A superconducting qubit was driven in an ultrastrong fashion by an oscillatory microwave field, which was created by coupling via the nonlinear Josephson energy. The observed Stark shifts of the `atomic' levels are so pronounced that corrections even beyond the lowest-order Bloch-Siegert shift are needed to properly explain the measurements. The quasienergies of the dressed two-level system were probed by resonant absorption via a cavity, and the results are in agreement with a calculation based on the Floquet approach.
@article{arxiv.1005.3446,
title = {Stark effect and generalized Bloch-Siegert shift in a strongly driven two-level system},
author = {Jani Tuorila and Matti Silveri and Mika Sillanpaa and Erkki Thuneberg and Yuriy Makhlin and Pertti Hakonen},
journal= {arXiv preprint arXiv:1005.3446},
year = {2015}
}