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Measurement-induced state transitions in a superconducting qubit: Beyond the rotating wave approximation

Alexander Korotkov
Amit Vainsencher
Andrew Dunsworth
Anthony Megrant
Ben Chiaro
Brooks Campbell
Charles Neill
Jim Wenner
John Martinis
Josh Mutus
Mostafa Khezri
Rami Barends
Yu Chen
Zijun Chen
Physical Review Letters (2016)
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Many superconducting qubit systems use the dispersive interaction between the qubit and a coupled harmonic resonator to perform quantum state measurement. Previous works have found that such measurements can induce state transitions in the qubit if the number of photons in the resonator is too high. We investigate these transitions and find that they can push the qubit out of the two-level subspace. Furthermore, these transitions show resonant behavior as a function of photon number. We develop a theory for these observations based on level crossings within the Jaynes-Cummings ladder, with transitions mediated by terms in the Hamiltonian which are typically ignored by the rotating wave approximation. We confirm the theory by measuring the photon occupation of the resonator when transitions occur while varying the detuning between the qubit and resonator.

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