Coupling 4H-SiC spins to a microwave resonator at millikelvin temperatures

Coupling microwave cavity modes with spin qubit transitions is crucial for enabling efficient qubit readout and control, long-distance qubit coupling, quantum memory implementation, and entanglement generation. We experimentally observe the coupling of different spin qubit transitions in silicon carbide (Si⁢C) material to a three-dimensional microwave (MW) resonator mode around 12.6 GHz at a temperature of 10 mK. Tuning the spin resonances across the cavity resonance via magnetic field sweeps, we perform microwave cavity transmission measurements. We observe spin transitions of different spin defects that are detuned from each other by around 60–70 MHz. By optically exciting the Si⁢C sample placed in the MW cavity with an 810-nm laser, we observe the coupling of an additional spin resonance to the MW cavity, also detuned by around 60–70 MHz from the center resonance. We perform complementary confocal optical spectroscopy as a function of temperature from 4 to 200 K, using a part of the same sample used for the cavity measurements. Combining the confocal spectroscopy results and a detailed analysis of the MW-resonator-based experiments, we attribute the observed spin resonances to two different paramagnetic defects, the negatively charged silicon-vacancy spins located at the 𝑉₁ and 𝑉₂ lattice sites. The 𝑉₁ and 𝑉₂ lines in Si⁢C are interesting qubit transitions since they are known to be robust to decoherence. Consequently, the demonstration of the joint coupling of these spin qubits to a MW cavity mode could lead to interesting new modalities: The microwave cavity could act as an information bus and mediate long-range coupling between the spins, with potential applications in quantum computing and quantum communication, which is an especially attractive proposition in a complementary metal-oxide semiconductor–compatible material such as Si⁢C.