  Energy-efficient optical control of photonic device properties is crucial for
diverse photonic signal processing. Here we demonstrate extremely efficient
optical tuning of photonic nanocavities, with only photon-level optical energy.
With a lithium niobate photonic crystal nanocavity with an optical Q up to 1.41
million and an effective mode volume down to 0.78$(\lambda/n)^3$, we are able
to achieve a resonance tuning rate of about 88.4~MHz/photon (0.67~GHz/aJ),
which allows us to tune across the whole cavity resonance with only about 2.4
photons on average inside the cavity. Such a photon-level resonance tuning is
of great potential for energy-efficient optical switching, wavelength routing,
and reconfiguration of photonic devices/circuits that are indispensable for
future photonic interconnect.
