Open-Cavity in Closed-Cycle Cryostat as a Quantum Optics Platform

The introduction of an optical resonator can enable efficient and precise interaction between a photon
and a solid-state emitter. It facilitates the study of strong light-matter interaction, polaritonic physics and
presents a powerful interface for quantum communication and computing. A pivotal aspect in the progress
of light-matter interaction with solid-state systems is the challenge of combining the requirements of cryo-
genic temperature and high mechanical stability against vibrations while maintaining sufficient degrees of
freedom for in situ tunability. Here, we present a fiber-based open Fabry-Pérot cavity in a closed-cycle
cryostat exhibiting ultrahigh mechanical stability while providing wide-range tunability in all three spatial
directions. We characterize the setup and demonstrate the operation with the root-mean-square cavity-
length fluctuation of less than 90 pm at temperature of 6.5 K and integration bandwidth of 100 kHz.
Finally, we benchmark the cavity performance by demonstrating the strong-coupling formation of exciton
polaritons in monolayer WSe2 with a cooperativity of 1.6. This set of results manifests the open cavity in
a closed-cycle cryostat as a versatile and powerful platform for low-temperature cavity QED experiments.