Performance of an on-chip superconducting circulator for quantum microwave systems

Microwave circulators enforce a single propagation direction for signals in an electrical network.~ Unfortunately, commercial circulators are bulky, lossy, and cannot be integrated close to superconducting circuits because they require strong (\textasciitilde kOe) magnetic fields produced by permanent magnets.~ Here we report on the performance of an on-chip, active circulator for superconducting microwave circuits, which uses no permanent magnets.~ Non-reciprocity is achieved by actively modulating reactive elements around 100 MHz, giving roughly a factor of 50 in the separation between signal and control frequencies, which facilitates filtering. The circulator's active components are dynamically tunable inductors constructed with arrays of dc-SQUIDs in series. Array inductance is tuned by varying the magnetic flux through the SQUIDs with fields weaker than 1 Oe. Although the instantaneous bandwidth of the device is narrow, the operation frequency is tunable between 4 and 8 GHz. This presentation will describe the device's theory of operation and compare its measured performance to design goals.