We study transport in an asymmetric SQUID which is composed of a loop withthree capacitively and resistively shunted Josephson junctions: two in seriesin one arm and the remaining one in the other arm. The loop is threaded by anexternal magnetic flux and the system is subjected to both a time-periodic anda constant current. We formulate the deterministic and, as well, the stochasticdynamics of the SQUID in terms of the Stewart-McCumber model and derive anequation for the phase difference across one arm, in which an effectiveperiodic potential is of the ratchet type, i.e. its reflection symmetry isbroken. In doing so, we extend and generalize earlier study by Zapata et al.[Phys. Rev. Lett. 77, 2292 (1996)] and analyze directed transport in wideparameter regimes: covering the over-damped to moderate damping regime up toits fully under-damped regime. As a result we detect the intriguing features ofa negative (differential) conductance, repeated voltage reversals, noiseinduced voltage reversals and solely thermal noise-induced ratchet currents. Weidentify a set of parameters for which the ratchet effect is most pronouncedand show how the direction of transport can be controlled by tailoring theexternal magnetic flux.
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