# Approaching ultra-strong coupling in Transmon circuit-QED using a high-impedance resonator

## TL;DR

This paper demonstrates a superconducting Transmon qubit coupled to a high-impedance resonator, achieving near ultra-strong coupling, which opens pathways for exploring new quantum regimes and enhancing quantum information processing.

## Contribution

The study introduces a high-impedance resonator coupled to a Transmon qubit, reaching a coupling ratio close to the ultra-strong regime, and suggests potential for even stronger coupling with advanced architectures.

## Key findings

- Achieved a large vacuum Rabi splitting of 910 MHz.
- Reached a coupling ratio of 0.071, near ultra-strong coupling.
- Potential to reach a coupling ratio of 0.45 with improved design.

## Abstract

In this experiment, we couple a superconducting Transmon qubit to a high-impedance $645\ \Omega$ microwave resonator. Doing so leads to a large qubit-resonator coupling rate $g$, measured through a large vacuum Rabi splitting of $2g\simeq 910$ MHz. The coupling is a significant fraction of the qubit and resonator oscillation frequencies $\omega$, placing our system close to the ultra-strong coupling regime ($\bar{g}=g/\omega=0.071$ on resonance). Combining this setup with a vacuum-gap Transmon architecture shows the potential of reaching deep into the ultra-strong coupling $\bar{g} \sim 0.45$ with Transmon qubits.