Phys. Rev. Lett. 109, 103603 (2012)

Jan Gieseler and colleagues from Spain, the USA and Switzerland have cooled a silica nanoparticle from room temperature to around 50 mK using an optical gradient trap. Traditional optical cooling requires a laser beam in every direction of motion; that is, six beams for three-dimensional trapping. However, a gradient force pointing towards the centre of a trap requires only one laser beam. To cool the centre-of-mass motion, the researchers employed parametric feedback after pumping down the chamber. The feedback effectively opposes the particle's motion by increasing the trap stiffness when the particle moves away from the trap centre. The researchers put this approach to use by performing modulation at twice the trap frequency, with an appropriate phase shift. The cooling action is in competition with reheating due to collisions with air molecules, which sets an upper limit to cooling at a given pressure. Although the technique suffers from a trade-off between measurement uncertainty and recoil heating, theoretical analysis indicates that temperatures close to the quantum ground state may be achievable.