, Professor at MIT is elected Mercator-Fellow of the recently funded DFG Research Group “Diamond Materials for Quantum Application”. Prof. Cappellaro is an internationally known expert in quantum spin physics with major contributions to NMR and diamond spin physics. In particular she focussed on the analysis of diamond-based nano magnetometry. Recently, she concentrated on applications to nuclear spin polarisation in combination with NMR structure analysis of single proteins. The Mercator-Fellowship endows the mutual research project and cooperation among Prof. Cappellaro and members of the research group. (Photo: Research Laboratory of Electronics)

Nanoengineered Diamond Waveguide as a Robust Bright Platform for Nanomagnetometry Using Shallow Nitrogen Vacancy Centers

Photonic structures in diamond are key to most of its application in quantum technology. Here, we demonstrate tapered nanowaveguides structured directly onto the diamond substrate hosting shallow-implanted nitrogen vacancy (NV) centers. By optimization based on simulations and precise experimental control of the geometry of these pillar-shaped anowaveguides, we achieve a net photon flux up to ~ 1.7 × 10 ⁶ s ^-1. This presents the brightest monolithic bulk diamond structure based on single NV centers so far. We observe no impact on excited state lifetime and electronic spin dephasing time (T ₂) due to the nanofabrication process. Possessing such high brightness with low background in addition to preserved spin quality, this geometry can improve the current nanomagnetometry sensitivity ~ 5 times. In addition, it facilitates a wide range of diamond defects-based magnetometry applications. As a demonstration, we measure the temperature dependency of T ₁ relaxation time of a single shallow NV center electronic spin. We observe the two-phonon Raman process to be negligible in comparison to the dominant two-phonon Orbach process.

with chemical contrast by nanoscale nuclear magnetic resonance

Scanning probe microscopy is one of the most versatile windows into the nanoworld, providing imaging access to a variety of electronic, dielectric, magnetic and topographic sample properties. We demonstrate a scanning probe imaging method that extends the range of accessible quantities to label-free imaging of chemical species while operating on arbitrary samples – including insulating materials – under ambient conditions. Moreover, its sensitivity extends below the surface of a sample, allowing for imaging of subsurface features. We achieve these results by recording NMR signals from a sample surface with a recently introduced scanning probe, a single nitrogen-vacancy centre in diamond. We demonstrate NMR imaging with 10 nm resolution and achieve chemically specific contrast by separating fluorine from hydrogen-rich regions. Our result opens the door to scanning probe imaging of the chemical composition and molecular structure of arbitrary samples.

Reference: Häberle, T., Schmid-Lorch, D., Reinhard, F. & Wrachtrup, J. Nanoscale nuclear magnetic imaging with chemical contrast. Nat. Nanotechnol. advance on, (2015), DOI: 10.1038/nnano.2014.299.

Spins in solids are cornerstone elements of quantum spintronics. Leading contenders such as defects in diamond or individual phosphorus dopants in silicon have shown spectacular progress, but either lack established nanotechnology or an efficient spin/photon interface. Silicon carbide (SiC) combines the strength of both systems: it has a large bandgap with deep defects and benefits from mature fabrication techniques. We report the characterization of photoluminescence and optical spin polarization from single silicon vacancies in SiC, and demonstrate that single spins can be addressed at room temperature. We also show coherent control of a single defect spin and find long spin coherence times under ambient conditions. Our study provides evidence that SiC is a promising system for atomic-scale spintronics and quantum technology.

The DFG research group FOR 1493 “Diamond Materials and Quantum Applications” goes into its second funding period.

FOR1493 is a national research consortium funded by the Deutsche Forschungs-gemeinschaft. The strategic aim of the research group is to foster quantum material science and build strong ties between material science and quantum applications. In total it hosts 11 partner universities with 20 PIs. In addition, the research group names Prof. Paola Cappellaro as Mercator Fellow. Paola is a world-known expert in spin quantum physics with a broad range of activities ranging from diamond magnetometry to NMR quantum information processing.

