arXiv subjects

Jörg Wrachtrup

Publications and source records attributed to Jörg Wrachtrup.

At least 37 records · Page 2Linked to original sources

arXiv2025

Efficient Detection of Statistical RF Fields at High Magnetic Field with a Quantum Sensor

Nuclear magnetic resonance (NMR) spectroscopy is widely used in fields ranging from chemistry, material science to neuroscience. Nanoscale NMR spectroscopy using Nitrogen-vacancy (NV) centers in diamond has emerged as a promising platform due to an unprecedented sensitivity down to the single spin level. At the nanoscale, high nuclear spin polarization through spin fluctuations (statistical polarization) far outweighs thermal polarization. However, until now efficient NMR detection using coherent averaging techniques could not be applied to the detection of statistical polarization, leading to long measurement times. Here we present two protocols to enable coherent averaging of statistical oscillating signals through rectification. We demonstrate these protocols on an artificial radiofrequency signal detected with a single NV center at 2.7 T. Through this, the signal-to-noise scaling with number of measurements $N$ is improved from $N^{0.5}$ to $N^1$, improving the measurement time significantly. The relevance of rectification for the detection of statistical polarization using NV ensembles is outlined, paving the way for efficient nanoscale NMR spectroscopy.

quant-ph↗

arXiv2025

Cavity enhancement of V2 centers in 4H-SiC with a fiber-based Fabry-Pérot microcavity

Silicon vacancy centers in 4H-silicon carbide (SiC) host a long-lived electronic spin and simultaneously possess spin-resolved optical transitions, making them a great candidate for implementing a spin-photon interface. These interfaces are an important building block of quantum networks, which in turn promise to enable secure communication and distributed quantum computing. To achieve this goal, the rate of coherently scattered photons collected from a single emitter needs to be maximized, which can be achieved by interfacing the emitter with an optical cavity. In this work, we integrate V2 centers inside a SiC membrane into a fiber-based Fabry-Pérot microcavity. We find that SiC lends itself well to membrane fabrication, as evidenced by low surface roughness $σ_{\mathrm{RMS}}$ ~ 400 pm, high reproducibility, and consequentially a high cavity finesse F ~ 40 000. At cryogenic temperatures, we observe individual emitters coupling to cavity modes. We confirm their single-emitter character by measuring the second-order autocorrelation function and investigate the Purcell factor by measuring the optical lifetime as a function of the cavity-emitter detuning. We find a 13.3-fold enhancement of photons scattered into the zero-phonon line (ZPL), which could be further increased by using optimized mirror coatings, potentially opening the path towards deterministic spin-photon interaction.

quant-ph↗

arXiv2025

Two-Photon Interference of Photons from Remote Tin-Vacancy Centers in Diamond

Scalable quantum networks rely on optical connections between long-lived qubits to distribute entanglement. Tin vacancies in diamond have emerged as promising long-lived qubits, offering extended spin coherence times at liquid helium temperatures and spin-dependent, highly coherent optical transitions for effective photon-based communication. Connecting remote nodes requires quantum interference of indistinguishable photons, which is challenging in an inhomogeneous solid-state environment. Here, we demonstrate a two-node experiment with tin vacancies in diamond, which exhibit a resonant frequency distribution spanning approximately 8 GHz. To overcome the frequency mismatch, we tune the resonant frequencies of one node using the Stark effect. We achieve tunability up to 4 GHz while maintaining optical coherence. As a demonstration, we achieve detuning-dependent remote two-photon interference between separate nodes, obtaining 80(6)% interference visibility without postprocessing when the defects' optical transitions are tuned into resonance, and 63(8)% with detuning up to 20 times their natural linewidths. These results highlight the potential of tin-vacancy centres in diamond for establishing robust optical links between remote quantum registers.

quant-ph↗

arXiv2025

Remote Cooling of Spin-ensembles through a Spin-mechanical Hybrid Interface

We present a protocol for the ground-state cooling of a tripartite hybrid quantum system, in which a macroscopic oscillator acts as a mediator between a single probe spin and a remote spin ensemble. In the presence of weak dispersive coupling between the spins and the oscillator, cooling of the oscillator and the ensemble spins can be achieved by exploiting the feedback from frequent measurements of the single probe spin. We explore the parameter regimes necessary to cool the ensemble, the oscillator, or both to their thermal ground states. This novel cooling protocol shows that, even with only weak dispersive coupling, energy transfer-like effects can be obtained by simply manipulating the probe spin. These results not only contribute to the development of a practical solution for cooling/polarizing large spin ensembles, but also provide a relatively simple means of tuning the dynamics of a hybrid system. The proposed cooling protocol thus has broader implications for advancing various quantum technology applications, such as macroscopic quantum state generation and remote sensing.

