Measurement-Induced State Transitions in a Superconducting Qubit: Beyond the Rotating Wave Approximation.

Area of Science:

• Quantum computing
• Superconducting circuits
• Quantum measurement

Background:

• Superconducting qubits commonly use dispersive readout via coupled harmonic resonators.
• High photon counts during measurement can induce unintended qubit state transitions.

Purpose of the Study:

• Investigate qubit transitions induced by high photon numbers in resonators.
• Develop a theoretical model for these transitions.
• Identify the underlying physical mechanisms driving these transitions.

Main Methods:

• Theoretical modeling based on Jaynes-Cummings ladder and Hamiltonian terms beyond the rotating wave approximation.
• Experimental measurements of resonator photon occupation during qubit transitions.
• Systematic variation of qubit-resonator detuning.

Main Results:

• Qubit transitions can occur when photon numbers are high, pushing qubits out of the two-level subspace.
• These transitions exhibit resonant behavior as a function of photon number.
• An unexpected broken symmetry in the qubit potential significantly contributes to the transitions.

Conclusions:

• The observed qubit transitions are explained by level crossings in the Jaynes-Cummings ladder.
• Non-rotating wave approximation terms, particularly those arising from broken symmetry, are crucial for understanding these transitions.
• Experimental results confirm the theoretical predictions, validating the model.