List of participating institutes and principal investigators
Fraunhofer Institut für Angewandte Festkörperphysik (C. Nebel, M. Wolfer, O. Ambacher)
Julius-Maximilians-Universität Würzburg (A. Krüger)
Universität Leipzig (J. Meijer)
Johannes Gutenberg-Universität Mainz (F. Schmidt-Kaler, K.Singer)
Technische Universität München (M. Stutzmann, M. Brandt, J.A. Garrido)
Westfälische Wilhelms-Universität Münster (R. Bratschitsch, S. Michaelis de Vasconcellos)
Humboldt-Universität zu Berlin (O. Benson)
Ludwig-Maximilians-Universität München (H. Weinfurter, M. Weber)
Universität Stuttgart (J. Wrachtrup, P. Neumann)
Universität Ulm (F. Jelezko, M. Plenio)
Universität des Saarlandes (C. Becher, M. Bienert)

Dr. Amit Finkler obtained his bachelor degrees in Electrical Engineering and Physics at Tel Aviv University.

He then moved to the Weizmann Institute of Science, doing his Master's thesis under the supervision of Profs. Amir Yacoby and Eli Zeldov, developing a unique superconducting quantum interference device (SQUID).

In his doctoral thesis, under the supervision of Prof. Zeldov, he worked on a novel scanning SQUID microscope for the study of magnetic phenomena on the nanoscale, which nowadays reaches a single electron spin sensitivity.

After a short interlude in industry in the field of thermoelectrics he moved back to the Weizmann Institute of Science for a post-doctoral position at the atomic scale physics laboratory before arriving in Stuttgart. (Picture: Grafiklabor Physik)

The Max-Planck-Gesellschaft and the Alexander von Humboldt Foundation honour pioneering work in the field of quantum nanoscience

Rare-earth-doped crystals are excellent hardware for quantum storage of photons. Additional functionality of these materials is added by their waveguiding properties allowing for on-chip photonic networks. However, detection and coherent properties of rare-earth single-spin qubits have not been demonstrated so far. Here, we present experimental results on high-fidelity optical initialization, effcient coherent manipulation, and optical readout of a single electron spin of Ce ³⁺ ion in a YAG crystal. Under dynamic decoupling, spin coherence lifetime reaches T ₂ = 2 ms and is almost limited by the measured spin-lattice relaxation time T ₁ = 4.5 ms. Strong hyperfine coupling to aluminium nuclear spins suggests that cerium electron spins can be exploited as an interface between photons and long-lived nuclear spin memory. Combined with high brightness of Ce ³⁺ emission and a possibility of creating photonic circuits out of the host material, this makes cerium spins an interesting option for integrated quantum photonics.

Future quantum communication will rely on the integration of single-photon sources, quantum memories and systems with strong single-photon nonlinearities. Two key parameters are crucial for the single-photon source: a high photon flux with a very small bandwidth, and a spectral match to other components of the system. Atoms or ions may act as single-photon sources—owing to their narrowband emission and their intrinsic spectral match to other atomic systems—and can serve as quantum nonlinear elements. Unfortunately, their emission rates are still limited, even for highly efficient cavity designs. Single solid-state emitters such as single organic dye molecules are significantly brighter and allow for narrowband photons; they have shown potential in a variety of quantum optical experiments but have yet to be interfaced with other components such as stationary memory qubits. Here we describe the optical interaction between Fourier-limited photons from a single organic molecule and atomic alkali vapours, which can constitute an efficient quantum memory. Single-photon emission rates reach up to several hundred thousand counts per second and show a high spectral brightness of 30,000 detectable photons per second per megahertz of bandwidth. The molecular emission is robust and we demonstrate perfect tuning to the spectral transitions of the sodium D line and efficient filtering, even for emitters at ambient conditions. In addition, we achieve storage of molecular photons originating from a single dibenzanthanthrene molecule in atomic sodium vapour. Given the large set of molecular emission lines matching to atomic transitions, our results enable the combination of almost ideal single-photon sources with various atomic vapours, such that experiments with giant single-photon nonlinearities, mediated, for example, by Rydberg atoms, become feasible.

Nanoscale Detection of a Single Fundamental Charge in Ambient Conditions Using the NV- Center in Diamond

Single charge detection with nanoscale spatial resolution in ambient conditions is a current frontier in metrology that has diverse interdisciplinary applications. Here, such single charge detection is demonstrated using two nitrogen-vacancy (NV) centers in diamond. One NV center is employed as a sensitive electrometer to detect the change in electric field created by the displacement of a single electron resulting from the optical switching of the other NV center between its neutral (NV0) and negative (NV-) charge states.

