  We demonstrate the tunable quantum beat of single photons through the
co-development of core nonlinear nanophotonic technologies for frequency-domain
manipulation of quantum states in a common physical platform. Spontaneous
four-wave mixing in a nonlinear resonator is used to produce non-degenerate,
quantum-correlated photon pairs. One photon from each pair is then frequency
shifted, without degradation of photon statistics, using four-wave mixing Bragg
scattering in a second nonlinear resonator. Fine tuning of the applied
frequency shift enables tunable quantum interference of the two photons as they
are impinged on a beamsplitter, with an oscillating signature that depends on
their frequency difference. Our work showcases the potential of nonlinear
nanophotonic devices as a valuable resource for photonic quantum information
science.
