[Paper Review] Extreme field physics and QED

T. Heinzl, Anton Ilderton|ArXiv.org|Sep 19, 2008

Quantum Information and Cryptography3 citations

TL;DR

This paper proposes that a 10 Petawatt upgrade of the Vulcan laser would enable experimental tests of strong-field Quantum Electrodynamics (QED) in extreme electromagnetic fields, where nonlinear effects like nonlinear Compton scattering, electron-positron pair production, and vacuum birefringence become accessible. The key contribution is identifying that such high-intensity lasers can probe the long-untested low-energy, strong-field regime of QED, with vacuum birefringence offering a measurable signal of order 10⁻⁷ to 10⁻⁴ for future Exawatt-class facilities like ELI or HiPER.

ABSTRACT

We give a brief overview of the most important QED effects that can be studied in the presence of extreme fields such as those expected at the Vulcan laser upgraded to a power of 10 Petawatts.

Motivation & Objective

• To explore the untested low-energy, strong-field regime of QED using ultra-intense lasers.
• To identify measurable quantum electrodynamics effects in extreme laser fields that are inaccessible with current facilities.
• To assess the feasibility of detecting vacuum birefringence and pair production in upcoming high-power laser experiments.
• To bridge the gap between standard QED (tested in weak fields) and strong-field QED, which remains experimentally unverified.

Proposed method

• Uses the dimensionless laser amplitude $ a_0 = \frac{eE\lambda_L}{mc^2} $ to characterize field strength, with $ a_0 \sim 1 $ marking the onset of relativistic electron dynamics.
• Applies strong-field QED formalism, treating electrons as Volkov states dressed by laser photons, represented by 'fat' lines in Feynman diagrams.
• Analyzes vacuum polarization effects via the Heisenberg-Euler effective Lagrangian, deriving expressions for refractive indices $ n_\pm $ and ellipticity signals $ \delta^2 $.
• Derives the ellipticity signal $ \delta^2 = 3.2 \times 10^5 \left( \frac{d}{\mu\text{m}} \epsilon^2 \nu \right)^2 $, where $ \epsilon^2 \sim (10^{-6}a_0)^2 $, to quantify vacuum birefringence.
• Evaluates signal strengths across laser facilities using the rule of thumb $ a_0^2 \simeq 5 \times 10^3 P/\text{PW} $, predicting $ \delta^2 \sim 10^{-7} $ to $ 10^{-4} $ for ELI/HiPER.
• Considers Compton backscattering from high-energy electron beams (e.g., Diamond) as a route to produce polarized MeV photons for enhanced sensitivity.

Experimental results

Research questions

• RQ1 Can vacuum birefringence be measured in a laser-generated hot spot using a linearly polarized X-ray probe beam?
• RQ2 What is the expected signal strength for vacuum birefringence in 10 PW and Exawatt-class laser facilities?
• RQ3 How do suppression factors, such as those related to the Schwinger exponent, affect the observability of pair production in extreme fields?
• RQ4 Can nonlinear Compton scattering be observed without threshold suppression in ultra-intense laser fields?
• RQ5 What role do Kramers-Kronig relations play in linking real (dispersive) and imaginary (absorptive) parts of vacuum polarization in strong fields?

Key findings

• The ellipticity signal for vacuum birefringence is predicted to reach $ \delta^2 \sim 2 \times 10^{-9} $ at the 10 PW Vulcan upgrade and $ \delta^2 \sim 10^{-7} \text{ to } 10^{-4} $ at Exawatt-class facilities like ELI or HiPER.
• For X-ray probes with $ \omega = 5 \text{ keV} $, the ellipticity signal grows quadratically with probe frequency $ \nu $, spot size $ d $, and intensity $ \epsilon^2 $, making it maximally sensitive to these parameters.
• The theoretical limit of X-ray polarimetry is $ \sim 10^{-11} $, suggesting that only Exawatt-class lasers can achieve detectable signals with current technology.
• Pair production and vacuum birefringence are suppressed by factors of order $ 10^{-6} $, which correspond to the inverse of the Schwinger exponent, making them challenging but potentially observable with future upgrades.
• Using Compton backscattering to generate polarized MeV photons could enhance the signal by several orders of magnitude, enabling access to the frequency dependence of refractive indices.
• The paper confirms that the 10 PW upgrade of the Vulcan laser is a crucial step toward probing the low-energy, strong-field regime of QED, where new physics such as WISPs might emerge.