The nanoscale detection of a single fundamental charge allows to gain insight into the charge dynamics of the NV center and its surrounding experiment. E.g. the field data suggests, that the charge is ionized to the conducting band, and not as suggested in other models is transferred to a close by charge trap. This technique therefore allows to gain insight into the local nanoscale charge distribution and opens up a pathway to detect the charge distribution outside of diamond using shallow implanted NV centers or nanodiamonds.

Reference:
Nanoscale Detection of a Single Fundamental Charge in Ambient Conditions Using the NV- Center in Diamond
Florian Dolde, Marcus W. Doherty, Julia Michl, Ingmar Jakobi, Boris Naydenov, Sebastien Pezzagna, Jan Meijer, Philipp Neumann, Fedor Jelezko, Neil B. Manson, and Jörg Wrachtrup
Physical Review Letters 112, 097603 2014 – Published March 3, 2014

Perfect alignment and preferential orientation of nitrogen vacancy centers during CVD growth of diamond on (111) surfaces

Quantum error correction in a small spin-based quantum computer in diamond

Mapping Spin Coherence of a Single Rare-Earth Ion in a Crystal onto a Single Photon Polarization State

We report on optical detection of a single photostable Ce ³⁺ ion in an yttrium aluminium garnet (YAG) crystal and on its magneto-optical properties at room temperature. The spin quantum state of the emitting level of a single cerium ion in YAG can be initialized by circularly polarized laser pulse. Coherent precession of the electron spin is read out by observing temporal behaviour of circularly polarized fluorescence of the ion. This implies direct mapping of the spin quantum state of Ce ³⁺ ion onto the polarization state of the emitted photon and represents quantum interface between a single spin and a single photon.

Reference:
Mapping Spin Coherence of a Single Rare-Earth Ion in a Crystal onto a Single Photon Polarization State
Phys. Rev. Lett. 111, 120502 (2013)
Roman Kolesov, Kangwei Xia, Rolf Reuter, Mohammad Jamali, Rainer Stöhr, Tugrul Inal, Petr Siyushev, and Jörg Wrachtrup

Sensing and imaging paramagnetic species under physiological conditions is a key technology in chemical and biochemical analytics, cell biology, and medical sciences. At submicrometer length scales, nitrogen-vacancy (NV) centers in diamond offer atom-sized probes for magnetic fields. We show that spin relaxation of an ensemble NV sensor allows sensing of adsorbed and freely diffusing manganese(II) ions and adsorbed ferritin. Sensitivities approach 175 Mn ions and 10 ferritin proteins per diffraction limited spot under ambient conditions.

Citation:
“Highly Sensitive Detection of Physiological Spins in a Microfluidic Device”
Ziem, F. C., Götz, N. S., Zappe, A., Steinert, S. & Wrachtrup, J., Nano letters (2013). doi:10.1021/nl401522a

Single Defect Center Scanning Near-Field Optical Microscopy on Graphene

¹³C nuclear spins coupled to an NV center are a valuable resource for realizing long lived solid state quantum registers. However, the number of nuclear spins that are addressable is limited by the inhomogeneous linewidth of the magnetic resonance transition, which is in turn limited by the nuclear spin bath. If we could address a nuclear spin that is hidden in the broadened resonance line - i.e. the hyperfine coupling is much smaller than the inhomogeneous linewidth - the number of addressable nuclear spins would greatly increase. In this work we demonstrate that this is possible by dynamically decoupling the electron spin from the surrounding nuclear spin bath. By carefully tuning the direction and strength of an applied magnetic field, the electron spin couples selectively only to specific bath spins. Thereby, we can map out the otherwise hidden nuclear spins around an NV center. Using isotopically enriched diamond (99.99% ¹²C), our method is capable of detecting nuclear spins coupled as weak as 300 Hz.

