A Tunable Coupler for Superconducting Microwave Resonators Using A Nonlinear Kinetic Inductance Transmission Line
Jiansong Gao, Michael R. Vissers, Johannes Hubmayr, Bockstiegel Clint, Saptarshi Chaudhuri
We present a tunable coupler scheme that allows us to tune the coupling strength between a feedline and a superconducting resonator in situ over a wide range. In this scheme, we shunt the feedline by a 50-ohm lumped-element nonlinear transmission line made from a 20 nm NbTiN film. By injecting a DC current, the nonlinear kinetic inductance is changed and the effective impedance shunting the resonator periodically varies from a short to an open, which tunes the coupling strength and coupling quality factor Qc. We have demonstrated Qc tuning over a factor of 40, between Qc~5.5e4 and Qc~2.3e6, for a 4.5 GHz resonator by applying a DC current less than 3.3 mA. Our tunable coupler scheme is easy to implement and may find broad applications in superconducting detector and quantum computing/information experiments.