Scientists Demonstrate Strong-Field QED Regime with Just 2 PW Laser Power

Researchers are now exploring innovative methods to access the strong-field quantum electrodynamics (QED) regime, and a new study led by Robert Babjak (GoLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, and Institute of Plasma Physics, Czech Academy of Sciences) and Marija Vranic (GoLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa) details a single-laser scheme for achieving this using direct laser acceleration. Their work demonstrates that laser pulses with powers as low as 2 petawatts could initiate the creation of electron-positron pairs via nonlinear Breit-Wheeler processes , a significant step towards experimentally verifying QED predictions and potentially revolutionising fields like high-energy physics and astrophysics. This research, supported by particle-in-cell simulations, offers a potentially feasible pathway for probing these extreme quantum effects with existing multi-petawatt laser facilities, paving the way for groundbreaking experiments.

Laser-driven electron acceleration and pair production are promising

Scientists have demonstrated a novel single-laser scheme capable of reaching the strong-field quantum electrodynamics (QED) regime, utilising direct laser acceleration (DLA) of electrons followed by head-on collision with the reflected laser pulse. This breakthrough relies on accelerating electrons within an underdense plasma using a relativistic laser pulse, subsequently interacting with the reflected field to generate high-energy photons via nonlinear Compton scattering. These photons then decay into electron-positron pairs through the nonlinear Breit-Wheeler process, opening new avenues for probing fundamental physics. The research team employed analytical scalings, validated by quasi-3D particle-in-cell simulations incorporating QED effects, to establish the feasibility of this approach with laser pulses as low as 2 PW.
The study reveals that a laser pulse with a power of just 2 PW is sufficient to achieve the quantum regime, specifically characterised by a dimensionless parameter χe exceeding 1. For even higher laser powers, the number of generated positrons increases rapidly and nonlinearly, exceeding 2 nC for a 10 PW laser pulse possessing an energy of approximately 1.1 kJ. A semi-analytical model was developed to accurately estimate the positron yield, exhibiting strong agreement with the simulation results, thus validating the theoretical framework. This precise modelling is crucial for predicting and optimising experimental outcomes.

Researchers further investigated the influence of laser depletion and the precise positioning of the reflecting foil on the efficiency of pair production, identifying key parameters for maximising positron generation. The simulations show that careful control of these factors can significantly enhance the yield of electron-positron pairs, making the process more efficient and detectable. This optimisation is vital for translating the theoretical potential into practical experimental results. The presented scheme offers an experimentally feasible platform for probing strong-field QED effects, utilising multi-petawatt laser systems currently available at leading research facilities.

This innovative approach circumvents the need for simultaneous access to both a relativistic-intensity laser and a multi-GeV electron accelerator, a significant limitation of previous methods. Instead of relying on complex setups involving both laser-wakefield acceleration and external electron beams, this single-laser scheme streamlines the process, making it more accessible and cost-effective. The work opens exciting possibilities for generating pair plasmas using strong lasers, enabling laboratory studies of their kinetic properties and furthering our understanding of fundamental interactions in extreme conditions. This research establishes a promising pathway for verifying radiation reaction models, with implications for astrophysical phenomena such as gamma-ray burst formation and magnetic reconnection.

Laser-driven electron acceleration and pair production are promising

An energy of 0.1 kilojoules (kJ) was considered. A semi-analytical model was implemented to estimate the positron yield, showing strong agreement with simulation results and validating the underlying theoretical framework. Experiments employed an overcritically dense foil to reflect the laser pulse, ensuring a head-on collision with the DLA-accelerated electrons. The team systematically investigated the influence of laser depletion and foil positioning on pair-production efficiency, optimizing these parameters to maximize yield.

To facilitate efficient laser wakefield acceleration (LWFA) in low-density gas, an initial laser spot size on the order of several tens of microns was used, enabling stable laser propagation and electron acceleration. However, recognizing the requirement for higher laser intensity, the study demonstrated that a moderate spot size of approximately 10 microns is optimal for direct laser acceleration (DLA) with multi-petawatt laser systems. Furthermore, the research showed that employing higher plasma densities on the order of 0.1 nanocoulombs enhances the acceleration process.

Experiments revealed a rapid, nonlinear increase in positron production with increasing laser power, culminating in the generation of more than 2 nanocoulombs of positrons using a 10 PW laser pulse with an energy of approximately 1.1 kJ. Measurements of the positron yield confirmed a nonlinear scaling with laser power, indicating a highly efficient pair-creation mechanism.

The data show that the scheme relies on energetic electrons that are first accelerated in an underdense plasma by a relativistic laser pulse and subsequently interact with the reflected laser field to emit high-energy photons. These photons then decay into electron–positron pairs through the nonlinear Breit–Wheeler process, a fundamental interaction in strong-field quantum electrodynamics. Measurements confirm the viability of this all-optical approach, which circumvents the need for simultaneous access to ultra-intense lasers and multi-GeV electron beams, a major limitation of previous experimental schemes.

The study further details how the DLA mechanism, extensively investigated in recent years, efficiently accelerates electrons in low-density gas, enabling the production of high-energy particles. The presented work establishes a platform for testing radiation-reaction models and has important implications for understanding astrophysical phenomena such as gamma-ray burst formation and magnetic reconnection. Overall, this innovative scheme opens new avenues for laboratory studies of strong-field quantum electrodynamics using currently available multi-petawatt laser systems, providing unprecedented opportunities to probe the quantum vacuum.

Laser-foil collisions generate high-energy positrons

Researchers have demonstrated a single-laser scheme capable of reaching the strong-field quantum electrodynamics (QED) regime. This innovative approach utilises direct laser acceleration (DLA) of electrons, followed by head-on collision with the reflected laser pulse from an overdense foil, to generate high-energy positrons via the nonlinear Breit-Wheeler process. Analytical scalings, supported by quasi-3D particle-in-cell simulations incorporating QED effects, indicate that a laser pulse with a power as low as 2 petawatts is sufficient to achieve conditions where the quantum nonlinearity parameter exceeds unity. The study reveals that this scheme can produce over 2 nanocoulombs of positrons with a 10 petawatt laser pulse possessing approximately 1.1 kilojoules of energy.

A semi-analytical model accurately predicts the positron yield, aligning with simulation results showing a rate of 1.8 picocoulombs per joule of laser energy. The authors acknowledge limitations in fully understanding laser absorption and acceleration processes, suggesting that further detailed investigation is needed to optimise positron production. Future work could focus on refining analytical strategies once these fundamental aspects are better characterised. This research establishes an experimentally viable platform for investigating strong-field QED phenomena using currently available multi-petawatt laser systems. While the scheme’s efficiency is influenced by laser depletion and foil positioning, simulations demonstrate robust particle production across a range of configurations, enhancing its suitability for experimental validation. The presented findings are significant as achieving such parameters is more challenging with two-step colliding schemes, where electron energy loss can hinder effective interaction with the laser pulse.