Motional Quantum Ground State of a Levitated Nanoparticle from Room Temperature

We report quantum ground state cooling of a levitated nanoparticle in a room temperature environment. Using coherent scattering into an optical cavity we cool the center of mass motion of a $143$ nm diameter silica particle by more than $7$ orders of magnitude to $n_x=0.43\pm 0.03$ phonons along the cavity axis, corresponding to a temperature of $12\mu$K. We infer a heating rate of ${\mathrm{\Gamma }}_x\mathrm{/}2\pi =21\pm 3$ kHz, which results in a coherence time of $7.6\mu$s – or $15$ coherent oscillations – while the particle is optically trapped at a pressure of ${10}^{-6}$ mbar. The inferred optomechanical coupling rate of $g_x\mathrm{/}2\pi =71$ kHz places the system well into the regime of strong cooperativity ($C\approx 5$). We expect that a combination of ultra-high vacuum with free-fall dynamics will allow to further expand the spatio-temporal coherence of such nanoparticles by several orders of magnitude, thereby opening up new opportunities for macrosopic quantum experiments.