Cryogenic microwave loss in epitaxial Al/GaAs/Al trilayers for superconducting circuits

Epitaxially-grown superconductor/dielectric/superconductor trilayers have the potential to form high-performance superconducting quantum devices and may even allow scalable superconducting quantum computing with low-surface-area qubits such as the merged-element transmon. In this work, we measure the power-independent loss and two-level-state (TLS) loss of epitaxial, wafer-bonded, and substrate-removed Al/GaAs/Al trilayers by measuring lumped element superconducting microwave resonators at millikelvin temperatures and down to single photon powers. The power-independent loss of the device is \(4.8 pm 0.1\) times 10^-5 and resonator-induced intrinsic TLS loss is \(6.4 pm 0.2\) times 10^-5. Dielectric loss extraction is used to determine a lower bound of the intrinsic TLS loss of the trilayer of 7.2 times 10^-5. The unusually high power-independent loss is attributed to GaAs's intrinsic piezoelectricity.