Phys. Rev. Lett. 114, 123602 (2015)

Cooling schemes that employ levitation are potentially useful for applications where isolation from the environment is important. James Millen and co-workers at University College London in the UK have now demonstrated the cooling of a single, charged 400 nm silica nanosphere that is levitated inside a specially designed trap. Their set-up combines a Paul trap — which typically use radio frequency fields for trapping ions — and an optical cavity field. A key advantage of the system is that levitation is achieved not by the optical field alone, so the light intensities required are not sufficiently strong to risk melting or damaging the sample. A particle in the Paul trap, which is inside the optical cavity, can be captured by one of the potential wells of the optical field. This capture occurs when the particle motion is optomechanically damped by red-detuned light during its oscillation in the trap. By monitoring scattered light the nanosphere was observed to be cooled to ∼10 K. The particles are trapped for ∼200 ms but it only takes about 10 ms to cool them. And a nanosphere lost from the optical well is caught in the Paul trap and the cooling cycle commences again.