Laser cooling to the zero-point energy of a nanomechanical oscillator*

Silicon optomechanical crystals enable coupling of photons at telecommunication wavelengths to GHz mechanical modes, giving rise to optomechanical dynamics that can extend well into the resolved-sideband regime. Despite these promising characteristics, high-fidelity ground state preparation has to date only been achieved using passive cooling in a dilution refrigerator. Moreover, heating due to optical absorption has limited measurement protocols to short, low-energy optical pulses. Here, we demonstrate continuous-wave laser sideband cooling of a silicon optomechanical crystal to the zero-point energy, reaching a mean thermal occupancy of $0.09_{-0.01}^{+0.02}$ quanta, or 92\% ground state occupation, self-calibrated via motional sideband asymmetry. Our results overcome previous limitations due to optical absorption heating and highlight optomechanical crystals for quantum-enhanced continuous displacement measurements, low-added-noise quantum transducers, and integration with superconducting qubit technology.