Quantum acoustics with lithium niobate on silicon*

Over the past decade, a growing number of experiments have demonstrated coupling of superconducting circuits to mechanical degrees of freedom, enabling new hybrid devices for the study of acoustic waves in the quantum regime. Recently these have included propagating as well as localized surface and bulk acoustic waves on piezoelectric substrates, where it is possible to achieve strong, resonant coupling with superconducting charge qubits. Here we propose a new platform based on thin-film lithium niobate on silicon (LNOS), where all circuit layers rest on a low-loss, passive silicon substrate and the lithium niobate film serves as a two-dimensional functional layer in which we pattern nanomechanical structures. We present a computational technique to calculate the coupling strength to arbitrary structures and show that reaching the strong-coupling regime should be possible with modest microwave and mechanical losses. As a first demonstration, we present the fabrication of a one-dimensional phononic crystal defect cavity and a preliminary characterization of its mechanical spectrum, which is read out via a tunable SQUID-array resonator.