Design of a current-biased tunable coupler integrated with superconducting 3D-cavities

Superconducting circuits are a leading platform for scalable quantum information processing. These macroscopic circuits can be easily controlled using magnetic flux generated by delivering current to an off-chip coil or an integrated on-chip coil. However, flux sensitive devices can suffer from flux offsets and environmental magnetic field fluctuations, and cannot be operated inside of superconducting enclosures. Furthermore, scaling up flux-controlled devices can be difficult, as large amounts of current may be required to deliver sufficient flux to each device. One solution for improved operation and scalability is to use current biasing.

In this talk, we present the design of a current-controlled superconducting coupling element based on an inductive Wheatstone bridge. The inductors on the arms of the bridge are arrays of gradiometric rf SQUIDs, which are insensitive to uniform flux changes in the environment to first order. The bridge is controlled by a uniform trapped current in the outer loop and imbalanced by a gradiometric current. Numerical simulations of the bridge coupled to two superconducting 3D cavities predict that tunable coupling on the order of megahertz, with high on/off contrast, is achievable between the cavities.