Fundamental tests of physics with optically trapped microspheres

<i>Fundamental Tests of Physics with Optically Trapped Microspheres </i>details experiments on studying the Brownian motion of an optically trapped microsphere with ultrahigh resolution and the cooling of its motion towards the quantum ground state. <br><br>Glass microspheres were trapped in water, air, and vacuum with optical tweezers; and a detection system that can monitor the position of a trapped microsphere with Angstrom spatial resolution and microsecond temporal resolution was developed to study the Brownian motion of a trapped microsphere in air over a wide range of pressures. The instantaneous velocity of a Brownian particle, in particular, was measured for the very first time, and the results provide direct verification of the Maxwell-Boltzmann velocity distribution and the energy equipartition theorem for a Brownian particle. For short time scales, the ballistic regime of Brownian motion is observed, in contrast to the usual diffusive regime. <br><br>In vacuum, active feedback is used to cool the center-of-mass motion of an optically trapped microsphere from room temperature to a minimum temperature of about 1.5 mK. This is an important step toward studying the quantum behaviors of a macroscopic particle trapped in vacuum.