Optically accessible high-finesse millimeter-wave resonator for cavity quantum electrodynamics with atom arrays

Cavity quantum electrodynamics (QED) is a powerful tool in quantum science, enabling preparation of non-classical states of light and scalable entanglement of many atoms coupled to a single field mode. While the most coherent atom–photon interactions have been achieved using superconducting millimeter-wave cavities coupled to Rydberg atoms, these platforms so far lack the optical access required for trapping and addressing individual atomic qubits within them. We present a millimeter-wave Fabry–Pérot cavity with finesse 6 × 10⁷ at 1 K in a near-confocal geometry providing generous transverse optical access (numerical aperture 0.56) for high-resolution atomic trapping and imaging. Conflicting goals of cavity QED coherence and optical access motivate a near-confocal geometry, but upon approaching the confocal limit, we encounter excess cavity loss from stronger than expected hybridization with higher-order transverse modes. An understanding of the post-paraxial corrections to the mode structure of quasioptical cavities informed tuning of the cavity geometry to evade this loss, producing a high finesse that will enable cavity QED experiments with trapped atoms deep in the strong coupling regime.