Further details:

N. Zhao et al. Nature nanotechnology 152 (2012) doi:10.1038/nnano.2012.152

Looking at individual fundamental building blocks of nature, like atoms and molecules, is always intriguing though challenging task. Detecting individual ions of rare-earth elements in laser crystals is such a challenging task because they yield very low light output when illuminated with a laser. At the same time, these ions, if detected, could become excellent quantum bits (qubits) for quantum computer. Recently, we succeeded in detection of single ions of one rare-earth species, namely praseodymium, in an artificial gem-stone yttrium aluminum garnet crystal and in nanometer sized particles of that crystal. The most peculiar feature of praseodymium ions is that they can emit an ultraviolet light once illuminate with a blue laser. We have shown that this ultraviolet radiation can strongly affect the environment in close proximity to the emitting ions. In particular, we have demonstrated that cancerous cells die because of this radiation and that the emitted ultraviolet can expose photosensitive chemicals like photoresists used in chip production. Thus, apart from potential use in quantum computers, the discovered single rare-earth ions might become valuable additions to modern nano- and biophotonics.

Further details:
Optical detection of a single rare-earth ion in a crystal
R. Kolesov et al. Nature Communications 3, 1029 doi:10.1038/ncomms2034

The University of Stuttgart annually awards a prize to the three best PhD thesis of the year. This year Philipp Neumann is among the recipients. Philipp received the prize for his outstanding work on diamond quantum spintronics which resulted in several high profile publications over the past years. He for example for the first time demonstrated electron nuclear spin entanglement in diamond defect spin systems, showed coupling among spin impurities and single shot readout of a nuclear quantum state. In addition Philipp was awarded the annual thesis prize of the collaborative research center SFB/TR21 which has provided the funding for his thesis.

Single-photon sources that provide non-classical light states on demand have a broad range of applications in quantum communication, quantum computing and quantum metrology. Meanwhile, single-photon emission stimulated by optical laser excitation has been demonstrated for a large variety of single quantum systems like atoms, ions, molecules, diamond color centers and semiconductor quantum dots. However, from a technical point of view it is desirable to generate single photon emission by electrical excitation as it was successfully demonstrated for the first time in 2002 using single InAs quantum dots incorporated in a GaAs diode structure. However, a major obstacle of this approach is the requirement of cryogenic temperatures necessary for electrical operation due to the confinement of charge carriers inside the quantum dot at elevated temperatures.

In this context, we were able to design a novel diamond based p-i-n device which proved that the realization of a stable, electrically driven single-photon source from a single neutral nitrogen-vacancy center at room-temperature is feasible. By the demonstration of electroluminescence from a single, neutral NV center an additional central element was added to the quantum toolbox of diamond defects which thereby offers new opportunities in terms of integrating single-photon sources based on diamond defects into electronic control circuitry and spintronic applications for quantum communication and processing. Remarkably, the generation of electroluminescence follows kinetics fundamentally different to that of photoluminescence with intra-bandgap excitation. This phenomenon suggests that the electroluminescent process comprises of a precursor state during electron-hole recombination at the defect site which functions as a rate limiting step in consecutive reactions. A tentative explanation could be drawn from ab ignition calculations in which this additional state is attributed to a loosely bound hole in the valence band edge and a strongly bound electron at the NV center.

Our results thereby show the feasibility of generating single photon emission by electrical excitation at room temperature using single NV centers in the diamond lattice and further highlights that functional single defects can be integrated into electronic control structures, which is a crucial step towards elaborate quantum information devices.

One main application of the nitrogen-vacancy defect in diamond is a sensor which can detect weak magnetic and electric fields at the nanoscale. This is done by measuring the field dependent shift in the Lamor frequency of the spin states using Ramsey interferometry: First, a ?/2 pulse creates a superposition of two spin states. The phase of this superposition will then undergo so called Ramsey oscillations, i.e. it will evolve periodically from 0 to 2? with a field dependent frequency. After a certain phase accumulation time, a second ?/2 pulse maps the accumulated phase onto spin state population, which is then measured. From the measurement result, one can deduce the accumulated phase, the Lamor frequency, and the applied field. However, due to the periodic modulation of the phase, such a measurement is ambiguous, which means that the applied field must be known within certain limits to unambiguously determine its value. Effectively, this limits the dynamic range of the sensor.

