Gallium Phosphide as a Piezoelectric Platform for Quantum Optomechanics*

Piezoelectric materials expand the capabilities of optomechanical devices to include native electromechanical interaction, enabling conversion between microwave-frequency modes and low-loss optical telecom bands. Specifically, the combination of high-cooperativity interfaces and ground-state occupation of the mechanical mode allows for transduction of a quantum state. While cryogenically cooled piezoelectric optomechanical devices have demonstrated sub-phonon occupation [1], absorption induced heating has prevented the observation of non-classical behaviour. Here, we fabricate and operate an on-chip optomechanical device made from gallium phosphide [2], realising high-cooperativity interaction with a long-lived 3-GHz mechanical mode which remains in the ground state. We observe non-classical correlations between state-projecting photons and the confined mechanical mode, establishing GaP as a piezoelectric platform for noise-free quantum-state conversion between microwave and optical carriers. [1] Ramp et al. PRL 123 93603 (2019) Forsch et al. Nat. Phys. (2019) [2] Stockill et al. PRL 123 163602 (2019)