Impact of quantum entanglement induced by magnetic fields on primordial gravitational waves

TL;DR

This paper explores how magnetic fields generated during inflation can induce quantum entanglement between primordial gravitational waves and electromagnetic fields, affecting their quantum states and offering insights into early universe physics.

Contribution

It introduces a model where magnetic fields cause entanglement between gravitons and photons during inflation, analyzing the resulting mixed quantum states of primordial gravitational waves.

Findings

Primordial magnetic fields induce entanglement between gravitons and photons.

Gravitons are found to be in a mixed, not squeezed, quantum state.

Entanglement effects are currently unobservable but pose experimental challenges.

Abstract

There exist observational evidence to believe the existence of primordial magnetic fields generated during inflation. We study primordial gravitational waves (PGWs) during inflation in the presence of magnetic fields sustained by a gauge kinetic coupling. In the model, not only gravitons as excitations of PGWs, but also photons as excitations of electromagnetic fields are highly squeezed. They become entangled with each other through graviton to photon conversion and vice versa. We derive the reduced density matrix for the gravitons and calculate their entanglement entropy. It turns out that the state of the gravitons is not a squeezed state but a mixed state. Although witnessing such an entanglement is not feasible at present, it would be an important experimental challenge.