We observed the dynamics of a superconducting flux qubit coupled to an extrinsic quantum system (EQS). The presence of the EQS is revealed by an anticrossing in the spectroscopy of the qubit. The excitation of a two-photon transition to the third excited state of the qubit-EQS system allows us to extract detailed information about the energy level structure and the coupling of the EQS. We deduce that the EQS is a two-level system, with a transverse coupling to the qubit. The transition frequency and the coupling of the EQS changed during experiments, which supports the idea that the EQS is a two-level system of microscopic origin.
@article{arxiv.0810.0590,
title = {One- and two-photon spectroscopy of a flux qubit coupled to a microscopic defect},
author = {A. Lupascu and P. Bertet and E. F. C. Driessen and C. J. P. M. Harmans and J. E. Mooij},
journal= {arXiv preprint arXiv:0810.0590},
year = {2009}
}