Short answer: Optomechanical cooling uses light or microwaves to damp the vibration of a small mechanical object toward its quantum ground state. A 2011 Nature paper cooled a micromechanical membrane of about 10 MHz to 0.34 phonons using a superconducting microwave circuit, and a 2016 Physical Review Letters paper reports sideband cooling of a membrane to the quantum backaction limit. A 2021 theory paper proposes another cooling scheme. These are laboratory experiments and a proposal.
The idea
The 2011 abstract explains that laser cooling transformed the study of atomic systems, and that analogous techniques can prepare macroscopic objects in their motional ground state. Cavity optomechanical and electromechanical systems achieve sideband cooling through strong interaction between light and motion. Entering the quantum regime, with less than a single quantum of motion, was difficult because sideband cooling had not sufficiently overwhelmed the coupling of low-frequency mechanical systems to their hot environments.
2011: ground state with a microwave circuit
In the Nature paper, the authors demonstrate sideband cooling of an approximately 10 MHz micromechanical oscillator to the quantum ground state. This needed a large electromechanical interaction, which they obtained by embedding a micromechanical membrane in a superconducting microwave resonant circuit. To verify the cooling, they perform a near-Heisenberg-limited position measurement and report a phonon occupation of 0.34 with an uncertainty of 0.05. They also report strong coupling, allowing coherent exchange of microwave photons and mechanical phonons. The rest of the abstract was cut off here.
2016: the quantum backaction limit
The Physical Review Letters abstract says light pressure can damp and cool a mechanical resonator, but shot noise sets a limit on the minimum phonon occupation even if the light has no thermal component. In sideband cooling in a cavity, finite off-resonant Stokes scattering, set by the cavity linewidth, together with shot noise defines a quantum backaction limit, which the authors compare to the Doppler limit of atomic laser cooling. They sideband cool a micromechanical membrane to this limit. By monitoring the optical sidebands, they directly observe the mechanical object come to thermal equilibrium with the optical bath. The abstract is cut off after that point and the remainder is not described here.
2021: a pulse and continuous-wave scheme
A 2021 Journal of Physics B paper is titled "Efficient ground state cooling of a membrane by the combination of continuous-wave field and pulses". Only its title and authors were readable here, so its results are not described.
Limits
The two experiments use different platforms, a microwave circuit in 2011 and an optical cavity in 2016, so their numbers are not comparable. Reaching 0.34 phonons is near the ground state and does not by itself show a mechanical quantum state. The theory paper is unverified here beyond its title.
FAQ
What is sideband cooling?
Using light or microwaves detuned from a resonator to remove energy from its vibration.
How cold did the 2011 membrane get?
To 0.34 phonons, with an uncertainty of 0.05, per the authors.
What is the quantum backaction limit?
The minimum phonon occupation set by shot noise and off-resonant scattering in sideband cooling.
Why does this matter?
Ground-state mechanical objects are a starting point for quantum control of massive systems.
Is this quantum computing?
No. It is quantum control of mechanical motion.
All field notes