A tunable coupler for superconducting microwave resonators using a nonlinear kinetic inductance transmission line

摘要

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 with a 50-Ω lumped-element nonlinear transmission line made from a 20 nm NbTiN film. By injecting a DC current, the nonlinear kinetic inductance changes and the effective impedance shunting the resonator periodically varies from a short to an open, which tunes the coupling strength and coupling quality factor Q_c. We have demonstrated Q_c tuning over a factor of 40, between Q_c ～ 5.5 × 10~4 and Q_c ～ 2.3 × 10~6, 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.