Rui-Qi Qin | Searcharxiv

arXiv2026

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

Vacuum birefringence (VB), a fundamental prediction of nonlinear quantum electrodynamics (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 gamma-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 corresponds to a refractive index difference $Δn = 1.829 \times 10^{-4}$ over micron-scale paths, directly measurable as a high-contrast "X-shape" asymmetry in $e^+e^-$ pair distributions. The scheme provides a feasible path to first laboratory VB detection with current laser and accelerator technologies.

physics.plasm-ph↗

arXiv2025

All-Optical Generation of Dense, Multi-GeV, Longitudinally-Polarized Positron Beams

The production of high-yield, longitudinally polarized positron beams represents an outstanding challenge in advanced accelerator science. Laser-driven schemes offer a compact alternative but typically yield only transverse polarization, or require pre-polarized electron beams, and struggle to efficiently accelerate positrons to high energies. Here, we introduce an all-optical scheme that overcomes these limitations by integrating positron generation, acceleration, and spin manipulation in a unified framework. Through a head-on collision between an ultraintense, circularly polarized laser pulse and a counterpropagating unpolarized electron beam, we drive a robust QED cascade. The nonlinear Breit-Wheeler process within the cascade produces positrons that are born directly within the strong laser field. Crucially, these positrons are instantaneously captured and accelerated to multi-GeV energies (up to $\sim$9 GeV) via a direct laser acceleration mechanism, while their spins are simultaneously rotated to longitudinal alignment by the field dynamics. Our Monte-Carlo simulations confirm the simultaneous achievement of a high positron yield ($\sim$20 $e^+/e^-$), a high average longitudinal polarization ($\sim$50\%), and GeV-scale energies. This all-optical source, feasible at upcoming ultraintense laser facilities, presents a compact and efficient solution for applications in collider physics and fundamental high-energy experiments.

physics.acc-ph↗