Short answer: Levitated nanoparticles held in light have been cooled to their quantum ground state of motion, which opens a route to quantum experiments with large masses. A 2020 Science paper reports ground-state cooling of an optically trapped nanoparticle from room temperature using an optical cavity, a 2021 Nature paper reports feedback cooling of a femtogram particle to 0.65 motional quanta, and a 2023 Nature Physics paper reports two-dimensional ground-state cooling. These are laboratory demonstrations of motion control.
Why this matters
The 2021 abstract states that tests of quantum mechanics at a macroscopic scale need extreme control over mechanical motion and its decoherence. It says that optically levitated nanoparticles are particularly promising for matter-wave experiments with massive objects, since the trapping potential is fully controllable, unlike in mechanically tethered systems. The 2020 abstract adds that quantum control of large, massive objects offers opportunities for sensing and for tests of fundamental physics, while noting that realizing such extreme quantum states remains a major challenge.
2020: cavity cooling to the ground state
The Science paper demonstrates a quantum interface that combines optical trapping of solids with cavity-mediated light-matter interaction. The authors say that precise control over the trap laser frequency and position relative to the optical cavity allowed them to laser-cool a trapped nanoparticle into its quantum ground state of motion from room temperature. They compare the particle to current Bose-Einstein condensates in the number of atoms, while having the density of a solid. They suggest the cooling technique, combined with trap manipulation, may enable superposition states involving large masses. That suggestion is the authors' outlook.
2021: feedback cooling in cryogenic free space
The Nature paper optically levitates a femtogram dielectric particle in cryogenic free space, which suppresses thermal effects enough that measurement backaction becomes the dominant decoherence mechanism. With an efficient quantum measurement, the authors exert quantum control over the particle's dynamics. They cool its centre-of-mass motion by measurement-based feedback to an average occupancy of 0.65 motional quanta, which corresponds to a state purity of 0.43. They note that avoiding an optical resonator removes its bandwidth limits.
2023: two dimensions
The Nature Physics paper says that to overcome cross-coupling and decoherence, quantum control must extend to more dimensions, but a decoupled dark mode has so far blocked cavity-based ground-state cooling of multiple modes. Using coherent scattering into a cavity mode, the authors reduce the occupation numbers of two separate centre-of-mass modes to 0.83 and 0.81. By tuning the frequency separation and cavity coupling of the modes, they show the transition from one-dimensional to two-dimensional ground-state cooling. They say this may lead to quantum-limited orbital angular momentum states for rotation sensing and, with the third axis shown earlier, possibly full three-dimensional ground-state cooling.
Limits
An occupation of 0.65 or 0.83 quanta is near the ground state, not a demonstration of a macroscopic superposition. The papers describe cooling and control of motion, and the superposition and sensing uses are stated aims.
FAQ
What is ground-state cooling?
Reducing a mechanical motion to an average occupancy near zero quanta, the lowest energy state.
How cold did the particle get?
0.65 quanta by feedback in 2021, and 0.83 and 0.81 quanta in two dimensions in 2023, per the authors.
Why use levitation?
The trapping potential is fully controllable and there is no mechanical tether, per the 2021 abstract.
Is this a quantum computer?
No. It is quantum control of a massive object, aimed at sensing and fundamental tests.
What is the next goal?
The 2023 authors say full three-dimensional ground-state cooling may be possible.
All field notes