  While Kramers' rates have been studied for almost a century, the transition
path time between states has only recently received attention. Transition paths
between different energy levels are expected to be indistinguishable in shape
and have equal uphill and downhill lengths. This fundamental symmetry often
prevents directional sensing in experiments. Here, we report experimental
evidence for transition path time symmetry and its breakdown on the
single-molecule and mesoscale. In automated Brownian dynamics experiments, we
establish first-passage time symmetries of colloids driven by femtoNewton-range
forces in holographically-created optical landscapes confined in 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 for the symmetry breakdown. Our results can reveal directionality in
molecular transitions or translocations through membrane channels and
nanopores.
