Thermal performance of a compact cryostat with an in-vacuum objective for color center spin-photon interfaces in silicon carbide

Abstract

Color centers in wide-bandgap semiconductors are promising spin-photon interfaces for quantum repeater nodes, but their optical coherence is highly sensitive to the local cryogenic environment. We characterize the thermal performance of a closed-cycle microscope cryostat with an in-vacuum room-temperature objective using the V1 center in 4H-SiC. Using pulsed resonant photoluminescence excitation of single V1 centers, we extract the zero-phonon-line linewidth as a local probe of the effective sample temperature. Comparing linewidths for different mounting media and objective geometries at a fixed cold-stage thermometer reading below 6K, we find that both the thermal interface and the objective working distance strongly affect the thermal margin at the emitter. Linewidths vary by more than an order of magnitude between configurations, consistent with several-kelvin differences within a phonon-activated linewidth model. These results provide a simple spectroscopic diagnostic and practical guidelines for configuring cryogenic optical platforms for solid-state quantum networks. © 2026 Korean Physical Society