One- and two-photon spectroscopy of a flux qubit coupled to a microscopic defect
Adrian Lupaşcu, Patrice Bertet, E. F. C. Driessen, C. J. P. M. Harmans, Johan E. Mooij · Physical Review B · 2009
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.