Phonons, the quanta of sound, provide a compelling topic of study, both for understanding their basic science as well as for developing applications such as in quantum sensing. I will describe some recent experiments my group has pursued in this area, using individual surface acoustic wave (SAW) phonons. In a properly engineered system using superconducting qubits, single SAW phonons can be easily generated and detected, in an on-demand, controlled fashion. Using this approach, we have demonstrated good quantum control of individual microwave-frequency phonons in a SAW acoustic resonator; used individual phonons to transmit quantum states and generate quantum entanglement between remote superconducting qubits; and demonstrated a quantum information process known as “quantum erasure”. Most recently, in unpublished work, we have demonstrated a additional fundamental element, a phonon beamsplitter, which can convert a single incident phonon into a superposition (Bell) state for a phonon in the two beamsplitter output channels. We use this to demonstrate a single phonon interferometer, with phase control of the phonons achieved using superconducting qubits. We have also used the beamsplitter to demonstrate a fundamental quantum effect first seen with photons, the Hong-Ou-Mandel effect, which beautifully illustrates the wave-particle duality fundamental to quantum mechanics. Interestingly, this last development completes the toolbox needed for developing a platform for linear mechanical quantum computing, similar to that developed for optical photons, i.e. linear optical quantum computing.
