Nonreciprocal microwave transmission through a long Josephson junction

We have measured the bidirectional propagation of microwaves through a long Josephson junction in a flux-flow regime. We demonstrate that the transmitted microwave power depends on the direction of microwave propagation with respect to the direction of the flux flow. This nonreciprocal behavior is explained by the interaction of the microwave signal with the moving fluxon chain inside the junction. Thus a long junction may act as an on-chip isolator for external microwave signals, with its transmission properties being fully controlled by the bias current and in-plane magnetic field.