  From fundamental studies of light-matter interaction to applications in
quantum networking and sensing, cavity quantum electrodynamics (QED) provides a
platform-crossing toolbox to control interactions between atoms and photons.
The coherence of such interactions is determined by the product of the
single-pass atomic absorption and the number of photon round-trips. Reducing
the cavity loss has enabled resonators supporting nearly 1-million optical
roundtrips at the expense of severely limited optical material choices and
increased alignment sensitivity. The single-pass absorption probability can be
increased through the use of near-concentric, fiber or nanophotonic cavities,
which reduce the mode waists at the expense of constrained optical access and
exposure to surface fields. Here we present a new high numerical-aperture,
lens-based resonator that pushes the single-atom-single-photon absorption
probability per round trip close to its fundamental limit by reducing the mode
size at the atom below a micron while keeping the atom mm-to-cm away from all
optics. This resonator provides strong light-matter coupling in a cavity where
the light circulates only ~ 10 times. We load a single 87Rb atom into such a
cavity, observe strong coupling, demonstrate cavity-enhanced atom detection
with imaging fidelity of 99.55(6) percent and survival probability of 99.89(4)
percent in 130 microseconds, and leverage this new platform for a time-resolved
exploration of cavity cooling. The resonator's loss-resilience paves the way to
coupling of atoms to nonlinear and adaptive optical elements and provides a
minimally invasive route to readout of defect centers. Introduction of
intra-cavity imaging systems will enable the creation of cavity arrays
compatible with Rydberg atom array computing technologies, vastly expanding the
applicability of the cavity QED toolbox.
