Physics

Quantum dynamics

Quantum technology

Optical force

Optomechanics

Radiation pressure

Optics

Quantum decoherence

Quantum state

Magnet

Open quantum system

Optical tweezers

Quantum mechanics

Levitation

Quantum optics

Classical mechanics

Quantum

Quantum sensor

Nature

Tests of quantum mechanics on a macroscopic scale require extreme control over mechanical motion and its decoherence1–3. Quantum control of mechanical motion has been achieved by engineering the radiation–pressure coupling between a micromechanical oscillator and the electromagnetic field in a resonator4–7. Furthermore, measurement-based feedback control relying on cavity-enhanced detection schemes has been used to cool micromechanical oscillators to their quantum ground states8. In contrast to mechanically tethered systems, optically levitated nanoparticles are particularly promising candidates for matter-wave experiments with massive objects9,10, since their trapping potential is fully controllable. Here we optically levitate a femtogram (10−15&nbsp;grams) dielectric particle in cryogenic free space, which suppresses thermal effects sufficiently to make the measurement backaction the dominant decoherence mechanism. With an efficient quantum measurement, we exert quantum control over the dynamics of the particle. We cool its centre-of-mass motion by measurement-based feedback to an average occupancy of 0.65 motional quanta, corresponding to a state purity of 0.43. The absence of an optical resonator and its bandwidth limitations holds promise to transfer the full quantum control available for electromagnetic fields to a mechanical system. Together with the fact that the optical trapping potential is highly controllable, our experimental platform offers a route to investigating quantum mechanics at macroscopic scales11. Quantum control of an optically levitated nanoparticle with a mass of just one femtogram is demonstrated in a cryogenic environment by feedback-cooling the motion of the particle to the quantum ground state.

Quantum control of a nanoparticle optically levitated in cryogenic free space