  Harnessing nonlinearities strong enough to allow two single photons to
interact with one another is not only a fascinating challenge but is central to
numerous advanced applications in quantum information science. Currently, all
known approaches are extremely challenging although a few have led to
experimental realisations with attenuated classical laser light. This has
included cross-phase modulation with weak classical light in atomic ensembles
and optical fibres, converting incident laser light into a non-classical stream
of photon or Rydberg blockades as well as all-optical switches with attenuated
classical light in various atomic systems. Here we report the observation of a
nonlinear parametric interaction between two true single photons. Single
photons are initially generated by heralding one photon from each of two
independent spontaneous parametric downconversion sources. The two heralded
single photons are subsequently combined in a nonlinear waveguide where they
are converted into a single photon with a higher energy. Our approach
highlights the potential for quantum nonlinear optics with integrated devices,
and as the photons are at telecom wavelengths, it is well adapted to
applications in quantum communication.
