Phononic bandgap nano-acoustic cavity with ultralong phonon lifetime

TL;DR

This paper demonstrates a silicon nanobeam cavity with a phononic bandgap that achieves ultralong phonon lifetimes of up to 1.5 seconds at millikelvin temperatures, enabling highly coherent nanoscale mechanical resonators.

Contribution

The study introduces a phononic bandgap clamping structure in silicon nanobeams that significantly extends phonon lifetimes, reaching seconds, which is a substantial improvement over previous resonators.

Findings

Phonon lifetime increases exponentially with the number of bandgap shield periods.

Achieved phonon lifetime of approximately 1.5 seconds at 5 GHz.

Damping primarily attributed to 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.