  Optomechanical cavities in the well-resolved-sideband regime are ideally
suited for the study of a myriad of quantum phenomena with mechanical systems,
including backaction-evading measurements, mechanical squeezing, and generation
of non-classical states. For these experiments, the mechanical oscillator
should be prepared in its ground state; residual motion beyond the zero-point
motion must be negligible. The requisite cooling of the mechanical motion can
be achieved using the radiation pressure of light in the cavity by selectively
driving the anti-Stokes optomechanical transition. To date, however,
laser-absorption heating of optical systems far into the resolved-sideband
regime has prohibited strong driving. For deep ground-state cooling, previous
studies have therefore resorted to passive cooling in dilution refrigerators.
Here, we employ a highly sideband-resolved silicon optomechanical crystal in a
$^3$He buffer gas environment at $\sim 2\text{K}$ to demonstrate laser sideband
cooling to a mean thermal occupancy of $0.09_{-0.01}^{+0.02}$ quantum
(self-calibrated using motional sideband asymmetry), which is $-7.4\text{dB}$
of the oscillator's zero-point energy and corresponds to 92% ground state
probability. Achieving such low occupancy by laser cooling opens the door to a
wide range of quantum-optomechanical experiments in the optical domain.
