# Fundamental tests of physics with optically trapped microspheres

 Title: Fundamental tests of physics with optically trapped microspheres Author: Li, Tongcang Abstract: This dissertation details our experiments on studying the Brownian motion of an optically trapped microsphere with ultrahigh resolution , and cooling of its motion towards the quantum ground state . We have trapped glass microspheres in water , air and vacuum with optical tweezers . We developed a detection system that can monitor the position of a trapped microsphere with Angstrom spatial resolution and microsecond temporal resolution . We studied the Brownian motion of a trapped microsphere in air over a wide range of pressures . We measured the instantaneous velocity of a Brownian particle . Our 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 . We are currently developing a new detection system to measure the instantaneous velocity of a Brownian particle in water . In vacuum , we have used active feedback to cool the three center -of -mass vibration modes of a trapped microsphere from room temperature to millikelvin temperatures with a minimum mode temperature of 1 .5 mK , which corresponds to the reduction of the root mean square (rms ) amplitude of the microsphere from 6 .7 nm to 15 pm for that mode . The mean thermal occupation number of that mode is reduced from about 6 .8 $\times 10^8$ at 297 K to about 3400 at 1 .5 mK . URI: http : / /hdl .handle .net /2152 /ETD -UT -2011 -05 -2930 Date: 2011-07-06

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