  The integration of nanophotonics and atomic physics has been a long-sought
goal that would open new frontiers for optical physics. Here, we report the
development of the first integrated optical circuit with a photonic crystal
capable of both localizing and interfacing atoms with guided photons in the
device. By aligning the optical bands of a photonic crystal waveguide (PCW)
with selected atomic transitions, our platform provides new opportunities for
novel quantum transport and many-body phenomena by way of photon-mediated
atomic interactions along the PCW. From reflection spectra measured with
average atom number N = 1.1$\pm$0.4, we infer that atoms are localized within
the PCW by Casimir-Polder and optical dipole forces. The fraction of
single-atom radiative decay into the PCW is $\Gamma_{\rm 1D}/\Gamma'$ =
0.32$\pm$0.08, where $\Gamma_{1D}$ is the rate of emission into the guided mode
and $\Gamma'$ is the decay rate into all other channels. $\Gamma_{\rm
1D}/\Gamma'$ is quoted without enhancement due to an external cavity and is
unprecedented in all current atom-photon interfaces.
