  Many processes in chemistry, physics, and biology involve rare events in
which the system escapes from a metastable state by surmounting an activation
barrier. Examples range from chemical reactions, protein folding, and
nucleation events to the catastrophic failure of bridges. A challenge in
understanding the underlying mechanisms is that the most interesting
information is contained within the rare transition paths, the exceedingly
short periods when the barrier is crossed. To establish a model process that
enables access to all relevant timescales, although highly disparate, we probe
the dynamics of single dielectric particles in a bistable optical trap in
solution. Precise localization by high-speed tracking enables us to resolve the
transition paths and relate them to the detailed properties of the 3D potential
within which the particle diffuses. By varying the barrier height and shape,
the experiments provide a stringent benchmark of current theories of transition
path dynamics.
