  Cavity quantum electrodynamics (cQED), the interaction of a two-level system
with a high quality factor (Q) cavity, is a foundational building block in
different architectures for quantum computation, communication, and metrology.
The strong interaction between the atom and the cavity enables single photon
operation which is required for quantum gates and sources. Cold atoms, quantum
dots, and color centers in crystals are amongst the systems that have shown
single photon operations, but they require significant physical infrastructure.
Atomic vapors, on the other hand, require limited experimental infrastructure
and are hence much easier to deploy outside a laboratory, but they produce an
ensemble of moving atoms that results in short interaction times involving
multiple atoms, which can hamper quantum operations. A solution to this issue
can be found in nanophotonic cavities, where light-matter interaction is
enhanced and the volume of operation is small, so that fast single-atom,
single-photon operations are enabled. In this work, we study the interaction of
an atomically-clad microring resonator (ACMRR) with different-sized ensembles
of Rb atoms. We demonstrate strong coupling between an ensemble of ~50 atoms
interacting with a high-quality factor (Q > 4 x 10^5) ACMRR, yielding a
many-atom cooperativity C ~ 5.5. We continue to observe signatures of
atom-photon interaction for a few (< 3) atoms, for which we observe saturation
at the level of one intracavity photon. Further development of our platform,
which includes integrated thermo-optic heaters to enable cavity tuning and
stabilization, should enable the observation of interactions between single
photons and single atoms.
