Phononic bandgap nano-acoustic cavity with ultralong phonon lifetime

Abstract

We present measurements at millikelvin temperatures of the \nmicrowave-frequency acoustic properties of a crystalline silicon nanobeam \ncavity incorporating a phononic bandgap clamping structure for acoustic \nconfinement. Utilizing pulsed laser light to excite a co-localized optical mode \nof the nanobeam cavity, we measure the dynamics of cavity acoustic modes with \nsingle-phonon sensitivity. Energy ringdown measurements for the fundamental \n5 GHz acoustic mode of the cavity shows an exponential increase in phonon \nlifetime versus number of periods in the phononic bandgap shield, increasing up \nto τ_(ph,0) ≈ 1.5~seconds. This ultralong lifetime, corresponding to an \neffective phonon propagation length of several kilometers, is found to be \nconsistent with damping from non-resonant two-level system defects on the \nsurface of the silicon device. Potential applications of these ultra-coherent \nnanoscale mechanical resonators range from tests of various collapse models of \nquantum mechanics to miniature quantum memory elements in hybrid \nsuperconducting quantum circuits.