Thomas G. Blackburn

Experiments using high-power lasers and relativistic electron beams will soon be capable of precision testing of the theory of strong-field quantum electrodynamics. The comparison between experiment and theory always occurs via numerical simulations, but the lack of quantitative uncertainty bounds for the latter means that the origin of any discrepancies cannot be identified conclusively. Here, I present a simulation framework that can place meaningful bounds on the theory uncertainties inherent to its operation. This work shows that the use of the locally constant field approximation leads to a theory uncertainty of a few percent in the signals of quantum radiation reaction expected in near-term experiments.

Radiation reaction, the force experienced by an accelerated charge due to radiation emission, has long been the subject of extensive theoretical and experimental research. Experimental verification of a quantum, strong-field description of radiation reaction is fundamentally important, and has wide-ranging implications for astrophysics, laser-driven particle acceleration, next-generation particle colliders and inverse-Compton photon sources for medical and industrial applications. However, the difficulty of accessing regimes where strong field and quantum effects dominate inhibited previous efforts to observe quantum radiation reaction in charged particle dynamics with high significance. We report a high significance ( > 5σ) observation of strong-field radiation reaction on electron spectra where quantum effects are substantial. We obtain quantitative, strong evidence favouring the quantum-continuous and quantum-stochastic models over the classical model; the quantum models perform comparably. The lower electron energy losses predicted by the quantum models account for their improved performance. Model comparison was performed using a novel Bayesian framework, which has widespread utility for laser-particle collision experiments, including those utilising conventional accelerators, where some collision parameters cannot be measured directly.

Several high power laser facilities are reaching field strengths where leading order strong-field quantum electrodynamical (QED) processes can be measured in the non-perturbative regime for the first time. At very high, as yet unobtainable in the laboratory, field strengths, the contribution of higher-order processes is predicted to dominate, implying a breakdown of current calculational methods. Focusing on nonlinear showers and considering currently available experimental parameters, we find that if the momentum spectrum of the \emph{incident} particles is well known, asymmetries in the \emph{outgoing} particle spectrum may provide a useful signature of higher orders of nonlinear phototrident, trident and Compton scattering. These signatures could be used by experiment to test how accurate the current calculational framework is when applied to strong-field QED at higher orders.

Experiments using high-power lasers and relativistic electron beams will soon be capable of precision testing of the theory of strong-field quantum electrodynamics. The comparison between experiment and theory always occurs via numerical simulations, but the lack of quantitative uncertainty bounds for the latter means that the origin of any discrepancies cannot be identified conclusively. Here I present a simulation framework that can place meaningful bounds on the theory uncertainties inherent to its operation. This work shows that the use of the locally constant field approximation leads to a theory uncertainty of a few per cent in the signals of quantum radiation reaction expected in near-term experiments.

Ultra-intense laser pulses can create sufficiently strong fields to probe quantum electrodynamics effects in a novel regime. By colliding a 60 GeV electron bunch with a laser pulse focussed to the maximum achievable intensity of 10²³ W cm⁻², we can reach fields much stronger than the critical Schwinger field in the electron rest frame. When the ratio of these fields χ e ≫ 1 we find that the hard ( > 25 GeV) radiation from the electron has a substantial contribution from spin-light. 33% more photons are produced above this energy due to spin-light, the radiation resulting from the acceleration of the electron’s intrinsic magnetic moment. This increase in high-energy photons results in 14% more positrons produced with energy above 25 GeV. Furthermore, the enhanced photon production due to spin-light results in a 46% increase in the electron recoil radiation reaction. These observable signatures provide a potential route to observing spin-light in the strongly quantum regime ( χ e ≫ 1 ) for the first time.

Pair creation by a γ ray in a high-intensity electromagnetic field (the nonlinear Breit-Wheeler process) is sensitive to the γ -ray polarization. Here we study the stability required in order to measure this polarization dependence in a head-on collision between a high-power linearly polarized laser and a relativistic electron beam, where the γ rays produced via nonlinear Compton scattering are highly polarized. We find that the laser strength parameter a 0 has to be known to within a few percent and that the alignment of the laser and electron beam cannot fluctuate by more than half the spot size in order to attribute the reduction in positron yield to polarization effects. These collision parameters are difficult to achieve, but may be realized in future precision experiments that focus on beam stability. Published by the American Physical Society 2025

Observing multiphoton electron–positron pair production (the nonlinear Breit–Wheeler process) requires high-energy γ rays to interact with strong electromagnetic fields. In order for these observations to be as precise as possible, the γ rays would ideally be both mono-energetic and highly polarized. Here we perform Monte Carlo simulations of an experimental configuration that accomplishes this in two stages. First, a multi-GeV electron beam interacts with a moderately intense laser pulse to produce a bright, highly polarized beam of γ rays by inverse Compton scattering. Second, after removing the primary electrons, these γ rays collide with another, more intense, laser pulse in order to produce pairs. We show that it is possible to measure the γ-ray polarization dependence of the nonlinear Breit–Wheeler process in near-term experiments, using a 100 TW class laser and currently available electron beams. Furthermore, it would also be possible to observe harmonic structure and the perturbative-to-nonperturbative transition if such a laser were colocated with a future linear collider.