Nonreciprocity and Circulation in a Passive Josephson-Junction Ring.

Building large-scale superconducting quantum circuits will require miniaturization and integration of supporting devices including microwave circulators, which are currently bulky, stand-alone components. Here, we report the measurement of microwave scattering from a ring of Josephson junctions, with dc-only control fields. We detect the effect of quasiparticle tunneling, and dynamically classify the system at its operating design point into different quasiparticle sectors. We optimize the device within one of the quasiparticle sectors, where we observe an unambiguous signature of nonreciprocal 3-port scattering within that sector. This enables operation as a circulator, and at the optimal circulation point, we observe on-resonance insertion loss of 2 dB, isolation of 14 dB, power reflectance of -11  dB, and a bandwidth of 200 MHz, averaged over the 3 input ports.