quant-ph↗

arXiv2025

Single-molecule Scale Magnetic Resonance Spectroscopy using Nitrogen-Vacancy Centers in Diamond

Single-molecule technology stands as a powerful tool, enabling the characterization of intricate structural and dynamic information that would otherwise remain concealed within the averaged behaviors of numerous molecules. This technology finds extensive application across diverse fields including physics, chemistry, biology, and medicine. Quantum sensing, particularly leveraging nitrogen-vacancy (NV) centers within diamond structures, presents a promising avenue for single-molecule magnetic resonance, offering prospects for sensing and imaging technology at the single-molecule level. Notably, while significant strides have been made in single-molecule scale magnetic resonance using NV centers over the past two decades, current approaches still exhibit limitations in magnetic sensitivity, spectral resolution, and spatial resolution. Particularly, the full reconstruction of three-dimensional positions of nuclear spins within single molecules remains an unattained goal. This review provides a comprehensive overview of the current state-of-the-art in single-molecule scale magnetic resonance, encompassing an analysis of various relevant techniques involving NV centers. Additionally, it explores the optimization of technical parameters associated with these methods. This detailed analysis serves as a foundation for the development of new technologies and the exploration of potential applications.

quant-ph↗

arXiv2024

Discovery of ST2 centers in natural and CVD diamond

The ST2 center is an optically addressable point defect in diamond that facilitates spin initialization and readout. However, while this study presents the discovery of ST2 centers first observed in a natural diamond and provides a reliable technique for artificially creating them, its chemical structure remains unknown. To assess the potential of ST2, we map out its basic optical characteristics, reveal its electronic level structure, and quantify the intrinsic transition rates. Furthermore, we investigate its response to microwaves, static magnetic fields, and the polarization of excitation laser light, revealing twelve inequivalent orientations of the ST2 center. Simultaneous exposure to microwaves and static magnetic fields also reveals an exceptionally wide acceptance angle for sensing strong magnetic fields, unlike the well-established NV center, which is sensitive only within a narrow cone aligned with its symmetry axis. This finding establishes the ST2 center as a highly promising candidate for nanoscale quantum sensing.

physics.optics↗

arXiv2024

Readout of strongly coupled NV center-pair spin states with deep neural networks

Optically addressable electron spin clusters are of interest for quantum computation, simulation and sensing. However, with interaction length scales of a few tens of nanometers in the strong coupling regime, they are unresolved in conventional confocal microscopy, making individual readout problematic. Here we show that when using a single shot readout technique, collective states of the combined register space become accessible. By using spin to charge conversion of the defects we draw the connection between the intricate photon count statistics with spin state tomography using deep neural networks. This approach is particularly versatile with further scaling the number of constituent spins in a cluster due to complexity of the analytical treatment. We perform a proof of concept measurement of the correlated classical signal, paving the way for using our technique in realistic applications.

quant-ph↗

arXiv2024

Observation of liquid-solid transition of nanoconfined water at ambient temperature

Nanoconfined water plays an indispensable role in various phenomena in biology, chemistry, and engineering. It exhibits many abnormal properties compared to bulk water, especially under strong confinement. However, the origin of those anomalies is still elusive due to the lack of structural information on hydrogen-bonding networks. Considering the inhomogeneity of the nanocavity and the tiny amount of water molecules, conventional optical spectroscopies and nuclear magnetic resonance (NMR) fail to realize the structure analysis of nanoconfined water. Here, we addressed this issue by combining scanning probe microscopy (SPM) with advanced quantum sensing(QS) based on an atomic-size quantum sensor like nitrogen-vacancy (NV) center in diamond, which can apply the nanoscale-NMR for characterizing both the dynamics and structure of confined water at ambient conditions. We built a two-dimensional (2D) nanoconfined water system with a hexagonal-boron nitride (hBN) flake and a hydrophilic diamond surface. By using the SPM tip to measure the confinement size precisely, we observed a critical confinement size of ~2 nm, below which the water diffusion was significantly suppressed and the hydrogen-bonding network of water showed an ordered structure. Meanwhile, molecular dynamics (MD) simulation revealed a solid-like water contact layer on the diamond surface under strong confinement, which also reproduced the measured nanoscale-NMR spectra and confirmed the liquid-solid phase transition observed in the experiments. Notably, with this new SPM-QS platform, our results showed a promising way to elucidate the abnormal properties of nanoconfined water in future applications.

cond-mat.mes-hall↗

arXiv2024

Cooling limits of coherent refrigerators

Refrigeration limits are of fundamental and practical importance. We here show that quantum systems can be cooled below existing incoherent cooling bounds by employing coherent virtual qubits, even if the amount of coherence is incompletely known. Virtual subsystems, that do not necessarily correspond to a natural eigensubspace of a system, are a key conceptual tool in quantum information science and quantum thermodynamics. We derive universal coherent cooling limits and introduce specific protocols to reach them. As an illustration, we propose a generalized algorithmic cooling protocol that outperforms its current incoherent counterpart. Our results provide a general framework to investigate the performance of coherent refrigeration processes.

