New quantum information perspectives in the axion--photon and neutrino systems

Abstract

In this work, we broach a quantum information-theoretic treatment of axion--photon mixing. Motivated by an emerging class of quantum-enhanced axion searches, we analyse the two-level single-excitation sector of axion--photon oscillations, demonstrating how the coupled dynamics naturally generate bipartite axion--photon mode entanglement. We study in detail the ensuing aspects of entanglement entropy, concurrence, negativity, quantum mutual information and discord, and capacity of entanglement, and the corresponding neutrino analogues wherever novel and previously unaddressed. In particular, we highlight the characteristic features that connect maximal axion--photon entanglement to resonant or strong-mixing conversion, and the distinct thresholds for the extremal values attained by the quantum information measures. We study aspects of the Mandelstam--Tamm and Margolus--Levitin quantum speed limits for both the axion--photon and neutrino systems. While orthogonalisation occurs only at axion--photon resonance, or at maximal neutrino mixing, where the two bounds coincide, away from these limits, the Margolus--Levitin bound is saturated at maximal conversion, while the Mandelstam--Tamm bound is generally weaker. We also study an entanglement quantum speed limit for axion--photon conversion, that separates into detuning-dominated and magnetic-mixing-dominated regimes, and find that it is saturated for a period and then the bound becomes weak. The results in this work identify the quantum resources and limiting timescales intrinsic to axion--photon conversion, and connect axion phenomenology, neutrino oscillations and quantum information theory.