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

TL;DR

This paper investigates how measurement-induced state transitions in superconducting qubits can occur beyond the rotating wave approximation, revealing resonant behaviors and the role of symmetry-breaking terms in the Hamiltonian.

Contribution

It develops a theoretical model explaining measurement-induced transitions beyond RWA, highlighting the impact of symmetry-breaking terms in the qubit potential.

Findings

Transitions can push the qubit out of the two-level subspace

Resonant behavior as a function of photon number

Confirmation of theory through photon occupation measurements

Abstract

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, and that they 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 that are typically ignored by the rotating wave approximation. We find that the most important of these terms comes from an unexpected broken symmetry in the qubit potential. We confirm the theory by measuring the photon occupation of the resonator when transitions occur while varying the detuning between the qubit and resonator.