Wigner function properties for electromagnetic systems

TL;DR

This paper derives and compares different Wigner functions for electromagnetic quantum systems, revealing their properties and differences, especially regarding positivity and their relation to the Hudson theorem, within the Wigner-Vlasov formalism.

Contribution

It constructs exact 3D solutions for Wigner functions in electromagnetic fields and analyzes their properties, including positivity and relation to the Hudson theorem, using the Wigner-Vlasov formalism.

Findings

The second Wigner function can be positive over the entire phase space for non-Gaussian states.

The first Wigner function exhibits negative regions for Gaussian wave functions.

Comparison of Wigner function-based distributions with exact quantum distributions shows notable differences.

Abstract

Using the Wigner-Vlasov formalism, an exact 3D solution of the Schr\"odinger equation for a scalar particle in an electromagnetic field is constructed. Electric and magnetic fields are non-uniform. According to the exact expression for the wave function, the search for two types of the Wigner functions is conducted. The first function is the usual Wigner function with a modified momentum. The second Wigner function is constructed on the basis of the Weyl-Stratonovich transform in papers [Phys. Rev. A 35 2791 (1987)] or [Phys. Rev. B 99 014423 (2019)]. It turns out that the second function, unlike the first one, has areas of negative values for wave functions with the Gaussian distribution (Hudson's theorem). An example of electromagnetic quantum system described by a non-Gaussian wave function has successfully been found. The second Wigner function is positive over the whole phase space for the non-Gaussian wave function. This result is analogous to the Hudson theorem for the gage-invariant Wigner function. On the one hand, knowing the Wigner functions allows one to find the distribution of the mean momentum vector field and the energy spectrum of the quantum system. On the other hand, within the framework of the Wigner-Vlasov formalism, the mean momentum distribution and the magnitude of the energy are initially known. Consequently, the mean momentum distributions and energy values obtained according to the Wigner functions can be compared with the exact momentum distribution and energy values. This paper presents this comparison and describes the differences. The Vlasov-Moyal approximation of average acceleration flow has been built in phase space for a quantum system with electromagnetic field. The obtained approximation makes it possible to cut the Vlasov chain off at the second equation and also to analyze the Boltzmann H-function evolution.