Nanomechanical resonators have demonstrated great potential for use as versatile tools in a number of emerging quantum technologies. For such applications, the performance of these systems is restricted by the decoherence of their fragile quantum states, necessitating a thorough understanding of their dissipative coupling to the surrounding environment. In bulk amorphous solids, these dissipation channels are dominated at low temperatures by parasitic coupling to intrinsic two-level system (TLS) defects, however, there remains a disconnect between theory and experiment on how this damping manifests in dimensionally-reduced nanomechanical resonators. Here, we present an optomechanically-mediated thermal ringdown technique, which we use to perform simultaneous measurements of the dissipation in four mechanical modes of a cryogenically-cooled silicon nanoresonator, with resonant frequencies ranging from 3 - 19 MHz. Analyzing the device's mechanical damping rate at fridge temperatures between 10 mK - 10 K, we demonstrate quantitative agreement with the standard tunneling model for TLS ensembles confined to one dimension. From these fits, we extract the defect density of states (P0∼ 1 - 4 × 1044 J−1 m−3) and deformation potentials (γ∼ 1 - 2 eV), showing that each mechanical mode couples on average to less than a single thermally-active defect at 10 mK.
@article{arxiv.1710.09439,
title = {Two-Level System Damping in a Quasi-One-Dimensional Optomechanical Resonator},
author = {B. D. Hauer and P. H. Kim and C. Doolin and F. Souris and J. Davis},
journal= {arXiv preprint arXiv:1710.09439},
year = {2018}
}