Quantum Acoustics with high-overtone bulk resonators and superconducting qubits: High-Q planar devices, phononlasers, and quantum ghosts

The field of quantum acoustics studies high frequency sounds generated at low temperatures such that quantum mechanical effects become relevant. The studies mainly revolves around propagating quantized sound waves, or phonons, a collective excitation of atoms in solids or liquids. In quantum acoustics, the engineering and design tools described by circuit quantum
acoustodynamics (cQAD) are used to develop quantum acoustic devices that are coupled to superconducting qubits. cQAD enabled the demonstrations of quantum ground state cooling mechanical objects, generating mechanical Fock-states, and Schrödinger cat states of motion. This makes quantum acoustic devices appealing candidates for applications such as quantum metrology, information processing, and quantum memory.

This thesis focuses on the coupling between a planar superconducting transmon qubit and a high-overtone bulk acoustic resonator (HBAR) and explore its possibilities. Here,experimental demonstrations are shown where the transmon is used to drive the HBAR into a phonon lasing state making it a superconducting single-atom phonon laser. Furthermore, the transmon-HBAR device is used to probe the nature of ghost modes observed in strongly driven nonlinear systems.