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

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.