Probing vacuum birefringence in an Ultrastrong Laser Field via High-energy Gamma-ray Polarimetry

Vacuum birefringence (VB), a fundamental prediction of nonlinear QED, has eluded direct laboratory detection due to its extreme weakness. We propose a compact, ”self-probing” scheme where a GeV electron beam collides head-on with a petawatt laser pulse. Circularly polarized γ -ray photons, generated via nonlinear Compton scattering in the same pulse, then probe the birefringent vacuum it induces. This integrated design bypasses the stringent synchronization and beam transport requirements of traditional pump–probe setups. Our nonperturbative strong-field QED simulations reveal a clear VB signature: conversion of circular to linear polarization, with the induced Stokes parameter S 1 reaching ∼ 0.019 within the selected angular range. This enables a 5 σ detection within only two shots, with the signal imprinted as a high-contrast “X-shape” asymmetry in the e + e − pair azimuthal distribution. The scheme provides a feasible path to first laboratory VB detection with current laser and accelerator technologies.