One- and two-photon spectroscopy of a flux qubit coupled to a microscopic defect

Abstract

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.

accepted in Physical Review B

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• Phase qubits fabricated with trilayer junctions
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• Ideal Quantum non-demolishing measurement of a flux qubit at variable bias
• Flux-qubit and the law of angular momentum conservation
• Properties of two level systems in current-carrying superconductors