  Mechanical resonators can act as excellent intermediaries to interface single
photons in the microwave and optical domains due to their high quality factors.
Nevertheless, the optical pump required to overcome the large energy difference
between the frequencies can add significant noise to the transduced signal.
Here we exploit the remarkable properties of thin-film gallium phosphide to
demonstrate bi-directional on-chip conversion between microwave and optical
frequencies, realized by piezoelectric actuation of a Gigahertz-frequency
optomechanical resonator. The large optomechanical coupling and the suppression
of two-photon absorption in the material allows us to operate the device at
optomechanical cooperativities greatly exceeding one. Alternatively, when using
a pulsed upconversion pump, we demonstrate that we induce less than one thermal
noise phonon. We include a high-impedance on-chip matching resonator to mediate
the mechanical load with the 50-Ohm source. Our results establish gallium
phosphide as a versatile platform for ultra-low-noise conversion of photons
between microwave and optical frequencies.
