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

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 $|m=\pm 3/2\rangle$ and $|m=\pm 1/2\rangle$ states, and optical pumping does not create a population difference between $|+1/2\rangle$ and $|-1/2\rangle$ states. Thus, operating one qubit (e.g., $\left{|+3/2\rangle, |+1/2\rangle \right}$ states) leaves the population in the remaining state ($|-1/2\rangle$) 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 $\left{ |+3/2\rangle, |+1/2\rangle \right}$ and $\left{ |-1/2\rangle, |-3/2\rangle \right}$. Experimental results demonstrate that this approach doubles signal contrast in optical readout and improves sensitivity in AC magnetometry compared to simplex operation.