  Understanding the thermally activated escape from a metastable state is at
the heart of important phenomena such as the folding dynamics of proteins, the
kinetics of chemical reactions or the stability of mechanical systems. In 1940
Kramers calculated escape rates both in the high damping and the low damping
regime and suggested that the rate must have a maximum for intermediate
damping. This phenomenon, today known as the Kramers turnover, has triggered
important theoretical and numerical studies. However, to date there is no
direct and quantitative experimental verification of this turnover. Using a
nanoparticle trapped in a bi-stable optical potential we experimentally measure
the nanoparticle's transition rates for variable damping and directly resolve
the Kramers turnover. Our measurements are in agreement with an analytical
model that is free of adjustable parameters.
