T1: Vacuum Birefringence

One of the most fascinating predictions of quantum electrodynamics (QED) is vacuum birefringence in the presence of macroscopic electromagnetic fields. If an originally linearly polarized probe photon beam is sent through a strong-field region, some of the photons constituting the beam can be scattered into a perpendicularly polarized mode, whereas the majority of the probe photons traverse the strong field unaltered. This effectively supplements the probe beam with a tiny ellipticity, thereby attributing a birefringence phenomenon to the quantum vacuum in electromagnetic fields. The recent advances in high-intensity laser technology, x-ray polarimetry and in theory have substantiated the possibility to measure the seminal QED prediction of vacuum birefringence for the first time in a discovery experiment with x-ray free electron and optical high-intensity lasers.

In this research project, we aim at a thorough theoretical study of vacuum birefringence in manifestly inhomogeneous electromagnetic fields, ranging from immediate phenomenological applications, like identifying the optimal setup for its measurement accounting for all the real-world imperfections in experiment, to fundamental theoretical challenges, such as pushing forward non-perturbative first-principles calculations. To this end, the project partners will pursue both analytic calculations and dedicated numerical simulations.

Principal Investigators

  • Felix Karbstein

    HI Jena

  • Hartmut Ruhl

    ASC, LMU München

E1: High-precision diamond X-ray polarimeters for the detection of vacuum birefringence

We propose to realize an experiment for the detection of vacuum birefringence at the HiBEF beam line at the European XFEL. The central bottleneck for such an experiment is the extinction ratio of the required crossed X-ray polarimeter setup. The current state of the art is a polarization purity of 2 x 10^{-10} at 6.5 keV. Our goal is to improve this value by 2 orders of magnitude while increasing the photon energy to 9.8 keV. In this way, a 1000-fold improvement of the sensitivity of probing the birefringence of vacuum can be realized.

Principal Investigators

  • Gerhard G. Paulus

    IOQ, FSU Jena

  • Kai Sven Schulze

    IOQ, FSU Jena

T2: Quantum vacuum nonlinearities in the all-optical regime

This project investigates the consequences of the nonlinear nature of the quantum vacuum for general self-interaction phenomena of light. The goal is an identification and a comprehensive description of potential discovery experiments of these phenomena based on macroscopically controllable high-intensity laser fields.

For this, the challenge of a quantitative treatment of virtual quantum processes in realistic laser pulses and the induced signatures dynamically in space and time has to be met. The project uses and further develops effective field theory methods, investigates general correlation functions of quantum electrodynamics (QED) in strong-field environments, and aims at computing a variety of key observables for such experiments, as well as their differential dependence on accessible control parameters.

Principal Investigators

  • Holger Gies

    TPI, FSU Jena

  • Hartmut Ruhl

    ASC, LMU München

E2: Spacetime resolved few-photon detection schemes for vacuum processes

The goal over a six year period of this project is to investigate the recent intriguing predictions of signatures of quantum vacuum non-linearity such as quantum reflectivity and to lay the basis for precision experiments based on photon emission from the quantum vacuum. Precision measurements of the response of the quantum vacuum based on full description of the interacting fields and sensitive detection systems are within reach based on the development of intense lasers and the work proposed here. The developments in high power lasers in both peak intensity and average power hold the promise of very precise tests becoming possible after the first proof-of-principle measurements envisaged in the scope of this project. The experimental challenge lies in the detection of a very small number of photons resulting from QED vacuum processes against the background of intense laser radiation. We propose developing the experimental tools. Specifically single photon detection schemes capable of sampling only photons from an area of only few μm² and femtosecond temporal windows with high quantum efficiency and techniques that enable pure vacuum interactions of colliding laser pulses.

Principal Investigators

  • Jörg Schreiber

    LMU München

  • Matt Zepf

    IOQ, FSU Jena

T3: Electron-positron pair production

Photons colliding with each other can transform their energy into mass in the form of an electron-positron pair. Until now, this fundamental quantum process of matter creation purely from photon beams - the so-called Breit-Wheeler process – could not directly be observed in experiment, though, since the requirements for obtaining a detectable signal are very high.

The goal of the present project is to provide theoretical predictions for Breit-Wheeler pair production in focused high-intensity laser pulses. This poses a formidable challenge whose mastery necessitates a suitable combination of scientific methods. In this project, analytical S-matrix calculations as well as numerical simulations based on a many-body description will be carried out.

Aside from overcoming the computational challenges, our project also aims at finding answers to serious conceptual questions, e.g. how radiation reaction can be incorporated consistently into the theoretical treatment of pair production at ultrahigh intensities. Corresponding solutions can make important contributions to an improved understanding of the quantum vacuum under extreme-field conditions.

Principal Investigators

  • Carsten Müller

    ITP I, HHU Düsseldorf

  • Hartmut Ruhl

    ASC, LMU München

E3: Towards pair production in the non-perturbative regime: Ultra-relativistic beams, laser characterisation and pair detection

The equivalence of mass and energy according to Einstein's famous relation E = mc² is one of the central tenets of modern physics. The creation of pairs of matter and antimatter from massless photons in vacuum is the purest form of this process. By colliding an ultra-intense laser with an intense γ-ray burst this iconic phenomenon opens up to experimental observation for the first time and allows a direct investigation of non-perturbative quantum vacuum effects and the test of theoretical predictions of this regime.

We aim to lay the foundations for a first direct measurement of this fundamental response of the quantum vacuum to strong fields. The initial phase is dedicated to developing constituent components of the experiment: Laser-driven wakefield accelerators will be perfected to provide stable, high-charge, quasi-monoenergetic multi-GeV electron bunches to drive the γ-rays on the one hand. On the other hand we will develop the required detectors and experimental arrangements to detect a small signal rate against a background of an intense radiation field. Building on these foundations, the second phase is dedicated to a quantitative exploration of the Breit-Wheeler process in detail, which will become valuable input for theoretical modeling efforts.

Principal Investigators

  • Stefan Karsch

    LMU München

  • Matt Zepf

    IOQ, FSU Jena