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.
@article{arxiv.2007.00255,
title = {Photon-Dressed Bloch-Siegert Shift in an Ultrastrongly Coupled Circuit Quantum Electrodynamical System},
author = {Shuai-Peng Wang and Guo-Qiang Zhang and Yimin Wang and Zhen Chen and Tiefu Li and J. S. Tsai and Shi-Yao Zhu and J. Q. You},
journal= {arXiv preprint arXiv:2007.00255},
year = {2020}
}