Passive On-chip Microwave Circulator based on a Josephson Ring

Scaling up superconducting quantum computers requires downsized and integrated readout circuits of qubits. In particular, conventional nonreciprocal devices such as ferrite-based circulators and isolators are bulky and difficult to incorporate into chips. As alternatives to ferrite circulators, on-chip circulators realized through a time-dependent microwave drive have been demonstrated. However, it is more desirable to get rid of the drive fields for better scalability. Here, we present a passive on-chip microwave circulator based on a Josephson ring circuit. The circuit design has been improved from the original theoretical proposal for experimental feasibility. Particularly, the reduction of the number of relevant modes enables impedance matching with experimentally feasible parameters. We observe microwave nonreciprocity of the Josephson ring device in transmission spectra. We further investigate the microwave response as a function of the gate charge and external flux to the circuitry, showing a good agreement with the theory. We discuss the limiting factors of the circulator's performance focusing mainly on the effect of charge parity switching due to quasi-particle tunneling.