Measurement of the Instantaneous Velocity of a Brownian Particle

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Science  20 May 2010:
DOI: 10.1126/science.1189403


Brownian motion of particles impacts many branches of science. We report on the Brownian motion of micron-sized beads of glass held in air in an optical tweezer, over a wide range of pressures, and measure 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 is observed, in contrast to the usual diffusive regime. We discuss the applications of these methods towards cooling the center of mass motion of a bead in vacuum to the quantum ground motional state.