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Detonation nanodiamond (DND) is the smallest class of diamond nanocrystal capable of hosting various color centers with a size akin to molecular pores. Their negatively charged nitrogen-vacancy center (NV⁻) is a versatile tool for sensing a wide range of physical and even chemical parameters at the nanoscale. The NV⁻ is, therefore, attracting interest as the smallest quantum sensor in biological research. Nonetheless, recent NV⁻ enhancement in DND has yet to yield sufficient fluorescence per particle, leading to efforts to incorporate other group-IV color centers into DND. An example is adding a silicon dopant to the explosive mixture to create negatively charged silicon-vacancy centers (SiV⁻). In this paper, we report on efficient observation (∼50% of randomly selected spots) of the characteristic optically detected magnetic resonance (ODMR) NV⁻ signal in silicon-doped DND (Si-DND) subjected to boiling acid surface cleaning. The NV⁻ concentration is estimated by continuous-wave electron spin resonance spectroscopy to be 0.35 ppm without the NV⁻ enrichment process. A temperature sensitivity of 0.36 K/√HZ in an NV⁻ ensemble inside an aggregate of Si-DND is achieved via the ODMR-based technique. Transmission electron microscopy survey reveals that the Si-DNDs core sizes are ∼11.2 nm, the smallest among the nanodiamond’s temperature sensitivity studies. Furthermore, temperature sensing using both SiV⁻ (all-optical technique) and NV⁻ (ODMR-based technique) in the same confocal volume is demonstrated, showing Si-DND’s multimodal temperature sensing capability. The results of the study thereby pave a path for multi-color and multimodal biosensors and for decoupling the detected electrical field and temperature effects on the NV⁻ center.

Electrical detection of nitrogen-vacancy (N-V) centers in diamond is advantageous for developing and integrating quantum information processing devices and quantum sensors and has the potential to achieve a higher collection efficiency than that of optical techniques. However, the mechanism for the electrical detection of N-V spins is not fully understood. In this study, we observe positive contrast in photocurrent detected magnetic resonance (PDMR). Note that negative PDMR contrast is usually observed. To discuss the sign of the PDMR contrast, we numerically analyze the dynamics of photocarrier generation by N-V centers using a seven-level rate model. It is found that the sign of the PDMR contrast depends on the difference in the photocurrent generated from the excited states and the metastable state of N-V centers. Furthermore, we demonstrate ac magnetic field sensing using spin coherence with the PDMR technique. ac magnetic field measurement with the PDMR technique is still challenging because the noise from a fluctuating magnetic environment is greater than the measured signal. Here, we introduce noise suppression using a phase-cycling-based noise-canceling technique. We demonstrate electrically detected ac magnetic field sensing with a sensitivity of 29 nT Hz[−1/2]. Finally, we discuss sensitivity enhancement based on the proposed model.

Color centers in wide-bandgap semiconductors, including diamond and silicon carbide (SiC), are attractive systems for quantum information and quantum sensor devices with excellent spin properties at room temperature. In addition, nuclear spins in crystals are expected to serve as the quantum memory and to enhance the sensitivity of quantum sensors with the combination with color centers as a result of an extremely long spin coherence time. Although the spin state of both color centers and nuclear spins coupled through hyperfine interactions is usually optically read out, an electrical readout technique is important for miniaturizing and integrating devices. In the present study, we report the electrical detection of silicon vacancy (V2) centers in 4H-SiC by photocurrent-detected magnetic resonance (PDMR) using a frequency-sweep technique. We electrically observe the spin coherence of the V2 centers and clearly resolve the hyperfine splitting of the electron spin signal for the V2 centers coupled with next-nearest-neighbor ²⁹Si atoms. In addition, we apply PDMR to electron–nuclear double resonance (PD-ENDOR) to detect nuclear magnetic resonance of ²⁹Si at room temperature and find that this method can resolve nuclear spins coupled with neighboring electron spins in the V2 centers. The realization of PD-ENDOR is expected to be a critical step toward the development of electrically driven integrated quantum devices.

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 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. The measured rates well explain the results of spin-state polarization dynamics, and we further discuss the altered photoemission dynamics in resonant enhancement structures such as radiative lifetime shortening and Purcell enhancement. By providing a thorough description of the V1 center’s spin-optical dynamics, our work provides deep understanding of the system, which guides implementations of scalable quantum applications based on silicon vacancy centers in silicon carbide.

