# Multi-mode ultra-strong coupling in circuit quantum electrodynamics

## TL;DR

This paper reports the first realization of a transmon qubit in the ultra-strong coupling regime with multiple modes, enabling exploration of extreme light-matter interactions in circuit quantum electrodynamics.

## Contribution

The authors experimentally demonstrate multi-mode ultra-strong coupling in a superconducting circuit, reaching a coupling ratio of 0.19 and hybridizing with up to five resonator modes.

## Key findings

- Achieved ultra-strong coupling ratio of 0.19
- Demonstrated multi-mode hybridization up to five modes
- Introduced a novel vacuum-gap capacitance architecture

## Abstract

With the introduction of superconducting circuits into the field of quantum optics, many novel experimental demonstrations of the quantum physics of an artificial atom coupled to a single-mode light field have been realized. Engineering such quantum systems offers the opportunity to explore extreme regimes of light-matter interaction that are inaccessible with natural systems. For instance the coupling strength $g$ can be increased until it is comparable with the atomic or mode frequency $\omega_{a,m}$ and the atom can be coupled to multiple modes which has always challenged our understanding of light-matter interaction. Here, we experimentally realize the first Transmon qubit in the ultra-strong coupling regime, reaching coupling ratios of $g/\omega_{m}=0.19$ and we measure multi-mode interactions through a hybridization of the qubit up to the fifth mode of the resonator. This is enabled by a qubit with 88% of its capacitance formed by a vacuum-gap capacitance with the center conductor of a coplanar waveguide resonator. In addition to potential applications in quantum information technologies due to its small size and localization of electric fields in vacuum, this new architecture offers the potential to further explore the novel regime of multi-mode ultra-strong coupling.