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 initialize 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 a weakly coupled diatomic nuclear-spin bath.

Qudit-Based Spectroscopy for Measurement and Control of Nuclear-Spin Qubits in Silicon Carbide.

Nuclear spins with hyperfine coupling to single electron spins are highly valuable quantum bits. Here we probe and characterize the particularly rich nuclear-spin environment around single silicon vacancy color centers (V2) in 4H-SiC. By using the electron spin-3/2 qudit as a four level sensor, we identify several sets of ^{29}Si and ^{13}C nuclear spins through their hyperfine interaction. We extract the major components of their hyperfine coupling via optical detected nuclear magnetic resonance, and assign them to shells in the crystal via the density function theory simulations. We utilize the ground-state level anticrossing of the electron spin for dynamic nuclear polarization and achieve a nuclear-spin polarization of up to 98±6%. We show that this scheme can be used to detect the nuclear magnetic resonance signal of individual spins and demonstrate their coherent control. Our work provides a detailed set of parameters and first steps for future use of SiC as a multiqubit memory and quantum computing platform.

Fabrication and nanophotonic waveguide integration of silicon carbide colour centres with preserved spin-optical coherence.

Optically addressable spin defects in silicon carbide (SiC) are an emerging platform for quantum information processing compatible with nanofabrication processes and device control used by the semiconductor industry. System scalability towards large-scale quantum networks demands integration into nanophotonic structures with efficient spin-photon interfaces. However, degradation of the spin-optical coherence after integration in nanophotonic structures has hindered the potential of most colour centre platforms. Here, we demonstrate the implantation of silicon vacancy centres (VSi) in SiC without deterioration of their intrinsic spin-optical properties. In particular, we show nearly lifetime-limited photon emission and high spin-coherence times for single defects implanted in bulk as well as in nanophotonic waveguides created by reactive ion etching. Furthermore, we take advantage of the high spin-optical coherences of VSi centres in waveguides to demonstrate controlled operations on nearby nuclear spin qubits, which is a crucial step towards fault-tolerant quantum information distribution based on cavity quantum electrodynamics.

Electrical Charge State Manipulation of Single Silicon Vacancies in a Silicon Carbide Quantum Optoelectronic Device.

Color centers with long-lived spins are established platforms for quantum sensing and quantum information applications. Color centers exist in different charge states, each of them with distinct optical and spin properties. Application to quantum technology requires the capability to access and stabilize charge states for each specific task. Here, we investigate charge state manipulation of individual silicon vacancies in silicon carbide, a system which has recently shown a unique combination of long spin coherence time and ultrastable spin-selective optical transitions. In particular, we demonstrate charge state switching through the bias applied to the color center in an integrated silicon carbide optoelectronic device. We show that the electronic environment defined by the doping profile and the distribution of other defects in the device plays a key role for charge state control. Our experimental results and numerical modeling evidence that control of these complex interactions can, under certain conditions, enhance the photon emission rate. These findings open the way for deterministic control over the charge state of spin-active color centers for quantum technology and provide novel techniques for monitoring doping profiles and voltage sensing in microscopic devices.

Scalable Quantum Photonics with Single Color Centers in Silicon Carbide.

Silicon carbide is a promising platform for single photon sources, quantum bits (qubits), and nanoscale sensors 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. A commercially obtained substrate is irradiated with 2 MeV electron beams to create vacancies. Subsequent lithographic process forms 800 nm tall nanopillars with 400-1400 nm diameters. We obtain high collection efficiency of up to 22 kcounts/s optical saturation rates from a single silicon vacancy center while preserving the single photon emission and the optically induced electron-spin polarization properties. Our study demonstrates silicon carbide as a readily available platform for scalable quantum photonics architecture relying on single photon sources and qubits.

Coherent control of single spins in silicon carbide at room temperature.

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. Here, 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 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.

Ultra-narrow inhomogeneous spectral distribution of telecom-wavelength vanadium centres in isotopically-enriched silicon carbide.

Spin-active quantum emitters have emerged as a leading platform for quantum technologies. However, one of their major limitations is the large spread in optical emission frequencies, which typically extends over tens of GHz. Here, we investigate single V4+ vanadium centres in 4H-SiC, which feature telecom-wavelength emission and a coherent S = 1/2 spin state. We perform spectroscopy on single emitters and report the observation of spin-dependent optical transitions, a key requirement for spin-photon interfaces. By engineering the isotopic composition of the SiC matrix, we reduce the inhomogeneous spectral distribution of different emitters down to 100 MHz, significantly smaller than any other single quantum emitter. Additionally, we tailor the dopant concentration to stabilise the telecom-wavelength V4+ charge state, thereby extending its lifetime by at least two orders of magnitude. These results bolster the prospects for single V emitters in SiC as material nodes in scalable telecom quantum networks.

Asymmetric split-vacancy defects in SiC polytypes: a combined theoretical and electron spin resonance study.

Transition metal defects were studied in different polytypes of silicon carbide (SiC) by ab initio supercell calculations. We found asymmetric split-vacancy (ASV) complexes for these defects that preferentially form at only one site in hexagonal polytypes, and they may not be detectable at all in cubic polytype. Electron spin resonance study demonstrates the existence of ASV complex in niobium doped 4H polytype of SiC.

