  Electrically actuated optomechanical resonators provide a route to
quantum-coherent, bidirectional conversion of microwave and optical photons.
Such devices could enable optical interconnection of quantum computers based on
qubits operating at microwave frequencies. Here we present a novel platform for
microwave-to-optical conversion comprising a photonic crystal cavity made of
single-crystal, piezoelectric gallium phosphide integrated on pre-fabricated
niobium circuits on an intrinsic silicon substrate. The devices exploit
spatially extended, sideband-resolved mechanical breathing modes at $\sim$ 3.2
GHz, with vacuum optomechanical coupling rates of up to $g_0/2\pi \approx$ 300
kHz. The mechanical modes are driven by integrated microwave electrodes via the
inverse piezoelectric effect. We estimate that the system could achieve an
electromechanical coupling rate to a superconducting transmon qubit of $\sim$
200 kHz. Our work represents a decisive step towards integration of
piezoelectro-optomechanical interfaces with superconducting quantum processors.
