Critical Fluorescence of a Transmon at the Schmid Transition

Area of Science:

• Quantum computing
• Solid-state physics

Background:

• Transmon qubits are key components in superconducting quantum computers.
• High-impedance circuits can induce unique quantum phenomena.

Purpose of the Study:

• Investigate inelastic microwave photon scattering by a transmon qubit.
• Analyze the impact of circuit impedance on qubit behavior and scattering.
• Characterize the charge-localization (Schmid) transition in the transmon.

Main Methods:

• Simulating and analyzing inelastic microwave photon scattering.
• Measuring the fluorescence spectrum of the transmon qubit.
• Examining the qubit's response to varying circuit impedance.

Main Results:

• The transmon qubit exhibits a charge-localization (Schmid) transition at critical impedance.
• The fluorescence spectrum reveals signatures of this transition point.
• Quasielastic photon scattering becomes dominant at higher impedance, with its dependence on qubit and circuit parameters identified.

Conclusions:

• The transmon qubit's level structure provides a unique signature for the Schmid transition.
• Understanding photon scattering in high-impedance circuits is crucial for quantum device performance.
• The study quantifies the role of quasielastic scattering in this regime.