Design and initial tests of a 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 emit large stray magnetic fields.~ Here we report progress toward the development of a lossless, on-chip, active circulator for superconducting microwave circuits in the 4-8 GHz band.~ Non-reciprocity is achieved by actively modulating circuit elements on a slow time scale (10 -- 100 ns). Our circulator's active components are dynamically tunable inductors constructed with arrays of dc-SQUIDs in series. The array inductance is tuned by varying the magnetic flux through the SQUIDs with fields weaker than 1 Oe. Initial tests show that the device exhibits non-reciprocity, but performance is degraded by trapped magnetic flux in the circuit. Nevertheless, the device meets many design goals including a tunable center frequency between 4-8 GHz and a high (-93 dBm) saturation power.~ ~This presentation will describe these tests and a new layout designed to avoid trapped flux.