Phase space geometry of general quantum energy transitions

TL;DR

This paper explores the phase space geometry of quantum energy transitions, revealing how classical trajectories and compound orbits influence quantum transition probabilities in a semiclassical framework.

Contribution

It introduces a semiclassical description of quantum energy transitions using closed compound orbits formed by classical trajectories influenced by static and driving Hamiltonians.

Findings

Transition amplitudes are linked to closed compound orbits in phase space.

Families of these orbits originate where classical flows commute.

The approach unifies quantum and classical descriptions of energy transitions.

Abstract

The mixed density operator for coarsegrained eigenlevels of a static Hamiltonian is represented in phase space by the spectral Wigner function, which has its peak on the corresponding classical energy shell. The action of trajectory segments along the shell determine the phase of the Wigner oscillations in its interior. The classical transitions between any pair of energy shells, driven by a general external time dependent Hamiltonian, also have a smooth probability density. It is shown here that a further contribution to the transition between the corresponding pair of coarsegrained energy levels, which oscillates with either energy, or the driving time, is determined by four trajectory segments (two in the pair of energy shells and two generated by the driving Hamiltonian) that join exactly to form a closed compound orbit. In its turn, this sequence of segments belongs to the semiclassical expression of a compound unitary operator that combines four quantum evolutions: a pair generated by the static internal Hamiltonian and a pair generated by the driving Hamiltonian. The closed compound orbits are shown to belong to continuous families, which are initially seeded at points where the classical flow generated by both Hamiltonians commute.