Surface acoustic wave Fano interference in the quantum regime

Quantum acoustic systems, which integrate surface or bulk phonons with superconducting qubits, offer a unique opportunity to investigate phononic interference and scattering processes in the quantum regime. In particular the interaction between a superconducting qubit and a phononic oscillator allows the qubit to operate as a sensor of the oscillator's excitation spectra and underlying interference effects. Here we present measurements revealing the interference of a resonantly trapped piezoelectric surface acoustic wave (SAW) mode and a broad continuum of surface phonons in a system consisting of a SAW resonator coupled to a superconducting qubit. By populating the SAW device with phonons we leverage the resulting ac Stark shift of the qubit to extract the mean excitation number of the surface phonon coherent state as a function of frequency. In this fashion, the qubit functions as a spectroscopic sensor of the surface phonon environment in the hybrid device and allows us to reveal the interaction between phonons in the resonant SAW mode and the broader surface phonon background, which is well-described within the context of Fano interference of surface piezo-phonons.