Nano-acoustic resonator with ultralong phonon lifetime

TLDR

Ultra‑coherent nanoscale mechanical resonators can be used to test quantum collapse models and as miniature quantum memory elements in hybrid superconducting circuits. The study measures the microwave‑frequency acoustic properties of a crystalline silicon nanobeam cavity with a phononic bandgap clamping structure at millikelvin temperatures. Pulsed laser light excites a co‑localized optical mode of the cavity, enabling single‑phonon‑sensitive measurement of acoustic mode dynamics. Energy ringdown of the 5 GHz fundamental mode shows phonon lifetimes up to 1.5 s, an effective propagation length of several kilometers, and consistency with damping from surface two‑level system defects.

Abstract

We present measurements at millikelvin temperatures of the microwave-frequency acoustic properties of a crystalline silicon nanobeam cavity incorporating a phononic bandgap clamping structure for acoustic confinement. Utilizing pulsed laser light to excite a co-localized optical mode of the nanobeam cavity, we measure the dynamics of cavity acoustic modes with single-phonon sensitivity. Energy ringdown measurements for the fundamental $5$~GHz acoustic mode of the cavity shows an exponential increase in phonon lifetime versus number of periods in the phononic bandgap shield, increasing up to $\tau \approx 1.5$~seconds. This ultralong lifetime, corresponding to an effective phonon propagation length of several kilometers, is found to be consistent with damping from non-resonant two-level system defects on the surface of the silicon device. Potential applications of these ultra-coherent nanoscale mechanical resonators range from tests of various collapse models of quantum mechanics to miniature quantum memory elements in hybrid superconducting quantum circuits.