  We consider phase-locked dynamics of a Josephson junction driven by
finite-spectral-linewidth ac current. By means of a transformation, the effect
of frequency fluctuations is reduced to an effective additive noise, the
corresponding (large) dephasing time being determined, in the logarithmic
approximation, by the Kramers' expression for the lifetime. For sufficiently
small values of the drive's amplitude, direct numerical simulations show
agreement of the dependence of the dephasing activation energy on the
ac-drive's spectral linewidth and amplitude with analytical predictions.
Solving the corresponding Fokker-Planck equation analytically, we find a
universal dependence of a critical value of the effective phase-diffusion
parameter on the drive's amplitude at a point of a sharp transition from the
phase-locked state to an unlocked one. For large values of the drive amplitude,
saturation and subsequent decrease of the activation energy are revealed by
simulations, which cannot be accounted for by the perturbative analysis. The
same new effect is found for a previously studied case of ac-driven Josephson
junctions with intrinsic thermal noise. The work was performed in the framework
of a cooperation agreement between Consiglio Nazionale di Ricerca (Italy) and
the Israeli Ministry of Science and Technology.
