Optomechanical control of long-lived bulk acoustic phonons in the quantum regime

High-fidelity quantum optomechanical control of a mechanical oscillator requires the ability to perform efficient, low-noise operations on long-lived phononic excitations. Microfabricated high-overtone bulk acoustic wave resonators (μHBARs) support high-frequency mechanical modes above 10 GHz with coherence times exceeding one millisecond. Here we demonstrate a μHBAR-based cavity optomechanical system that permits quantum optomechanical control of individual high-coherence phonon modes. We perform laser cooling of the phonon modes from an occupation of approximately 22 phonons to fewer than 0.4, corresponding to laser-based ground-state cooling of a mechanical object with a mass of 7.5 μg. During the cooling process we do not observe any absorption-induced heating, demonstrating the resilience of the HBAR optomechanical systems against parasitic heating. Our work demonstrates that μHBARs are promising as the basis for quantum optomechanical systems with robustness to decoherence that is necessary for efficient, low-noise photon–phonon conversion.