Ultrastrong coupling in two-resonator circuit QED

TL;DR

This paper demonstrates ultrastrong coupling between a superconducting flux qubit and a two-resonator system, revealing a significant Bloch-Siegert shift and breakdown of the Jaynes-Cummings model at high coupling strengths.

Contribution

It reports the first realization of ultrastrong coupling in a two-resonator circuit QED system without using Josephson junction inductance, achieving a coupling strength of 17%.

Findings

Observed a 17% coupling strength of mode frequency.

Detected a pronounced Bloch-Siegert shift.

Revealed breakdown of the Jaynes-Cummings model.

Abstract

We report on ultrastrong coupling between a superconducting flux qubit and a resonant mode of a system comprised of two superconducting coplanar stripline resonators coupled galvanically to the qubit. With a coupling strength as high as 17% of the mode frequency, exceeding that of previous circuit quantum electrodynamics experiments, we observe a pronounced Bloch-Siegert shift. The spectroscopic response of our multimode system reveals a clear breakdown of the Jaynes-Cummings model. In contrast to earlier experiments, the high coupling strength is achieved without making use of an additional inductance provided by a Josephson junction.