  We discuss activated escape from a metastable state of a system driven by a
time-periodic force. We show that the escape probabilities can be changed very
strongly even by a comparatively weak force. In a broad parameter range, the
activation energy of escape depends linearly on the force amplitude. This
dependence is described by the logarithmic susceptibility, which is analyzed
theoretically and through analog and digital simulations. A closed-form
explicit expression for the escape rate of an overdamped Brownian particle is
presented and shown to be in quantitative agreement with the simulations. We
also describe experiments on a Brownian particle optically trapped in a
double-well potential. A suitable periodic modulation of the optical intensity
breaks the spatio-temporal symmetry of an otherwise spatially symmetric system.
This has allowed us to localize a particle in one of the symmetric wells.
