Ultrastrong coupling in two-resonator circuit QED

A Baust,J.-J García Ripoll,E Hoffmann,E Solano,M Haeberlein,F Wulschner,E P Menzel,L Zhong,G Romero,Frank Deppe ,Martin Schwarz ,Edwar Xie ,Fernando Quijandría ,Kirill G Fedorov ,Peter Eder ,Jan Goetz ,Laura García-Álvarez ,Achim Marx ,David Zueco ,Rudolf Gross

doi:10.1103/physrevb.93.214501

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.

Highlights

Circuit quantum electrodynamics (QED) [1] has become a versatile toolbox for quantum information processing [2,3] and quantum simulation [4,5,6] but is a powerful platform for the study of light-matter interaction [7,8] and fundamental aspects of quantum mechanics [9,10,11,12]
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
We demonstrate physics beyond the JaynesCummings approximation in a circuit QED architecture consisting of two coplanar stripline resonators and a superconducting flux qubit coupled galvanically to both of them

Summary

INTRODUCTION

Circuit quantum electrodynamics (QED) [1] has become a versatile toolbox for quantum information processing [2,3] and quantum simulation [4,5,6] but is a powerful platform for the study of light-matter interaction [7,8] and fundamental aspects of quantum mechanics [9,10,11,12]. Even the regime of ultrastrong coupling can be reached in superconducting circuits where the Jaynes-Cummings approximation breaks down [7]. In this situation, the interaction between light and matter can only be described correctly by the quantum Rabi model [13,14] which takes into account the counterrotating terms describing processes where the number of excitations is no longer conserved. Reaching the regime of ultrastrong coupling paves the way for various applications and the study of interesting phenomena. We find that ultrastrong coupling of a qubit to a distributed resonator structure can be reached solely by the geometrical configuration of the latter without making use of additional inductive elements realized for example by Josephson junctions

SAMPLE CONFIGURATION AND MEASUREMENT SETUP

MODE STRUCTURE

ULTRASTRONG COUPLING

BREAKDOWN OF THE JAYNES-CUMMINGS APPROXIMATION

Findings

CONCLUSIONS