quant-ph↗

arXiv2024

Single V2 defect in 4H Silicon Carbide Schottky diode at low temperature

Nanoelectrical and photonic integration of quantum optical components is crucial for scalable solid-state quantum technologies. Silicon carbide stands out as a material with mature quantum defects and a wide variety of applications in semiconductor industry. Here, we study the behaviour of single silicon vacancy (V2) colour centres in a metal-semiconductor (Au/Ti/4H-SiC) epitaxial wafer device, operating in a Schottky diode configuration. We explore the depletion of free carriers in the vicinity of the defect, as well as electrical tuning of the defect optical transition lines. By detecting single charge traps, we investigate their impact on V2 optical line width. Additionally, we investigate the charge-photon-dynamics of the V2 centre and find its dominating photon-ionisation processes characteristic rate and wavelength dependence. Finally, we probe the spin coherence properties of the V2 system in the junction and demonstrate several key protocols for quantum network applications. Our work shows the first demonstration of low temperature integration of a Schottky device with optical microstructures for quantum applications and paves the way towards fundamentally scalable and reproducible optical spin defect centres in solids.

quant-ph↗

arXiv2024

Efficient nuclear spin - photon entanglement with optical routing

Quantum networks and distributed quantum computers rely on entanglement generation between photons and long-lived quantum memories. For large-scale architectures, one of the most crucial parameters is the efficiency at which entanglement can be created and detected. Here, we maximize the efficiency for the detection of hybrid entanglement between a nuclear spin qubit in diamond with a photonic time-bin qubit. Our approach relies on an optimal implementation of the photonic qubit analyzer, for which we use a high-speed electro-optic deflector to direct photons deterministically along the optimal interferometer paths. This way, we completely eliminate all cases in which photons are randomly lost due to the propagation in the wrong interferometer path. In this first demonstration experiment, we use nitrogen-vacancy center in diamond, for which we immediately demonstrate the presence of the entanglement. An extension to other spin-photon entanglement systems is straightforward. Further, our scheme can be used in the framework of quantum repeater networks, including spectral and temporal multiplexing strategies. Our results thus pave the way for the future high-performance quantum networks.

quant-ph↗

arXiv2024

Studying Critical Parameters of Superconductor via Diamond Quantum Sensors

Critical parameters are the key to superconductivity research, and reliable instrumentations can facilitate the study. Traditionally, one has to use several different measurement techniques to measure critical parameters separately. In this work, we develop the use of a single species of quantum sensor to determine and estimate several critical parameters with the help of independent simulation data. We utilize the nitrogen-vacancy (NV) center in the diamond, which recently emerged as a promising candidate for probing exotic features in condensed matter physics. The non-invasive and highly stable nature provides extraordinary opportunities to solve scientific problems in various systems. Using a high-quality single-crystalline YBa$_{2}$Cu$_{4}$O$_{8}$ (YBCO) as a platform, we demonstrate the use of diamond particles and a bulk diamond to probe the Meissner effect. The evolution of the vector magnetic field, the $H-T$ phase diagram, and the map of fluorescence contour are studied via NV sensing. Our results reveal different critical parameters, including lower critical field $H_{c1}$, upper critical field $H_{c2}$, and critical current density $j_{c}$, as well as verifying the unconventional nature of this high-temperature superconductor YBCO. Therefore, NV-based quantum sensing techniques have huge potential in condensed matter research.

cond-mat.mes-hall↗

arXiv2024

Fluorescence enhancement of single V2 centers in a 4H-SiC cavity antenna

Solid state quantum emitters are a prime candidate in distributed quantum technologies since they inherently provide a spin-photon interface. An ongoing challenge in the field, however, is the low photon extraction due to the high refractive index of typical host materials. This challenge can be overcome using photonic structures. Here, we report the integration of V2 centers in a cavity-based optical antenna. The structure consists of a silver-coated, 135 nm thin 4H-SiC membrane functioning as a planar cavity with a broadband resonance yielding a theoretical photon collection enhancement factor of 34. The planar geometry allows us to identify over 20 single V2 centers at room temperature with a mean (maximum) count rate enhancement factor of 9 (15). Moreover, we observe 10 V2 centers with a mean absorption linewidth below 80MHz at cryogenic temperatures. These results demonstrate a photon collection enhancement that is robust to the lateral emitter position.

