A two-level system traversing a level anticrossing has a small probability to make a so-called Landau-Zener (LZ) transition between its energy bands, in deviation from simple adiabatic evolution. This effect takes on renewed relevance due to the observation of quantum coherence in superconducting qubits (macroscopic Schrodinger cat devices). We report an observation of LZ transitions in an Al three-junction qubit coupled to a Nb resonant tank circuit.
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