A Fully Spin and Polarization Resolved Strong Field QED Algorithm for Particle-in-Cell Codes

TL;DR

This work delivers a fully spin- and polarization-resolved SFQED extension to a PIC code (OSIRIS) for extreme laser-plasma interactions, enabling ab initio simulations of spin and polarization dynamics in strong fields. It employs a hierarchical, conditional Monte Carlo sampling strategy that handles the nine-dimensional spin/polarization spectrum by sequentially sampling $\lambda$, final lepton spin $\pmb{s}_f$, and photon polarization $\boldsymbol{\xi}$, within a spin basis defined by $(\hat{\varepsilon},\hat{\beta},\hat{k})$ and a Stokes-vector description of photons. The code is extensively validated against analytical results (Sokolov–Ternov, classical polarization), cross-checked against Ptarmigan, and benchmarked across multiple published scenarios including bichromatic and elliptically polarized lasers, polarized gamma-ray generation, helicity transfer, and polarized QED cascades, showing strong agreement. This spin- and polarization-aware modeling reveals significant effects on polarization transfer and cascade dynamics, underscoring its importance for interpreting next-generation laser-plasma experiments and certain astrophysical environments. The framework opens pathways to studying vacuum birefringence and linear Compton/Breit–Wheeler processes with polarization resolution, enhancing predictive capability for experiments at ultra-intense laser facilities and high-energy astrophysical contexts.

Abstract

Modern ultra-intense laser facilities can generate electromagnetic fields strong enough to accelerate particles to near-light speeds over micron-scale distances and also approach the QED critical field, resulting in highly nonlinear and relativistic quantum phenomena. For such conditions, ab-initio modeling techniques are required that capture the electromagnetic, relativistic particle, and quantum emission processes in the plasma. One such technique is particle-in-cell (PIC) simulation. In this paper, we describe the underlying theory for and development, validation, and verification of an extension to standard QED-PIC in the OSIRIS framework to include spin- and polarization-resolved QED processes central to next-generation laser-plasma experiments. This code can advance the current understanding of spin- and polarization-dependent QED phenomena in ultra-intense laser-plasma interactions.

Figures (28)

• Figure 1: Schemetic of spin measurement under the basis ($\pmb{\hat{\varepsilon}}$, ${\pmb{\hat{\beta}}}$, $\pmb{\hat{k}}$)
• Figure 2: Photon polarization basis. A high-energy lepton with momentum in direction $\pmb{\hat{u}}$ emits a photon whose momentum direction $\pmb{\hat{k}} = \pmb{\hat{u}}$. The polarization basis vector for the photon is $\pmb {\hat{\varepsilon}}$, $\pmb{\hat{\beta}}$. When $\pmb {\hat{\kappa}} = -\pmb {\hat{k}}$, $\pmb\sigma$ and $\pmb\pi$ are in the same direction as $\pmb{\hat{\varepsilon}}$ and $\pmb{\hat{\beta}}$.
• Figure 3: Flow chart for spin-polarized QED algorithm
• Figure 4: Flow chart for spin-polarized NLCS algorithm
• Figure 5: Flow chart for spin-polarized NBW algorithm