Quantum transport and the phase space structure of the Wightman functions

TL;DR

This paper investigates the phase space structure of quantum Wightman functions in time-varying systems, revealing additional coherence shells and non-local correlations, and introduces a quantum transport formalism applicable to interacting systems.

Contribution

It provides a derivation of the cQPA quantum transport equations that include local quantum coherence and compares their effectiveness with exact and semiclassical methods.

Findings

Coherence shells around zero frequency encode quantum coherence.

Semiclassical approximation performs well even with sharp temporal features.

The cQPA formalism effectively captures quantum transport phenomena.

Abstract

We study the phase space structure of exact quantum Wightman functions in spatially homogeneous, temporally varying systems. In addition to the usual mass shells, the Wightman functions display additional coherence shells around zero frequency $k_0=0$, which carry the information of the local quantum coherence of particle-antiparticle pairs. We find also other structures, which encode non-local correlations in time, and discuss their role and decoherence. We give a simple derivation of the cQPA formalism, a set of quantum transport equations, that can be used to study interacting systems including the local quantum coherence. We compute quantum currents created by a temporal change in a particle's mass, comparing the exact Wightman function approach, the cQPA and the semiclassical methods. We find that the semiclassical approximation, which is fully encompassed by the cQPA, works surprisingly well even for very sharp temporal features. This is encouraging for the application of semiclassical methods in electroweak baryogenesis with strong phase transitions.