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arXiv:2411.10208 · npj Quantum Information

Quantum sensing with duplex qubits of silicon vacancy centers in SiC at room temperature

Kosuke Tahara, Shin-ichi Tamura, Haruko Toyama, Jotaro J. Nakane, Katsuhiro Kutsuki, Yuichi Yamazaki, Takeshi Ohshima

Published Apr 4, 2025• 15 citations
arXivHTMLPDF

Abstract

The silicon vacancy center in Silicon Carbide (SiC) provides an optically addressable qubit at room temperature in its spin- $$\frac{3}{2}$$ electronic state. However, optical spin initialization and readout are less efficient compared to those of spin-1 systems, such as nitrogen-vacancy centers in diamond, under non-resonant optical excitation. Spin-dependent fluorescence exhibits contrast only between $$| m=\pm 3/2\left.\right\rangle$$ and $$| m=\pm 1/2\left.\right\rangle$$ states, and optical pumping does not create a population difference between $$| +1/2\left.\right\rangle$$ and $$| -1/2\left.\right\rangle$$ states. Thus, operating one qubit (e.g., $$\left\{| +3/2\left.\right\rangle ,| +1/2\left.\right\rangle \right\}$$ states) leaves the population in the remaining state ( $$| -1/2\left.\right\rangle$$ ) unaffected, contributing to background in optical readout. To mitigate this problem, we propose a sensing scheme based on duplex qubit operation in the quartet, using microwave pulses with two resonant frequencies to simultaneously operate $$\left\{| +3/2\left.\right\rangle ,| +1/2\left.\right\rangle \right\}$$ and $$\left\{| -1/2\left.\right\rangle ,| -3/2\left.\right\rangle \right\}$$ . Experimental results demonstrate that this approach doubles signal contrast in optical readout and improves sensitivity in AC magnetometry compared to simplex operation.

DOI
10.1038/s41534-025-01011-2

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The identification of a microscopic configuration of point defects acting as quantum bits is a key step in the advance of quantum information processing and sensing. Among the numerous candidates, silicon-vacancy related centers in silicon carbide (SiC) have shown remarkable properties owing to their particular spin-3/2 ground and excited states. Although, these centers were observed decades ago, two competing models, the isolated negatively charged silicon vacancy and the complex of negatively charged silicon vacancy and neutral carbon vacancy [Phys. Rev. Lett. 115, 247602 (2015)], are still argued as an origin. By means of high-precision first-principles calculations and high-resolution electron spin resonance measurements, we here unambiguously identify the Si-vacancy related qubits in hexagonal SiC as isolated negatively charged silicon vacancies. Moreover, we identify the Si-vacancy qubit configurations that provide room-temperature optical readout.
The V2 silicon vacancy defect in 4H-SiC has emerged as a promising system for quantum technologies due to its favorable optical and spin properties and the advantages of the SiC host. However, the readout contrast - an important benchmark for quantum sensing - of V2 ensembles for optically-detected magnetic resonance (ODMR) is relatively low, usually <1%\%% at room temperature. To overcome this challenge, we resonantly excite the V2 ensembles at cryogenic temperatures and compare the results with the off-resonant case. We report a maximum ODMR contrast of 50%\%% with only 2 μμμW of resonant laser power, almost 100 times improvement over off-resonant excitation. We attribute this high readout contrast to a subset of V2 centers that have one spin-selective optical transition resonant with the laser. The ODMR contrast decreases with temperature, approaching the non-resonant contrast by 60 K, likely due to broadening of the optical transition linewidths. We achieve a maximum sensitivity of 100 nT/Hz\sqrt{Hz}Hz​ with a resonant laser power of 300 μμμW, while 100 times more non-resonant excitation power is needed to achieve comparable sensitivity.
Optically addressable solid-state color center spin qubits have become important platforms for quantum information processing, quantum networks and quantum sensing. The readout of color center spin states with optically detected magnetic resonance (ODMR) technology is traditionally based on Stokes excitation, where the energy of the exciting laser is higher than that of the emission photons. Here, we investigate an unconventional approach using anti-Stokes excitation to detect the ODMR signal of silicon vacancy defect spin in silicon carbide, where the exciting laser has lower energy than the emitted photons. Laser power, microwave power and temperature dependence of the anti-Stokes excited ODMR are systematically studied, in which the behavior of ODMR contrast and linewidth is shown to be similar to that of Stokes excitation. However, the ODMR contrast is several times that of the Stokes excitation. Coherent control of silicon vacancy spin under anti-Stokes excitation is then realized at room temperature. The spin coherence properties are the same as those of Stokes excitation, but with a signal contrast that is around three times greater. To illustrate the enhanced spin readout contrast under anti-Stokes excitation, we also provide a theoretical model. The experiments demonstrate that the current anti-Stokes excitation ODMR approach has promising applications in quantum information processing and quantum sensing.
Silicon carbide is an emerging platform for quantum technologies that provides wafer scale and low-cost industrial fabrication. The material also hosts high-quality defects with long coherence times that can be used for quantum computation and sensing applications. Using an ensemble of nitrogen-vacancy centers and an XY8-2 correlation spectroscopy approach, we demonstrate a room-temperature quantum sensing of an artificial AC field centered at ~900 kHz with a spectral resolution of 10 kHz. Implementing the synchronized readout technique, we further extend the frequency resolution of our sensor to 0.01 kHz. These results pave the first steps for silicon carbide quantum sensors toward low-cost nuclear magnetic resonance spectrometers with a wide range of practical applications in medical, chemical, and biological analysis.
We report the realization of the optically induced inverse population of the ground-state spin sublevels of the silicon vacancies (V(Si)) in silicon carbide (SiC) at room temperature. The data show that the probed silicon vacancy spin ensemble can be prepared in a coherent superposition of the spin states. Rabi nutations persist for more than 80 μs. Two opposite schemes of the optical alignment of the populations between the ground-state spin sublevels of the silicon vacancy upon illumination with unpolarized light are realized in 4H- and 6H-SiC at room temperature. These altogether make the silicon vacancy in SiC a very favorable defect for spintronics, quantum information processing, and magnetometry.
Abstract High-fidelity projective readout of a qubit’s state in a single experimental repetition is a prerequisite for various quantum protocols of sensing and computing. Achieving single-shot readout is challenging for solid-state qubits. For Nitrogen-Vacancy (NV) centers in diamond, it has been realized using nuclear memories or resonant excitation at cryogenic temperature. All of these existing approaches have stringent experimental demands. In particular, they require a high efficiency of photon collection, such as immersion optics or all-diamond micro-optics. For some of the most relevant applications, such as shallow implanted NV centers in a cryogenic environment, these tools are unavailable. Here we demonstrate an all-optical spin readout scheme that achieves single-shot fidelity even if photon collection is poor (delivering less than 10 3 clicks/second). The scheme is based on spin-dependent resonant excitation at cryogenic temperature combined with spin-to-charge conversion, mapping the fragile electron spin states to the stable charge states. We prove this technique to work on shallow implanted NV centers, as they are required for sensing and scalable NV-based quantum registers.