Quantum Effects in Strong Classical Potentials – Why we need Ultrahigh Intensity Lasers

In its 2003 report “Connecting Quarks with the Cosmos” the National Research Council identified “11 science questions for the new century”. Similar questions have been posed by other panels and are discussed in current literature. One of these questions, that ultrahigh intensity lasers are uniquely poised to address is question of the validity of the known physical laws and the search for new physical laws in extreme environments. Of direct interest in the context of ultrahigh intensity lasers is the specific question raised in the report of “Is a New Theory of Matter and Light Needed at the Highest Energies [and intensities[1]]?” The question arises with particular relevance when high energy conditions, conductive to quantum phenomena are superimposed with strong classical fields.

Currently there are no successful non-perturbative, dynamic quantum field theories, and such theories are necessary to calculate quantum effects in the presence of strong classical potentials. Important problems involving strong classical potentials are found in many areas of physics including Quantum Electrodynamics, Quantum Chromodynamics and Gravity. Example problems include: determining the Parton distribution function (i.e., the distribution of Quarks and Gluons in a proton), modeling the transition of colliding hadrons to a Quark-Gluon-Plasma (QGP), or describing particle creation in the vicinity of a black hole, electron motion and radiation emission in the magnetic field of a neutron star, and spontaneous pair creation from the quantum vacuum in the presence of a strong electromagnetic field. Developing and testing a theory in one sub-field, e.g. QED, allows the methods employed in that theory to be transferred to other fields.

An ultra-intense laser can be used to test a non-perturbative quantum field theory in the QED context. A 100-PW laser can create ultrastrong fields of >>10¹⁵ V/m to form a strong classical potential for QED processes. Moreover, it can create this potential in a controlled fashion, allowing testing of the theory over a range of parameters.

One ansatz to developing such a dynamical quantum field theory is based on the hierarchical separation of scales. In such a theory, observables can be defined in a way that cancel the non-perturbative contributions and make the theory testable with ultrahigh intensity lasers. Current 1PW class lasers can reach fields of ~10¹⁵ V/m and just start to scratch at the threshold of the QED regime, possibly allowing the observation of classical Radiation Reactions, the onset of pair creation and nonlinear Compton scattering. To develop and test a full theory, including Quantum Plasma Dynamic (QPD) effects, a larger area of parameter space has to be accessible. New 5-10 PW lasers under construction in Europe and Asia will have the capability to perform these experiments and extend deeper into the QED parameter space. Seeing the most interesting quantum effects become dominant, like pair cascades, will likely require fields on the order of a percent of the critical field ( = E_L/E_c ~ 0.01, E_L ~10¹⁶ V/m) requires a 50-100 PW laser system.

Existing laser systems like the Texas Petawatt Laser have been upgraded to reach an intensity of ~3x10²² W/cm² at a contrast sufficient to limit preplasma effects. First experiments exploring this new extreme field regime on this system are about to start. Furthermore, new, multi-PW systems are currently under construction, like the 5PW OPCPA laser at SIOM, to become operational end of 2016 and the 10PW lasers at ELI-NP, Romania and ELI-BL, Prague, which are scheduled to become operational for experiments at the end of 2018. These experiments are a first step across the threshold of nonlinear QED and will pave the way for future work on stronger lasers, which will more fully explore the regime of non-perturbative, dynamic quantum field theories.

Once QED contributions are well understood, any observed deviations from them indicate an effect outside of QED, which would motivate searches for axion-like pseudo-scalar and light scalar particles that also couple to gravity. Coupling to gravity would be indicated if we see experimentally that particles accelerated strongly in an EM field have a horizon, thus building a bridge to quantum effects in strong gravity fields as found around black holes.  Since these effects would manifest themselves in experiments as fluctuations on a QED background, it is imperative to develop first a complete understanding of the QED case.

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[1] “… and intensities” added by author.