Optical Coherence Time of Silicon Vacancy Color Centers in 4H-SiC Measured by Two-Pulse Coherent Excitation

Abstract Silicon vacancy color centers in 4H-SiC are a promising solid-state platform for quantum information technologies, yet the optical coherence associated with their zero-phonon lines (ZPLs) remains largely uncharacterized. Here, we present a method for measuring the optical coherence time T2 of the V1′ ZPL of an ensemble of color centers in 4H-SiC based on two-pulse coherent excitation with phase-sensitive lock-in detection, together with the theoretical framework required to extract coherence parameters from the measured signal. Two phase-modulated femtosecond pulses excite the emitters, and the resulting fluorescence is detected with a lock-in amplifier referenced to the beat note of the same pulse pair, so that the relative phase between the excitation pulses is monitored and corrected in real time. We develop a model based on the density matrix formalism that yields closed-form expressions for the lock-in outputs. The model shows that Gaussian inhomogeneous broadening adds a quadratic term to the logarithm of the lock-in output signal, so that homogeneous and inhomogeneous contributions are separable from a single time-domain spectrum. We obtain a coherence time T2 = 12.59 ± 0.23 ps, far below the lifetime limit, indicating strong pure dephasing. This may arise from interactions with the defects created by the high electron irradiation fluence used to prepare the sample. The inhomogeneous broadening is negligible at low excitation power and grows above 0.75 mW, possibly due to linear Stark shifts or other excitation-induced changes in the environment of the emitters. These results establish two-pulse coherent excitation as a robust route to the optical coherence time of color centers, applicable to emitters in diamond, silicon, and two-dimensional materials.