Quantum Acoustics with Multimode Surface Acoustic Wave Cavities

Control over the quantum state of macroscopic mechanical oscillators promises advances in the understanding of fundamental physics as well as the development of new quantum technologies. Surface acoustic waves are an attractive mechanical system in a quantum context as they can be designed to interact with many popular quantum platforms, including superconducting qubits. Moreover, surface acoustic waves are already a mature commercial technology widely used in classical signal processing where the slow speed of sound (km/s) means structures with long delays and therefore  ne frequency features can be engineered in chip-scale geometries.

In this thesis, I will describe how quantum acoustics with surface acoustic waves coupled to superconducting qubits can leverage these long delays to build cavities with high densities of resonant modes and qubit-phonon interactions precisely tailored in the frequency domain to suit experimental demands. I  first demonstrate resonant interaction between a transmon qubit and a multi-mode surface acoustic wave resonator where the qubit-resonator coupling strength exceeds not only the decay rates of the qubit and resonator but also the spacing between resonant modes of the cavity. As a natural extension of this result, I describe how intentional shaping of the qubit-cavity coupler in real space leads to a desirable frequency-dependent interaction strength. This hybrid system can achieve interaction strengths large enough for the single-phonon Stark shift to exceed the relevant dissipation rates, leading to the resolution of phonon number states in the qubit spectrum. I will conclude by evaluating the prospects for improved qubit and acoustic performance that would enable a host of experiments, in particular showing that the dominant acoustic loss mechanism, phonon diffraction, can be eliminated by implementing focusing acoustic cavities