In the light of increased commercial and scientific applicability of organic electronic circuitry and in particular organic light emitting diodes (OLEDs), it is of great interest to study fundamental electrical excitation processes and charge carrier dynamics on molecular length scales. Single molecule studies thereby offer the possibility and material compatibility to perform measurements on the level of a single quantum object in organic electronic devices. By doping of single Dibenzoterrylene (DBT) guest molecules into the charge carrier recombination zone of polymeric OLEDs, we were able to demonstrate that a single fluorescent dopant can be used as nanoscale sensor for current densities with nanometer spatial resolution.

While monitoring laser induced photoluminescence of a single DBT molecule during current injection into the emissive layer of a PPV based OLED, we observed that the PL intensity decreases with increasing current density. In contrast to optical laser excitation, electron-hole recombination mechanisms on a single molecule result in a probability of 75% to excite the triplet state manifold compared to only 25% of singlet state stimulation. This phenomenon is related to spin statistics of two recombining fermionic particles (electron and hole) resulting in either a spin 0 or spin 1 exciton. Therefore, electron-hole recombination processes in organic matrices result in a predominant pumping to the molecule’s dark and long living triplet state which leads to a reduction of photoluminescence intensity with increasing recombination rate and current density. Related to the dielectric constant of the polymer matrix, charge carriers only recombine on a single molecular dopant once they are as close as 17 nm to the dopant. Therefore, by measuring voltage dependent single molecule quenching behaviors, a determination of current densities within the molecular environment, or capture cross section, of the molecular sensor is possible. By the study of photon correlations during charge injection it is further possible to measure current densities solely by optical means without the need of any ampere meter.

In contrast to conductive AFM techniques, this approach thereby enables the non-invasive determination of current densities on nanometer length scales in operational OLED devices.

Optical Sensing of Current Dynamics in Organic Light-Emitting Devices at the Nanometer Scale
CHEMPHYSPHEM 10.1002/cphc.201100442

M. Nothaft ¹, S. Höhla ², A. Nicolet ³, F. Jelezko ⁴, N. Frühauf ², J. Pflaum ⁵, J. Wrachtrup ¹

¹ 3rd Physics Institute and Research Center SCoPE, University of Stuttgart, 70550 Stuttgart (Germany)
² Institute for Large Area Microelectronics and Research Center SCoPE, University of Stuttgart, 70550 Stuttgart (Germany)
³ Laboratoire National des Champs Magnétiques Intenses, 38042 Grenoble (France)
⁴ Institut für Quantenoptik, University of Ulm, 89069 Ulm (Germany)
⁵ Experimentelle Physik VI, University of Würzburg and ZAE Bayern, 97074 Würzburg (Germany)

Prof. Carlos Meriles is visiting the institute as a Humboldt fellow for the upcomming year. Carlos is a specialist in spin physics with major contributions in the field of NMR and single spin physics. He graduated from Universidad Nacional de Cordoba and spent his post doc with Alex Pines in UC Berkeley. Currently Carlos is a professor of physics at City College New York – CUNY. Recently he worked on nuclear induced Faraday rotation, optical spin pumping in GaAs and NV center spin physics. Carlos will spend one year at the institute funded by the Humboldt foundation working on field sensing with diamond spin systems.

Among the most puzzling aspects of quantum physics is the measurement process being most drastically posed by Schrödinger’s cat paradox. Schrödinger envisaged a (quantum) cat trapped inside a box equipped with some device which may, randomly, kill the cat. Quantum mechanics tells us that at any time, if unobserved, the cat is both dead and alive. However, if the system is probed, for instance by opening the box, we will find the cat in one of the two possible states, either alive or dead. According to the formalism of quantum mechanics, this result is nondeterministic; i.e. the outcome of our measurement – and therefore the (classical) state of the
cat – is not even defined before opening the box. But how can we know that quantum mechanics is not simply incomplete, and that there are not other ‘‘hidden’’ variables on which the state of the cat depends, thus rendering the measurement deterministic? When formulated in the context of spatially separated subsystems, this type of argument leads to the celebrated Bell’s inequality, which limits the strength of the statistical correlations between distant subsystems within a local realistic theory. On the other hand, in contexts where locality is not relevant, the focus is on the premises of realism and the characterization of the type of temporal (vs spatial) correlations that would emerge within this realist description.
This issue was first investigated by Leggett and Garg in 1985 and led to the formulation of the so-called temporal Bell inequalities (TBI). The major difference to the original Bell inequalities is that instead of correlations between states of two spatially separated systems, one is now concerned with correlations of the state of a single system at different points in time. A measurable inequality is obtained from the assumptions of macroscopic realism (a macroscopic system is always in one of its macroscopically distinct states) and thus the possibility to perform noninvasive measurements (measurements do not influence the dynamics of the system). It is clear that these assumptions, and therefore the resulting inequality, are typically violated by quantum mechanics. However, we would expect these assumptions to become valid at some point as the considered system becomes closer and closer to a truly macroscopic object. Within this view TBIs which have been measured in a recent publication in PRL [Waldherr et al. Phys. Rev. Lett 107, 129901 2011] provide a criterion to characterize the boundary between the quantum and classical domains and the possible identification of macroscopic quantum coherence.

