Cavity quantum electrodynamics in a high numerical aperture 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 the 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 by reducing the mode size at the atom to order lambda. This
resonator provides a single-atom cooperativity of 1.6 in a cavity where
the light circulates only~10times. We load a single 87Rb atom into this
cavity, observe strong coupling, and demonstrate cavity-enhanced detection
with fidelity of 99.55(6)% and survival of 99.89(4)% in 130 microseconds.
Introducing intra-cavity imaging systems will enable cavity arrays
compatible with Rydberg atom arrays computing technologies, expanding the
the applicability of the cavity QED toolbox.