  The ability to interface multiple optical quantum devices is a key milestone
towards the development of future quantum networks that are capable of sharing
and processing quantum information encoded in light. One of the requirements
for any node of these quantum networks will be cascadability, i.e. the ability
to drive the input of a node using the output of another node. Here, we report
the cascading of quantum light-matter interfaces by storing few-photon level
pulses of light in warm vapor followed by the subsequent storage of the
retrieved field onto a second ensemble. We demonstrate that even after the
sequential storage, the final signal-to-background ratio can remain greater
than 1 for weak pulses containing 8 input photons on average.
