An optomechanical disk in its fundamental state of motion

At room temperature, the atoms constituting an object oscillate constantly due to thermal motion. To reveal the quantum properties of a mechanical device, this thermal motion must be eliminated through cooling. One way to determine the effective temperature of the device is then to couple its motion to light and measure the latter. This is the approach taken by the LIME team to demonstrate the cooling of an optomechanical disk very close to its ground state of motion, the “phonon vacuum” (the quantum of vibration), in a paper published in Physical Review Letters.

The disks used are micrometer-sized objects made of a semiconductor material (gallium arsenide) that vibrate at high frequencies (GHz). Over the years, they have enabled numerous advances as sensors – in magnetometry, high-frequency fluid rheology, and the measurement of the mass of individual nanoscale objects – but only in the classical regime. To reach the quantum regime, the device was cooled in a dilution refrigerator down to a temperature of approximately 10 millikelvin. Its vibrational state was then probed using thermometry by photon-counting in the optomechanical sidebands: two laser beams are positioned alternately on either side of the optical resonance at a spectral distance of one vibrational quantum, allowing for the measurement of phonon creation and annihilation processes. When the resonator reaches its ground state, the annihilation of a phonon becomes extremely unlikely, since there is no vibration left to suppress. Comparing the occurrence rates of these two processes thus provides a direct measurement of the quantum occupancy of the mechanical mode. The measurements reveal an average occupancy of 0.66 ± 0.2 phonons in the resonator, corresponding to a probability of approximately 60% that the resonator is in its ground state. This recent work shows that a regime in which it is possible to manipulate, count, add, and subtract the quantum vibrations of optomechanical disks is now within reach.

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Contacts: Adrien Borne, Ivan Favero