Probing dynamical axion quasiparticles with two-photon correlations

TL;DR

This paper proposes a method to detect dynamical axion quasiparticles in topological materials by analyzing the hyperentangled two-photon states resulting from axion decay, revealing unique polarization and momentum correlations.

Contribution

It extends the Weisskopf-Wigner theory to quantum field theory of axion decay, linking two-photon correlations to axion properties and proposing experimental detection techniques.

Findings

Two-photon states exhibit hyperentanglement in momentum and polarization.

Momentum correlations of polarization asymmetry can be measured via coincident detection.

Two-photon correlations serve as a direct probe of dynamical axion quasiparticles.

Abstract

Dynamical axion (quasi) particles are emergent collective excitations in topological magnetic insulators that break parity and time reversal invariance or in Weyl semimetals. They couple to electromagnetism via a topological Chern-Simons term, leading to their decay into two photons. We extend the Weisskopf-Wigner formulation of atomic spontaneous emission to the quantum field theory of dynamical axion quasiparticles, allowing us to obtain the quantum two-photon state emerging from axion decay in real time. This state features \emph{hyperentanglement} in momentum and polarization with a distinct polarization pattern, a consequence of the parity and time reversal breaking of the axion-photon interaction. Polarization aspects of this two-photon state are studied by introducing quantum Stokes operators. Whereas the two-photon quantum state features vanishing \emph{averages} of the degree of polarization and polarization asymmetry, there are non-trivial momentum correlations of the Stokes operators. In particular momentum correlations of the \emph{polarization asymmetry} can be obtained directly from coincident momentum and polarization resolved two photon detection. Correlations of Stokes operators are directly related to momentum and polarization resolved Hanbury-Brown Twiss second order coherences. This relationship suggests two-photon correlations as a direct probe of dynamical axion quasiparticles. Similarities and differences with parametrically down converted photons and other systems where spontaneous emission yield hyperentangled two photon states are recognized, suggesting experimental avenues similar to tests of Bell inequalities to probe dynamical axion quasiparticles with coincident two photon detection.