Thin-film quartz for high-coherence piezoelectric phononic crystal resonators

By combining long-lived mechanical resonators with superconducting circuits, the emergent field of circuit quantum acoustodynamics (cQAD) has demonstrated impressive quantum control over phonons. Yet, this control has been hampered by the fast decay of qubits that occurs when integration with piezoelectric materials results in their spontaneous emission into spurious acoustic modes. Phononic crystal defect resonators, fabricated from thin-film piezoelectric materials, attempt to mitigate this problem by means of an engineered acoustic density of states. Although ultralong phonon lifetimes have been demonstrated with this platform, mechanical quality factors at single phonon levels remain limited by the ever-present coupling to a dissipative ensemble of two-level systems (TLS).



We address this problem by exploring a new material platform: thin-film quartz. The high crystallinity and excellent bulk mechanical properties achievable in quartz make it an attractive alternative to traditional thin-film piezoelectrics. We demonstrate that high quality factors (Q>160,000) at mK temperatures and single-phonon occupancies of the mechanical mode can be achieved, a necessary benchmark for cQAD applications. Although the dominant loss mechanism still arises from coupling to a TLS bath, this work already represents an order of magnitude improvement over existing piezoelectric thin-film devices. Building on this progress, we report on the design and fabrication of a hybrid device which galvanically couples thin-film quartz resonators to a fluxonium qubit on sapphire in a flip-chip architecture.