Quantum acoustics: creation and control of multi-phonon Fock states*

Quantum states of mechanical motion can be important resources for quantum information, metrology, and studies of fundamental physics. There have recently been several new demonstrations that advance our ability to generate, control, and measure individual quanta of motion. In our approach, we combine a superconducting qubit with a piezoelectric transducer, which can couple to high quality factor bulk acoustic resonators [1]. In analogy with cavity QED, this system allows for strong coupling between the electrical excitations of the qubit and single Gigahertz phonons trapped in a single-crystal substrate, opening new capabilities for manipulation of mechanical degrees of freedom in the quantum domain. First, we show that a single excitation can be controllably swapped back and forth between the qubit and the acoustic resonator. Next, by employing a flip-chip geometry and a phononic “supercavity” made by curving one side of the substrate, we observe increased phonon lifetimes approaching 100 microseconds, comparable to state-of-the-art qubit coherence times. Using this system, we can then carry out a protocol to climb the phonon ladder, creating multi-photon Fock states or superpositions of Fock states, and measuring the Wigner function of the results using the qubit. I will talk about the prospects of using these mechanical degrees of freedom as multi-mode memories and as probes of the fundamental mechanisms of decoherence in quantum systems.


This work performed in collaboration with Yiwen Chu, Peter Rakich, Taekwan Yoon, Vijay Jain, and Prashanta Kharel.

[1] Y. Chu, et al., Quantum acoustics with superconducting qubits, Science 358, 6360 (2017).
[2] Y. Chu, et al., Creation and control of multi-phonon Fock states in a bulk acoustic-wave resonator, Nature 563, 666-670 (2018).