Fig. 1

Magnetic-based isolator versus the MPIJIS. A microwave isolator, whose circuit symbol is shown in a, is a unidirectional, two-port microwave device. It transmits signals from port 1 to 2 (as indicated by the arrow) and blocks signals propagating in the opposite direction. To break the transmission-coefficient symmetry between source (input) and detector (output), commercial isolators rely on magneto-optical effects, which require magnetic materials and strong magnets. One common realization of such isolators is a three-port circulator in which one of its ports (i.e., 3) is terminated by a 50 Ω load as shown in b. In this example, signals entering port 1 are directed towards port 2, whereas signals entering port 2 are dissipated in the matched load attached to the internal port 3. c A circuit symbol for the Josephson-based isolator realized and measured in this work, which is based on a different reciprocity-breaking mechanism. In this device, schematically shown in d, two active Josephson mixers are coupled via beam-splitters and driven by a microwave pump source. Unidirectional transmission from port 1 to 2 is generated by constructive and destructive wave interference between different paths in the device. The interference pattern is controlled by the phase difference φ = −π/2 between the pumps feeding the two mixing stages. Signals propagating from port 2 to 1 are dissipated in the matched loads connected to the internal ports 3 and 4