Single photons role in the development of quantum science and
technology. They can carry quantum information over extended distances
to act as the backbone of a future quantum Internet(1) and can be
manipulated in advanced photonic circuits, enabling scalable photonic
quantum computing(2,3). However, more sophisticated devices and
protocols need access to multi-photon states with particular forms of
entanglement. Efficient light-matter interfaces offer a route to
reliably generating these entangled resource states(4,5). Here we
utilize the efficient and coherent coupling of a single quantum emitter
to a nanophotonic waveguide to realize a quantum nonlinear interaction
between single-photon wavepackets. We demonstrate the control of a
photon using a second photon mediated by the quantum emitter. The
dynamical response of the two-photon interaction is experimentally
unravelled and reveals quantum correlations controlled by the pulse
duration. Further development of this platform work, which constitutes a
new research frontier in quantum optics(6), will enable the tailoring of
complex photonic quantum resource states.