Moyal Quantum Mechanics: The Semiclassical Heisenberg Dynamics

TL;DR

This paper explores the phase space formulation of quantum mechanics using Moyal--Weyl representation, presenting methods to derive semiclassical expansions of quantum dynamics that avoid complex singularities and multiple trajectories.

Contribution

It introduces two novel methods for constructing semiclassical expansions of quantum evolution in phase space, extending Groenewold's formula and simplifying the semiclassical analysis.

Findings

The expansion coefficients are globally defined differential operators.

The methods avoid singularities, Hamilton-Jacobi equations, and multiple trajectories.

The approach provides a regular asymptotic series for quantum evolution.

Abstract

The Moyal--Weyl description of quantum mechanics provides a comprehensive phase space representation of dynamics. The Weyl symbol image of the Heisenberg picture evolution operator is regular in $\hbar$. Its semiclassical expansion `coefficients,' acting on symbols that represent observables, are simple, globally defined differential operators constructed in terms of the classical flow. Two methods of constructing this expansion are discussed. The first introduces a cluster-graph expansion for the symbol of an exponentiated operator, which extends Groenewold's formula for the Weyl product of symbols. This Poisson bracket based cluster expansion determines the Jacobi equations for the semiclassical expansion of `quantum trajectories.' Their Green function solutions construct the regular $\hbar\downarrow0$ asymptotic series for the Heisenberg--Weyl evolution map. The second method directly substitutes such a series into the Moyal equation of motion and determines the $\hbar$ coefficients recursively. The Heisenberg--Weyl description of evolution involves no essential singularity in $\hbar$, no Hamilton--Jacobi equation to solve for the action, and no multiple trajectories, caustics or Maslov indices.