Towards the quantum regime with propagating acoustic waves: Local probing in a GHz echo chamber

Abstract

In the same way that micro-mechanical resonators resemble guitar strings and drums, Surface Acoustic Waves (SAW) resemble the sound these instruments produce, but moving over a solid surface rather than through air. In recent pioneering experiments, the displacements of string-like resonators have been measured near the quantum mechanical limit and a drum-like resonator has been made to vibrate quantum-coherently with a superconducting qubit. Propagating mechanical waves, on the other hand, have not been studied at this ultimate level, for good reasons: They are elusive to their very nature, and the displacement due to an unconfined phonon is too small to detect with conventional capacitive and optical techniques. Here, we demonstrate local and time-resolved probing of propagating surface acoustic waves with a displacement sensitivity of 30am/\surdHz RMS, at a mechanical frequency of 932MHz. The improvement three orders of magnitude compared to previous studies of SAW. Extreme displacement sensitivity has been achieved in other systems, but only at lower frequencies, where optical probing techniques are efficient. The high sensitivity allows us to resolve pulses below the single-phonon level, using averaging, and we project that single-shot resolution is feasible at the level of single propagating phonons. Our probe is a piezoelectrically coupled RF-SET, which is sufficiently fast, non-destructive and localized to let us track an acoustic pulse as it echoes back and forth in a long acoustic cavity, interfering with itself and ringing the cavity up and down. By strong coupling to quantum circuits and read-out on the single-phonon level, we project that the unique features of SAW will lead to novel quantum hybrids. Acoustic coupling to superconducting qubits and quantum investigations of phonon-phonon collisions are some of the exciting prospects