Ratchet effect in dc SQUIDs

We analyzed voltage rectification for de SQUIDs biased with ac current with zero mean value. We demonstrate that the reflection symmetry in the 2-dimensional SQUID potential is broken by an applied flux and with appropriate asymmetries in the de SQUID. Depending on the type of asymmetry, we obtain a rocking or a simultaneously rocking and flashing ratchet. the latter showing multiple sign reversals in the mean voltage with increasing amplitude of the ac current. Our experimental results are in agreement with numerical solutions of the Langevin equations for the asymmetric de SQUID.