Optical trapping and cooling of glass microspheres

摘要

We report on experiments studying the Brownian motion of an optically trapped bead in air, and observe for the firsttime the short-time regime of ballistic motion. Einstein predicted this effect in 1907, but said the experiment would beimpossible in practice. Our measurements were enabled by our development of a new detection system that is capable ofreal-time tracking of the motion of a trapped bead on unprecedented short time scales and correspondingly small lengthscales. We used the data to measure the average kinetic energy of a Brownian particle, and find good agreement withthe energy equipartition theorem of statistical mechanics. Measurement of the instantaneous velocity also allows us tostably trap beads in vacuum, using active feedback to control and cool the center of mass motion to mK temperatures inthree dimensions. The system of an optically trapped bead in vacuum can serve as a testing ground for macroscopicquantum superpositions and the role of decoherence. In the opposite extreme, a trapped bead in a fluid can be used to testbasic questions in statistical mechanics, and fluid dynamics on the smallest scales.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.