Temporal Bell inequality as measured by a single nitrogen nuclear spin of a diamond defect. The curve shows the temporal correlations of measurements on a single spin as this spin is subject to forced coherent oscillations.

Electrical fields play a pivotal role in numerous cases in both nature and technical areas: by changing the electrical field, impulses of nerves are transmitted and modern data storage operates by saving electrical charges the so-called Flash Memories. An ultra-precise reading of electrical fields, however, is still a challenge for physical measurement techniques. Researchers from Stuttgart succeeded in measuring electrical fields with the aid of one single defect center in diamond. This research report has now been published by Nature Physics.

There are different ways in which electrical charges control almost all physical, chemical or biological processes. For example, the exact distribution of electrons on DNA is crucial for the precise transmission of genetic information and modern electric circuits actuate electric currents up to single electrons.

However, measuring those minor electronic fields connected to the charge is still one of the most challenging tasks of measurement technology. Researchers from Stuttgart developed a novel sensor consisting of just one single atom. This nitrogen atom is an impurity captured in diamond. The diamond lattice fixes the atom, simultaneously allowing a laser to address the nuclear vacancy center. The interaction of the atom with the measured field can be determined by the light emitted by the impurity and, therefore, electrical fields can be measured which are just a fracture of the electrical field of an elementary charge in 0,1 µm distance. Since the sensor itself is only about the size of an atom, electrical fields can also be measured with the same spacial precision.

Quantum Technology with Electron Spins, Prof. Jörg Wrachtrup

Due to the advancing miniaturization of atomically structured solids as well as the integration of optical, mechanical and electronic components, quantum mechanical phenomena can be detected and used in a novel direction. The SQUTEC project will exploit this development in order to process and transfer information with extremely high speed and to design sensors of hitherto unequalled sensitivity. The key to success will be a material known for its special hardness and optical transparency: diamond. By precise doping with foreign substances, such as nitrogen for example, specific color centers can be produced. Physically speaking, those centers behave like embedded atoms: They are optically addressable and shielded from the environment by the diamond lattice. S atoms provide an excellent source for quantum technology. By using electron spins of a particular sort of diamond defects, quantum states can be prepared which are a prerequisite for data information transfer and processing. One particularity of this method is of great importance for practical applications: The diamond lattice protects the electron spins to such an extent that it allows the experiments to be performed at room temperature.

The ERC Grant will enable the realization of many technically demanding stages such as the positioning of nuclear impurities with nearly atomic accuracy. A purpose-built implanter will allow for the implementation of single atoms through extremely small apertures. With this technique, it will be possible to engineer large clusters of interacting impurities. Based on this principle, complex quantum states could be created and exploited for further applications.

Moreover, quantum arrays can be used for sensor applications since diamond defect centres have proven to be excellent sensors of magnetic fields. Built into nano probes, they permit detection of magnetic fields of single electron and nuclear spins with sub nanometer precision. This could be the first step to decoding structures of complex materials or molecules – such as proteins – with unprecedented accuracy. At the same time, diamonds of only few nanometers in size can be implanted into cells or tissue to serve as local probes for ph-value or the concentration of cell-damaging substances. Together with scientific work on the application of nanodiamonds as genetic or pharmaceutical ingredient carriers, the Grant allows for discovering new and attractive perspectives of usage.

