Noise, coherence, and reversibility in Josephson arrays

The noise sensitivity of Josephson-junction arrays is greatly enhanced in the presence of a so-called ``reversibility'' symmetry. We use a simple dynamical picture corresponding to a diffusion process in phase space to calculate the power spectrum of the total array voltage. Breaking the reversibility symmetry leads to a crossover from incoherent to coherent oscillations, reflected by improved scaling of the voltage with respect to array size. In the limit where the in-phase orbit is strongly attracting, the results agree with previous theoretical and experimental work.