A 5-micrometer glass bead levitated in air by a single laser beam from below. This optical trap is formed by the force from the laser beam and the gravitational force on the bead. Tongcang Li, et. al. used a similar optical trap to study the Brownian motion of a trapped bead in air with ultra-high resolution. Their paper is published in Science. Image Credit: Tongcang Li
Paper: Measurement of the Instantaneous Velocity of a Brownian Particle
Authors: Tongcang Li, Simon Kheifets, David Medellin and Mark G. Raizen
Abstract:
"Brownian motion of particles affects many branches of science. We report on the Brownian motion of micrometer-sized beads of glass held in air by an optical tweezer, over a wide range of pressures, and we measured the instantaneous velocity of a Brownian particle. Our results provide direct verification of the energy equipartition theorem for a Brownian particle. For short times, the ballistic regime of Brownian motion was observed, in contrast to the usual diffusive regime. We discuss the applications of these methods toward cooling the center-of-mass motion of a bead in vacuum to the quantum ground motional state."
An applet on Brownian Motion:
I've met Tongcang Li and Mark Raizen in Austin, honored with a tour of their lab at UT.
Blog about their paper:
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