Optical Trapping and Cooling of Dielectric Particles and Space-time Crystals

Quantum optomechanics has attracted increasing attention in recent years due to its broad applications. In 2008, we started a pioneering experiment to trap and cool a glass microsphere in vacuum towards the quantum ground state of an optical tweezer, and to create a quantum-limited microscopic detector. This novel system eliminates the physical contact inherent to clamped cantilevers and can allow ground-state cooling from room temperature. Moreover, the optical trap can be switched off, allowing a particle to undergo free-fall in vacuum after cooling. This system is ideal for studying macroscopic quantum mechanics, gravity induced quantum effects, and creating an ultrasensitive detector with force sensitivity on the order of 10-22 N/Hz1/2.
We have optically trapped glass microspheres in air and vacuum, built an ultrasensitive detector to monitor their Brownian motion, and performed feedback cooling. With a glass microsphere levitated in air, we measured the instantaneous velocity of a Brownian particle, a task that was said to be impossible by Albert Einstein in 1907. Our results provide direct verification of the energy equipartition theorem and the Maxwell-Boltzmann velocity distribution for a Brownian particle. This result was published in Science, and has been included in undergraduate curricula. In vacuum, we have used active feedback to cool the center-of-mass motion of a trapped microsphere from room temperature to a minimum temperature of 1.5 mK, which is an important step towards creating large Schrödinger’s cat states of massive objects.
Unlike conventional optomechanical systems such as clamped cantilevers, microspheres levitated in vacuum may rotate freely. It is interesting to ask how the rotation of a system consisting of millions of strongly interacting atoms may behave nonclassically. This line of thought led us to propose an experimental scheme to create space-time crystals of trapped ions by confining a large number of identical ions in a ring trap with a static magnetic field. We are currently working on this experiment.