# High-Energy Vacuum Birefringence and Dichroism in an Ultrastrong Laser Field

## TL;DR

This paper explores the theoretical prediction and experimental feasibility of observing vacuum birefringence and dichroism caused by quantum electrodynamics effects in an ultrastrong laser field, using high-energy probe photons.

## Contribution

It derives the influence of a polarized probe beam on vacuum birefringence and dichroism in a strong laser field and proposes an experimental scheme with circularly polarized photons for high-confidence detection.

## Key findings

- Feasibility of measuring vacuum birefringence at 5σ confidence within days using 10 PW lasers.
- Demonstrates that dichroism and anomalous dispersion effects are detectable in vacuum at these facilities.
- Proposes using circularly polarized high-energy photons to improve measurement sensitivity.

## Abstract

A long-standing prediction of quantum electrodynamics, yet to be experimentally observed, is the interaction between real photons in vacuum. As a consequence of this interaction, the vacuum is expected to become birefringent and dichroic if a strong laser field polarizes its virtual particle--antiparticle dipoles. Here, we derive how a generally polarized probe photon beam is influenced by both vacuum birefringence and dichroism in a strong linearly polarized plane-wave laser field. Furthermore, we consider an experimental scheme to measure these effects in the nonperturbative high-energy regime, where the Euler-Heisenberg approximation breaks down. By employing circularly polarized high-energy probe photons, as opposed to the conventionally considered linearly polarized ones, the feasibility of quantitatively confirming the prediction of nonlinear QED for vacuum birefringence at the $5\sigma$ confidence level on the time scale of a few days is demonstrated for upcoming 10 PW laser systems. Finally, dichroism and anomalous dispersion in vacuum are shown to be accessible at these facilities.