Resolved-sideband and cryogenic cooling of an optomechanical resonator

Cooling a mechanical oscillator to its quantum ground state enables the exploration of the quantum nature and the quantum-classical boundary of an otherwise classical system(1-7). In analogy to laser cooling of trapped ions(8), ground-state cooling of an optomechanical system can in principle be achieved by radiation-pressure cooling in the resolved-sideband limit where the cavity photon lifetime far exceeds the mechanical oscillation period(9-11). Here, we report the experimental demonstration of an optomechanical system that combines both resolved-sideband and cryogenic cooling. Mechanical oscillations of a deformed silica microsphere are coupled to optical whispering-gallery modes that can be excited through free-space evanescent coupling(12,13). By precooling the system to 1.4 K, a final average phonon occupation as low as 37 quanta, limited by ultrasonic attenuation in silica, is achieved. With diminishing ultrasonic attenuation, we anticipate that the ground-state cooling can be achieved when the resonator is precooled to a few hundred millikelvin in a He-3 cryostat.