Although they enabled several advances in the field of optomechanics, optomechanical disk resonators have not yet been qualified for operation in the quantum regime of motion. We present the experimental demonstration of an optomechanical disk resonator prepared in the quantum ground state. With a gigahertz frequency, the mechanical breathing mode of the investigated semiconductor disk reaches a level of excitation below a single phonon when cooled in a dilution refrigerator. We quantify the phonon occupancy by performing sideband thermometry: a conical optical fiber is evanescently coupled to the disk optical whispering-gallery mode, and Stokes and anti-Stokes photons scattered by phonon emission and absorption are counted on a single-photon detector. We measure a suppression of the absorption process corresponding to a mean phonon occupancy of $0.66\pm 0.20$. Beyond ground-state cooling, we experimentally investigate the mechanisms ruling laser-induced heating, which ultimately limits the lowest measurable phonon occupancy, and witness both intracavity and extracavity heating.

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• Optomechanics
• Nanomechanical devices
• Semiconductor microcavities
• Photon counting
• Whispering gallery mode resonators