  Quantum information technology strongly relies on coupling of optical photons
with narrowband quantum systems, such as quantum dots, color centers, and
atomic systems. This coupling requires matching the optical wavelength and
bandwidth to the desired system, which presents a considerable problem for most
available sources of quantum light. Here we demonstrate coupling of alkali
dipole transitions with a tunable source of photon pairs. Our source is based
on spontaneous parametric down-conversion in a triply-resonant
whispering-gallery mode resonator. For this, we have developed novel wavelength
tuning mechanisms, which allow for a coarse tuning to either cesium or rubidium
wavelength with subsequent continuous fine-tuning to the desired transition. As
a demonstration of the functionality of the source, we performed a heralded
single photon measurement of the atomic decay. We present a major advance in
controlling the spontaneous down-conversion process, which makes our bright
source of single photons now compatible with a plethora of narrow-band resonant
systems.
