Experimental evidence of symmetry breaking of transition-path times.

While thermal rates of state transitions in classical systems have been studied for almost a century, associated transition-path times have only recently received attention. Uphill and downhill transition paths between states at different free energies should be statistically indistinguishable. Here, we systematically investigate transition-path-time symmetry and report evidence of its breakdown on the molecular- and meso-scale out of equilibrium. In automated Brownian dynamics experiments, we establish first-passage-time symmetries of colloids driven by femtoNewton forces in holographically-created optical landscapes confined within microchannels. Conversely, we show that transitions which couple in a path-dependent manner to fluctuating forces exhibit asymmetry. We reproduce this asymmetry in folding transitions of DNA-hairpins driven out of equilibrium and suggest a topological mechanism of symmetry breakdown. Our results are relevant to measurements that capture a single coordinate in a multidimensional free energy landscape, as encountered in electrophysiology and single-molecule fluorescence experiments.(J.G.) European Training Network (ETN) Grant No. 674979-NANOTRANS (J.G.) Winton Programme for the Physics of Sustainability (M.R.C.) European Unions Horizon 2020 Marie Skłodowska-Curie grant agreement No. 749944 (U.F.K.) ERC Consolidator Grant (DesignerPores 647144) (F.R.) Spanish Research Council [FIS2016-80458-P] (F.R.) Catalan Government [Icrea Academia prize 2013] (F.R.) EU [Proseqo, FETOPEN, Proposal 687089