Cavity QED in a high NA resonator.

From fundamental studies of light-matter interaction to applications in quantum networking and sensing, cavity quantum electrodynamics (QED) provides a toolbox to control interactions between atoms and photons. The coherence of interactions is determined by the single-pass atomic absorption and number of photon round-trips. Reducing the cavity loss has enabled resonators supporting 1 million roundtrips but with limited material choices and increased alignment sensitivity. Here, we present a high-numerical aperture, lens-based resonator that pushes the single-atom single-photon absorption probability near its fundamental limit, reducing the mode size at the atom to order λ. This resonator provides a single-atom cooperativity of 1.6 in a cavity where the light circulates ∼10 times. We load single Rb atoms into this cavity, observe strong coupling, and demonstrate cavity-enhanced atom detection with fidelity of 99.55(6)% and survival of 99.89(4)% in 130 μs. Introducing intracavity imaging systems will enable cavity arrays compatible with Rydberg atom array computing technologies, expanding the applicability of the cavity QED toolbox.