  Vacuum fluctuations are a source of irreversibility and decoherence. We
investigate the persistent current and its fluctuations in a ring with an
in-line quantum dot with an Aharonov-Bohm flux through the hole of the ring.
The Coulomb blockade leads to persistent current peaks at values of the gate
voltage at which two charge states of the dot have the same free energy. We
couple the structure to an external circuit and investigate the effect of the
zero-temperature (vacuum fluctuations) on the ground state of the ring. We find
that the ground state of the ring undergoes a crossover from a state with an
average persistent current much larger than the (time-dependent) mean squared
fluctuations to a state with a small average persistent current and large mean
squared fluctuations. We discuss the spectral density of charge fluctuations
and discuss diffusion rates for angle variables characterizing the ground state
in Bloch representation.