Colour centres are a promising quantum information platform, but coherence degradation after integration in nanostructures has hindered scalability. Here, the authors show that waveguide-integrated V-Si centres in SiC maintain spin-optical coherences, enabling nuclear high-fidelity spin qubit operations. 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 (V-Si) 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 V-Si 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., Funding Agencies|Ion Beam Centre at Helmholtz-Zentrum Dresden-Rossendorf; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2020-05444]; Swedish Energy AgencySwedish Energy AgencyMaterials & Energy Research Center (MERC) [43611-1]; EUEuropean Commission [862721]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2018.0071]; UC Davis Summer GSR Award; National Science FoundationNational Science Foundation (NSF) [CAREER-2047564]; EU-FET Flagship on Quantum Technologies [820394, 820445]; European Research Council (ERC) grant SMelEuropean Research Council (ERC); Max Planck SocietyMax Planck SocietyFoundation CELLEX; German Research FoundationGerman Research Foundation (DFG) [SPP 1601, FOR 2724]; German Federal Ministry of Education and Research (BMBF)Federal Ministry of Education & Research (BMBF) [16KIS0867]

In current long-distance communications, classical information carried by large numbers of particles is intrinsically robust to some transmission losses but can, therefore, 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 this end, considerable effort has been spent over the last few decades toward developing quantum repeaters that combine long-lived quantum memories with a source of indistinguishable single photons. 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, we give a brief overview on recent advances in developing optically active spin qubits in SiC and discuss challenges in applications for quantum repeaters and possible solutions. In view of the development of different material platforms, the perspective of SiC spin qubits in scalable quantum networks is discussed., Funding Agencies|Swedish Research CouncilSwedish Research Council [VR 2016-04068, VR 2016-05362]; Swedish Energy AgencySwedish Energy Agency [43611-1]; EUEuropean Union (EU) [862721]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2018.0071]; BMBF via Q.Link.XFederal Ministry of Education & Research (BMBF); AFOSRUnited States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-15-1-0029, FA9550-19-1-0358]; DARPAUnited States Department of DefenseDefense Advanced Research Projects Agency (DARPA) [D18AC00015KK1932]; ONROffice of Naval Research [N00014-17-1-3026]; EUEuropean Union (EU)

Silicon-vacancy qubits in silicon carbide (SiC) are emerging tools in quantum-technology applications due to their excellent optical and spin properties. In this paper, we explore the effect of temperature and strain on these properties by focusing on the two silicon-vacancy qubits, V1 and V2, in 4H-SiC. We apply density-functional theory beyond the Born-Oppenheimer approximation to describe the temperature-dependent mixing of electronic excited states assisted by phonons. We obtain a polaronic gap of around 5 and 22 meV for the V1 and V2 centers, respectively, which results in a significant difference in the temperature-dependent dephasing and zero-field splitting of the excited states, which explains recent experimental findings. We also compute how crystal deformations affect the zero-phonon line of these emitters. Our predictions are important ingredients in any quantum applications of these qubits sensitive to these effects., Funding Agencies|National Excellence Program of Quantum-Coherent Materials Project (Hungarian NKFIH Grant) [KKP129866]; European Union QuantERA Q-Magine Project [127889]; QuantERA Nanospin Project [127902]; European Union Horizon 2020 Quantum Technology Flagship project ASTERIQS [820394]; National Quantum Technology Program [2017-1.2.1NKP-2017-00001]; Swedish Research CouncilSwedish Research Council [VR 2016-04068]; Swedish Energy AgencySwedish Energy Agency [43611-1]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2018.0071]; European Union Horizon 2020 project QuanTELCO [862721]; Japan Society for the Promotion of ScienceMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of Science [JSPS KAKENHI 17H01056, JSPS KAKENHI 18H03770]; U.S. Department of Energy, Office of ScienceUnited States Department of Energy (DOE) [DE-SC0019174]; Swedish National Infrastructure for Computing at the National Supercomputer Centre [SNIC 2019/3667, LiU-2015-00017-60]

We present the DC Stark tuning of single Silicon Vacancies in SiC. We demonstrate static tuning across 200 GHz, exceeding the inhomogenous broadening, and dynamic tuning on timescales shorter than the optical decay rate. (C) 2020 The Author(s)

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., Funding Agencies|EU (ASTERIQS)European Union (EU); EU (ERC SMeL); BMBF (BrainQSens)Federal Ministry of Education & Research (BMBF); Max Planck SocietyMax Planck SocietyFoundation CELLEX; Volkswagen FoundationVolkswagen; Swedish Research CouncilSwedish Research Council [VR 2016-04068]; Carl Tryggers Stiftelse for Vetenskaplig Forskning [CTS 15:339]; Swedish Energy AgencySwedish Energy Agency [43611-1]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2018.0071]; Korea Institute of Science and Technology institutional programs [2E27231, 2E29580]; Japan Society for the Promotion of Science KAKENHIMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) [17H01056, 18H03770]