Two-level atom at finite temperature

TL;DR

This paper extends a QED-inspired method for analyzing a two-level atom to finite temperatures, using Matsubara formalism, revealing temperature effects on spectral properties and resonance sharpening without RWA.

Contribution

It generalizes a zero-temperature qubit description method to finite temperature using Matsubara formalism and analyzes spectral and polarizability properties.

Findings

Predicted sharpening of atomic resonance at finite temperature.

Established connection between temperature and real-time propagators.

Demonstrated effectiveness of the method in perturbation theory.

Abstract

Properties of a two-level atom coupled to the quantized electromagnetic field at finite temperature are determined. The analysis is based on a new method (inspired by QED) of describing qubits, developed previously at zero temperature (Phys. Rev. A 76, 062106 (2007)). In this paper, we make a generalization to finite temperature by introducing the Matsubara formalism and the temperature propagators. We analyze the spectral properties of different types of propagators and we derive a direct connection between the temperature propagators and the real time propagators. To show the effectiveness of this method, we calculate the temperature dependence of the polarizability of a two-level atom in the lowest order of perturbation theory and we predict an unexpected sharpening of the resonance. The whole discussion is carried out without the rotating wave approximation.