The researchers at the 3rd Institute of Physics of the University of Stuttgart are internationally renowned for their work on manipulation of single atoms in diamond and have achieved numerous publications notably in Nature, Nature Physics and Science. In 2009 they succeeded in synthesizing diamond out of methane plasma with purity 10.000 times higher than flawless diamonds (Pressemitteilung). A year ago, they achieved, in collaboration with the Ruhr Universität Bochum and colleagues from Austin/Texas, placement of two nitrogen atoms within only a few nanometers distance. Between the two atoms, they achieved quantum mechanical coupling through laser excitation thus setting a new milestone for quantum computing.
Professor Jörg Wrachtrup was born in 1961 in Herford and studied physics at the Freie Universität Berlin where he finished his doctorate in 1994 with his thesis on magnetic resonance in single molecules. As a postdoc, he continued his research at the Institute of Physics at the Technische Universität Chemnitz where he habilitated in 1998 working on ‘Optical Spectroscopy on Single Quantum Systems in Solid State’. He received offers for professorship from the universities in Hamburg, Göttingen, Leipzig and Stuttgart – the latter he accepted in January 2000. Since then he has directed the 3rd Physics Institute at the University of Stuttgart.

Being able to read out the state of a quantum mechanical object without subsequent destruction of that state is at the heart of numerous quantum feed forward protocols like error correction, entanglement purification. In general a dedicated setting of the measurement device is needed in order to couple strong enough for the readout process to yield a sufficient signal for state identification and yet to preserve the state measured. For the nitrogen-vacancy spin defect in diamond such a setting is achieved for the nitrogen nucleus with an appropriately chosen magnetic field strength and orientation (see Neumann et al. Science 329 (2010) 542ff.). Single shot and quantum non demolition readout of electron spins in this system has not been conclusively demonstrated owing to fast repolarization of the electron spin due to laser excitation and fluorescence readout of the center. However the nitrogen nuclear spin with its hyperfine coupling tensor aligned with the symmetry axis of the electron spin wavefunction is sufficiently decoupled from the electron spin dynamics provided a magnetic field of sufficient strength and orientation along the C _3v center axis is chosen. The work presents a major step towards application of diamond spin defects in quantum science. It might facilitate scaling of spin arrays in the system and even lead to more sensitive parameter estimation in electric and magnetic field measurements.

Original publication:
Title: Single-Shot Readout of a Single Nuclear Spin
Author(s): Neumann,P.,Beck, J., Steiner, M., Rempp, F., Fedder, H., Hemmer, P.R., Wrachtrup, J., Jelezko, F.
Source: Science
Volume: 329
Issue: 5991
Pages: 542 – 544
Published: 30 July 2010

Die optische Detektion fluoreszenzmarkierter Proteine ermöglicht die Erforschung verschiedenster Prozesse in lebenden Zellen. Eine entscheidende Erweiterung ist die Fluoreszenzanalyse mittels Einzel-Molekül-Techniken wie z.B. die Fluoreszenz-Korrelations-Spektroskopie (FCS). Mit Hilfe von FCS haben wir die topologische Trennung der zwei membranständigen Rezeptorproteine TNFR1 und TNFR2 der Tumor-Necrosis-Factor (TNF) Familie nachweisen können. TNFR1 weist eine langsame Diffusionszeit von 1.2×10-9 cm2/s, welche sich durch Ligandenstimulierung mit TNF nicht verändert. Überraschenderweise erhöht sich die Diffusion signifikant sobald den Zellen Cholesterol entzogen wurde. Dies deutet auf eine Lokalisierung der TNFR1-Rezeptoren in Cholesterol-reichen Mikrodomänen hin. Im Gegensatz dazu ist die Diffusion von TNFR2 vergleichsweise schnell (D = 3.1×10-9 cm2/s). Desweiteren ist TNFR2 gekennzeichnet durch eine charakteristische Reduktion der Rezeptordiffusion nach Stimulierung mit TNF (D = 0.9×10-9 cm2/s). Fluoreszenzintensitätsanalysen haben gezeigt, dass diese Verlangsamung nicht durch die Formation von TNFR2 Clustern erklärt werden kann und dass selbst nach Stimulierung nur wenige Rezeptor Moleküle im Signalkomplex gebunden sind. Unsere Daten weisen deutlich auf eine räumliche Trennung der TNF Rezeptoren in verschiedenen Membrandomänen hin welche unabhängig von der zytoplasmatischen Signaldomäne sind.