  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.
