Ultrastrong tunable coupler between superconducting LC resonators

TL;DR

This paper presents an ultrastrong, tunable coupler for superconducting resonators with coupling exceeding 1 GHz, enabling advanced quantum circuit applications by breaking the rotating wave approximation.

Contribution

The work introduces a novel ultrastrong tunable coupler using rf-SQUID and dc-SQUID, achieving unprecedented coupling strength and wide tunability in superconducting resonator systems.

Findings

Coupling constant exceeds 1 GHz.

Achieves wide tunability from -1086 MHz to 604 MHz.

Experimental results align with the circuit model.

Abstract

We investigate the ultrastrong tunable coupler for coupling of superconducting resonators. Obtained coupling constant exceeds 1 GHz, and the wide range tunability is achieved both antiferromagnetics and ferromagnetics from $-1086$ MHz to 604 MHz. The ultrastrong coupler is composed of rf-SQUID and dc-SQUID as tunable junctions, which connected to resonators via shared aluminum thin film meander lines enabling such a huge coupling constant. The spectrum of the coupler obviously shows the breaking of the rotating wave approximation, and our circuit model treating the Josephson junction as a tunable inductance reproduces the experimental results well. The ultrastrong coupler is expected to be utilized in quantum annealing circuits and/or NISQ devices with dense connections between qubits.