  Hybrid quantum systems are essential for the realization of distributed
quantum networks. In particular, piezo-mechanics operating at typical
superconducting qubit frequencies features low thermal excitations, and offers
an appealing platform to bridge superconducting quantum processors and optical
telecommunication channels. However, integrating superconducting and
optomechanical elements at cryogenic temperatures with sufficiently strong
interactions remains a tremendous challenge. Here, we report an integrated
superconducting cavity piezo-optomechanical platform where 10-GHz phonons are
resonantly coupled with photons in a superconducting and a nanophotonic
cavities at the same time. Benefited from the achieved large piezo-mechanical
cooperativity ($C_\mathrm{em}\sim7$) and the enhanced optomechanical coupling
boosted by a pulsed optical pump, we demonstrate coherent interactions at
cryogenic temperatures via the observation of efficient microwave-optical
photon conversion. This hybrid interface makes a substantial step towards
quantum communication at large scale, as well as novel explorations in
microwave-optical photon entanglement and quantum sensing mediated by gigahertz
phonons.
