Thermally activated transitions in a bistable three-dimensional optical trap

Activated escape from a metastable state underlies many physical, chemical and biological processes: examples include diffusion in solids, switching i n superconducting junctions(1,2), chemical reactions(3,4) and protein foldi ng(5,6). Kramers presented the first quantitative calculation(7) of thermal ly driven transition rates in 1940. Despite widespread acceptance of Kramer s' theory(8), there have been few opportunities to test it quantitatively a s a comprehensive knowledge of the system dynamics is required. A trapped b rownian particle (relevant to our understanding of the kinetics, transport and mechanics of biological matter(9,10)) represents an ideal test system. Here we report a detailed experimental analysis of the brownian dynamics of a sub-micrometre sized dielectric particle confined in a double-well optic al trap. We show how these dynamics can be used to directly measure the ful l three-dimensional confining potential-a technique that can also be applie d to other optically trapped objects(11,12). Excellent agreement is obtaine d between the predictions of Kramers' theory and the measured transition ra tes, with no adjustable or free parameters over a substantial range of barr ier heights.