  Photons are ideal carriers of quantum information, as they can be easily
created and can travel long distances without being affected by decoherence.
For this reason, they are well suited for quantum communication. However, the
interaction between single photons is negligible under most circumstances.
Realising such an interaction is not only fundamentally fascinating but holds
great potential for emerging technologies. It has recently been shown that even
weak optical nonlinearities between single photons can be used to perform
important quantum communication tasks more efficiently than methods based on
linear optics, which have fundamental limitations. Nonlinear optical effects at
single photon levels in atomic media have been studied and demonstrated but
these are neither flexible nor compatible with quantum communication as they
impose restrictions on photons' wavelengths and bandwidths. Here we use a high
efficiency nonlinear waveguide to observe the sum-frequency generation between
a single photon and a single-photon level coherent state from two independent
sources. The use of an integrated, room-temperature device and telecom
wavelengths makes this approach to photon-photon interaction well adapted to
long distance quantum communication, moving quantum nonlinear optics one step
further towards complex quantum networks and future applications such as device
independent quantum key distribution.
