Wideband Isolation by Frequency Conversion in a Josephson-Junction Transmission Line

When making a quantum measurement, one must be careful not to disturb the system. Nonreciprocal elements called isolators allow microwave signals to propagate in one direction but not the reverse, and so can mitigate disturbance, but commercial isolators require strong permanent magnets that cannot be placed close to superconducting amplifiers or qubits. To this end, the authors demonstrate a microwave isolator with directionality provided by a strong, traveling ``pump'' wave. Its simple construction and large bandwidth afford this isolator integrability with existing superconducting quantum hardware.