Quantum Control of Bulk Acoustic Phonons

We describe new strategies to access ultrahigh coherence phonon modes within bulk acoustic wave resonators for use in both quantum optics and circuit QED. Efficient access to phonons with long coherence times could enable high-fidelity quantum memories, quantum transducers, and quantum sensors. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are intriguing for their ability to support phonon modes with record fQ products at high frequencies (>10GHz) while also minimizes unwanted surface interactions that can contribute to decoherence. In this work, we combine new non-invasive laser spectroscopy techniques with materials analysis to realize microfabricated HBAR resonators with Q-factors exceeding 140-million at 12.6 GHz frequencies, corresponding to phonon lifetimes of >1.6 milliseconds and record-level fQ products of 1.8e18 Hz. We describe complementary techniques to harness these phonons using optomechanical and electromechanical couplings. Piezoelectric coupling is used to interface these resonators with superconducting qubits to synthesize nonclassical states of sound. Using resonantly enhanced Brillouin interactions, we also demonstrate laser cooling of phonons to their ground state, a crucial first demonstration of quantum optical control.