We report on experiments studying the Brownian motion of an optically trapped bead in air, and observe for the first time the short-time regime of ballistic motion. Einstein predicted this effect in 1907, but said the experiment would be impossible in practice. Our measurements were enabled by our development of a new detection system that is capable of real-time tracking of the motion of a trapped bead on unprecedented short time scales and correspondingly small length scales. We used the data to measure the average kinetic energy of a Brownian particle, and find good agreement with the energy equipartition theorem of statistical mechanics. Measurement of the instantaneous velocity also allows us to stably trap beads in vacuum, using active feedback to control and cool the center of mass motion to mK temperatures in three dimensions. The system of an optically trapped bead in vacuum can serve as a testing ground for macroscopic quantum superpositions and the role of decoherence. In the opposite extreme, a trapped bead in a fluid can be used to test basic questions in statistical mechanics, and fluid dynamics on the smallest scales.

Date Published: 10 October 2012
PDF: 7 pages
Proc. SPIE 8458, Optical Trapping and Optical Micromanipulation IX, 84580B (10 October 2012); doi: 10.1117/12.928885

Published in SPIE Proceedings Vol. 8458:
Optical Trapping and Optical Micromanipulation IX
Kishan Dholakia; Gabriel C. Spalding, Editor(s)