Switchable ultrastrong coupling in circuit QED

TL;DR

This paper proposes a design for a switchable, ultrastrong coupling mechanism in circuit QED systems, enabling dynamic control over qubit-resonator interactions in the ultrastrong regime, which is crucial for advanced quantum microwave applications.

Contribution

It introduces a novel circuit design with tunable coupling in the ultrastrong regime, advancing the control of qubit-resonator interactions in superconducting quantum circuits.

Findings

Designs for circuits with switchable ultrastrong coupling

Modulation of coupling strength and type demonstrated

Potential for immediate experimental implementation

Abstract

Superconducting quantum circuits possess the ingredients for quantum information processing and for developing on-chip microwave quantum optics. From the initial manipulation of few-level superconducting systems (qubits) to their strong coupling to microwave resonators, the time has come to consider the generation and characterization of propagating quantum microwaves. In this paper, we design a key ingredient that will prove essential in the general frame: a swtichable coupling between qubit(s) and transmission line(s) that can work in the ultrastrong coupling regime, where the coupling strength approaches the qubit transition frequency. We propose several setups where two or more loops of Josephson junctions are directly connected to a closed (cavity) or open transmission line. We demonstrate that the circuit induces a coupling that can be modulated in strength and type. Given recent studies showing the accessibility to the ultrastrong regime, we expect our ideas to have an immediate impact in ongoing experiments.