Tunable coupling of transmission-line microwave resonators mediated by an rf SQUID

TL;DR

This paper demonstrates a method to achieve tunable coupling between superconducting microwave resonators using an rf SQUID, enabling precise control over interaction strength and enabling applications like parametric amplification.

Contribution

The study introduces a flux-tunable mutual inductance mediated by an rf SQUID for controlling coupling between transmission-line resonators, with experimental validation of variable coupling strength.

Findings

Coupling strength tunable from -320 MHz to 37 MHz.

Near-zero coupling reduces microwave cross transmission by nearly four orders of magnitude.

Observation of parametric amplification with an additional drive tone.

Abstract

We realize tunable coupling between two superconducting transmission line resonators. The coupling is mediated by a non-hysteretic rf SQUID acting as a flux-tunable mutual inductance between the resonators. From the mode distance observed in spectroscopy experiments, we derive a coupling strength ranging between -320MHz and 37 MHz. In the case where the coupling strength is about zero, the microwave power cross transmission between the two resonators can be reduced by almost four orders of magnitude compared to the case where the coupling is switched on. In addition, we observe parametric amplification by applying a suitable additional drive tone.