quant-ph↗

arXiv2024

Nanoscale Mapping of Magnetic Auto-oscillations with a single Spin Sensor

Magnetic auto-oscillations are damping-compensated magnetization precessions. They can be generated in spin Hall nano-oscillators (SHNO) among others. Current research on these devices is dedicated to create next generation energy-efficient hardware for communication technologies. However, the underlying physics governing the formation of auto-oscillation modes, their output power and line width in a single SHNO device have remained elusive so far. We image the sources of magnetic auto-oscillations in a metallic SHNO using a single spin quantum sensor. We directly measure the microwave field generated by an auto-oscillation spot at the nanoscale by driving the electron spin resonance transition of the sensor spin, enabling faster acquisition speed (100 ms/pixel). Instead of being defined by the points of the largest antidamping only, we experimentally demonstrate for the first time with quantitative magnetometry that the auto-oscillation spots are determined by the positions of the magnetic field minima. The latter act as local potential wells for confining spin-waves, thus supporting large amplitude auto-oscillations. By comparing the magnitude of the magnetic stray field at these spots, we decipher the different frequencies of the auto-oscillation modes. The insights gained regarding the interaction between auto-oscillation modes and spin-wave potential wells enable advanced engineering of real devices.

cond-mat.mes-hall↗

arXiv2024

Three-dimensional imaging of integrated-circuit activity using quantum defects in diamond

The continuous scaling of semiconductor-based technologies to micron and sub-micron regimes has resulted in higher device density and lower power dissipation. Many physical phenomena such as self-heating or current leakage become significant at such scales, and mapping current densities to reveal these features is decisive for the development of modern electronics. However, advanced non-invasive technologies either offer low sensitivity or poor spatial resolution and are limited to two-dimensional spatial mapping. Here we use near-surface nitrogen-vacancy centres in diamond to probe Oersted fields created by current flowing within a multi-layered integrated circuit in pre-development. We show the reconstruction of the three-dimensional components of the current density with a magnitude down to about $\approx 10 \,\rm μA / μm^2$ and sub-micron spatial resolution at room temperature. We also report the localisation of currents in different layers and observe anomalous current flow in an electronic chip. Our method provides, therefore a decisive step toward three-dimensional current mapping in technologically relevant nanoscale electronics chips.

physics.app-ph↗

arXiv2024

Spectral stability of V2 centres in sub-micron 4H-SiC membranes

Colour centres in silicon carbide emerge as a promising semiconductor quantum technology platform with excellent spin-optical coherences.However, recent efforts towards maximising the photonic efficiency via integration into nanophotonic structures proved to be challenging due to reduced spectral stabilities. Here, we provide a large-scale systematic investigation on silicon vacancy centres in thin silicon carbide membranes with thicknesses down to $0.25\,\rmμm$. Our membrane fabrication process involves a combination of chemical mechanical polishing, reactive ion etching, and subsequent annealing. This leads to highly reproducible membranes with roughness values of $3-4\,\rmÅ$, as well as negligible surface fluorescence. We find that silicon vacancy centres show close-to lifetime limited optical linewidths with almost no signs of spectral wandering down to membrane thicknesses of $0.7 \,\rmμm$. For silicon vacancy centres in thinner membranes down to $0.25\,\rmμm$, we observe spectral wandering, however, optical linewidths remain below $200\,\rm MHz$, which is compatible with spin-selective excitation schemes. Our work clearly shows that silicon vacancy centres can be integrated into sub-micron silicon carbide membranes, which opens the avenue towards obtaining the necessary improvements in photon extraction efficiency based on nanophotonic structuring.

quant-ph↗

arXiv2024

Precise characterization of a silicon carbide waveguide fiber interface

Emitters in high refractive index materials like 4H-SiC suffer from reduced detection of photons because of losses caused by total internal reflection. Thus, integration into efficient nanophotonic structures which couple the emission of photons to a well defined waveguide mode can significantly enhance the photon detection efficiency. In addition, interfacing this waveguide to a classical fiber network is of similar importance to detect the photons and perform experiments. Here, we show a waveguide fiber interface in SiC. By careful measurements we determine efficiencies exceeding 93 % for the transfer of photons from SiC nanobeams to fibers. We use this interface to create a bright single photon source based on waveguide integrated V2 defects in 4H-SiC and achieve an overall photon count rate of 181 kilo-counts per second. We observe and quantify the strain induced shift of the ground state spin states and demonstrate coherent control of the electron spin with a coherence time of T2=42.5 $\rmμ$s.

quant-ph↗

arXiv2024

High fidelity optical readout of a nuclear spin qubit in Silicon Carbide

Quantum state readout is a key requirement for a successful qubit platform. In this work we demonstrate a high fidelity quantum state readout of a V2 center nuclear spin based on a repetitive readout technique. We demonstrate up to 99.5$\,\%$ readout fidelity and 99$\,\%$ for state preparation. Using this efficient readout we initialise the nuclear spin by measurement and demonstrate its Rabi and Ramsey nutation. Finally, we use the nuclear spin as a long lived memory for quantum sensing application of weakly coupled diatomic nuclear spin bath.

quant-ph↗