Spin-controlled generation of indistinguishable and distinguishable photons from silicon vacancy centres in silicon carbide.

Quantum systems combining indistinguishable photon generation and spin-based quantum information processing are essential for remote quantum applications and networking. However, identification of suitable systems in scalable platforms remains a challenge. Here, we investigate the silicon vacancy centre in silicon carbide and demonstrate controlled emission of indistinguishable and distinguishable photons via coherent spin manipulation. Using strong off-resonant excitation and collecting zero-phonon line photons, we show a two-photon interference contrast close to 90% in Hong-Ou-Mandel type experiments. Further, we exploit the system's intimate spin-photon relation to spin-control the colour and indistinguishability of consecutively emitted photons. Our results provide a deep insight into the system's spin-phonon-photon physics and underline the potential of the industrially compatible silicon carbide platform for measurement-based entanglement distribution and photonic cluster state generation. Additional coupling to quantum registers based on individual nuclear spins would further allow for high-level network-relevant quantum information processing, such as error correction and entanglement purification.

Ligand hyperfine interactions at silicon vacancies in 4H-SiC.

The negative silicon vacancy ([Formula: see text]) in SiC has recently emerged as a promising defect for quantum communication and room-temperature quantum sensing. However, its electronic structure is still not well characterized. While the isolated Si vacancy is expected to give rise to only two paramagnetic centers corresponding to two inequivalent lattice sites in 4H-SiC, there have been five electron paramagnetic resonance (EPR) centers assigned to [Formula: see text] in the past: the so-called isolated no-zero-field splitting (ZFS) [Formula: see text] center and another four axial configurations with small ZFS: T V1a, T V2a, T V1b, and T V2b. Due to overlapping with 29Si hyperfine (hf) structures in EPR spectra of natural 4H-SiC, hf parameters of T V1a have not been determined. Using isotopically enriched 4H-28SiC, we overcome the problems of signal overlapping and observe hf parameters of nearest C neighbors for all three components of the S = 3/2 T V1a and T V2a centers. The obtained EPR data support the conclusion that only T V1a and T V2a are related to [Formula: see text] and the two configurations of the so-called isolated no-ZFS [Formula: see text] center, [Formula: see text] (I) and [Formula: see text] (II), are actually the central lines corresponding to the transition |-1/2〉 ↔ |+1/2〉 of the T V2a and T V1a centers, respectively.

Coherent electrical readout of defect spins in silicon carbide by photo-ionization at ambient conditions.

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.

Stabilization of point-defect spin qubits by quantum wells.

Defect-based quantum systems in wide bandgap semiconductors are strong candidates for scalable quantum-information technologies. However, these systems are often complicated by charge-state instabilities and interference by phonons, which can diminish spin-initialization fidelities and limit room-temperature operation. Here, we identify a pathway around these drawbacks by showing that an engineered quantum well can stabilize the charge state of a qubit. Using density-functional theory and experimental synchrotron X-ray diffraction studies, we construct a model for previously unattributed point defect centers in silicon carbide as a near-stacking fault axial divacancy and show how this model explains these defects' robustness against photoionization and room temperature stability. These results provide a materials-based solution to the optical instability of color centers in semiconductors, paving the way for the development of robust single-photon sources and spin qubits.

High-fidelity spin and optical control of single silicon-vacancy centres in silicon carbide.

Scalable quantum networking requires quantum systems with quantum processing capabilities. Solid state spin systems with reliable spin-optical interfaces are a leading hardware in this regard. However, available systems suffer from large electron-phonon interaction or fast spin dephasing. Here, we demonstrate 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, allowing exploitation of the spin selectivity of the optical transitions. In combination with millisecond-long spin coherence times originating from the high-purity crystal, we demonstrate high-fidelity optical initialization and coherent spin control, which we exploit to show coherent coupling to single nuclear spins with ∼1 kHz resolution. The summary of our findings makes this defect a prime candidate for realising memory-assisted quantum network applications using semiconductor-based spin-to-photon interfaces and coherently coupled nuclear spins.

Magnetic resonance identification of hydrogen at a zinc vacancy in ZnO.

Hydrogen (H) at a zinc vacancy (VZn) in ZnO is identified by electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM). In ZnO irradiated by 2 MeV electrons, a doublet EPR spectrum, labelled S1, is observed. The doublet structure and the accompanying weak satellites are shown to be the allowed and forbidden lines of the hyperfine structure due to the dipolar interaction between an electron spin S = 1/2 and a nuclear spin I = 1/2 of (1)H located at a VZn. The involvement of a single H atom in the S1 defect is further confirmed by the observation of the nuclear Zeeman frequency of (1)H in ESEEM experiments. We show that at a VZn, H prefers to make a short O-H bond with one O neighbour and is off the substitutional site, forming a low symmetry C1 defect. In this partly H passivated VZn, the unpaired electron localizes on the p orbital of another O neighbour of VZn, and not on the H.