Photon-Dressed Bloch-Siegert Shift in an Ultrastrongly Coupled Circuit Quantum Electrodynamical System

TL;DR

This paper reports the direct measurement of photon-dressed qubit transition frequencies up to four photons in an ultrastrongly coupled circuit-QED system, demonstrating the photon-dressed Bloch-Siegert shift and its dependence on photon number.

Contribution

It provides the first direct observation of the photon-dressed Bloch-Siegert shift in an ultrastrongly coupled circuit-QED system, highlighting the quantum Rabi model's photon number dependence.

Findings

Photon-dressed Bloch-Siegert shift observed up to four photons

Shift increases with photon number, confirming quantum Rabi model predictions

Demonstrates ultrastrong coupling effects in circuit-QED systems

Abstract

A cavity quantum electrodynamical (QED) system beyond the strong-coupling regime is expected to exhibit intriguing quantum phenomena. Here we report a direct measurement of the photon-dressed qubit transition frequencies up to four photons by harnessing the same type of state transitions in an ultrastrongly coupled circuit-QED system realized by inductively coupling a superconducting flux qubit to a coplanar-waveguide resonator. This demonstrates a convincing observation of the photon-dressed Bloch-Siegert shift in the ultrastrongly coupled quantum system. Moreover, our results show that the photon-dressed Bloch-Siegert shift becomes more pronounced as the photon number increases, which is a characteristic of the quantum Rabi model.