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photon excitation behaves similarly. The fits of ω 01 and ω 02 versus the flux bias
φ can be used to extract the actual values of the qubit’s inductance, capacitance,
and critical current.
The 2D-Spectroscopy scan also shows avoided level crossings that correspond
to the qubit interacting with a two-level defect in the tunnel barrier which responds
at a certain frequency [Cooper et al., 2004, Neeley et al., 2008a]. Since we
believe that these defects are the primary source of energy relaxation in our qubit
[Simmonds et al., 2004, Martinis et al., 2005], one should choose an operating bias
that places the qubit far (in frequency) from one of these defects.
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