Ultrastrong coupling dynamics with a transmon qubit

TL;DR

This paper proposes a method to achieve and study ultrastrong light-matter coupling using a Josephson junction array to enhance the impedance and enable complex, non-perturbative dynamics with a transmon qubit.

Contribution

It introduces a novel approach employing a Josephson junction array to realize ultrastrong coupling with a transmon, leveraging multi-mode structures and tunability.

Findings

Demonstrates how array impedance enhances coupling strength

Shows the feasibility of probing complex dynamics via multi-mode structures

Utilizes low-frequency modes for non-adiabatic ground state changes

Abstract

The interaction of light and matter is often described by the exchange of single excitations. When the coupling strength is a significant fraction of the system frequencies, the number of excitations are no longer preserved and that simple picture breaks down. This regime is known as the ultrastrong coupling regime and is characterized by non-trivial light-matter eigenstates and complex dynamics. In this work, we propose to use a an array Josephson junctions to increase the impedance of the light mode enabling ultrastrong coupling to a transmon qubit. We show that the resulting dynamics can be generated and probed by taking advantage of the multi-mode structure of the junction array. This proposal relies on the frequency tunability of the transmon and, crucially, on the use of a low frequency mode of the array, which allows for non-adiabatic changes of the ground state.