  Optically trapped nanoparticles have recently emerged as exciting candidates
for tests of quantum mechanics at the macroscale and as versatile platforms for
ultrasensitive metrology. Recent experiments have demonstrated parametric
feedback cooling, nonequilibrium physics, and temperature detection, all in the
classical regime. Here we provide the first quantum model for trapped
nanoparticle cooling and force sensing. In contrast to existing theories, our
work indicates that the nanomechanical ground state may be prepared without
using an optical resonator; that the cooling mechanism corresponds to nonlinear
friction; and that the energy loss during cooling is nonexponential in time.
Our results show excellent agreement with experimental data in the classical
limit, and constitute an underlying theoretical framework for experiments
aiming at ground state preparation. Our theory also addresses the optimization
of, and the fundamental quantum limit to, force sensing, thus providing
theoretical direction to ongoing searches for ultra-weak forces using levitated
nanoparticles.
