  Thermally activated transitions are ubiquitous in nature, occurring in
complex environments which are typically conceived as ideal viscous fluids. We
report the first direct observations of a Brownian bead transiting between the
wells of a bistable optical potential in a viscoelastic fluid with a single
long relaxation time. We precisely characterize both the potential and the
fluid, thus enabling a neat comparison between our experimental results and a
theoretical model based on the generalized Langevin equation. Our findings
reveal a drastic amplification of the transition rates compared to those in a
Newtonian fluid, stemming from the relaxation of the fluid during the particle
crossing events.
