Investigating superconducting qubit loss channels in a quantum acoustical device

The loss channels that limit superconducting qubit coherence times are presently not well understood. Mounting evidence shows that qubits couple to dissipative two-level system baths in dielectrics. In contrast, a less explored loss channel is from phonon radiation that is intrinsic to the Josephson junction superconductor-insulator boundary. Most dielectrics have a vanishing piezoelectric effect in the bulk from inversion symmetry, but this is not the case at surfaces. Theory predicts that the loss rate from this piezoelectric effect can be comparable to the lifetimes of state-of-the-art transmon qubits. Here we investigate phonon radiation from Josephson junctions to put an upper bound on its limitations on the transmon's coherence time.