Polarization effects in modified versions of the QED vacuum

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Exploring the quantum vacuum is central to obtain an in-depth and clear understanding of the fundamental laws of nature. While the classical vacuum simply describes a region devoid of matter, quantum electrodynamics reveals that it is characterized by fluctuations of charged quantum fields. This allows the quantum vacuum to be understood as a plasma of "virtual" electron-positron pairs, which behaves like a weakly nonlinear, dielectric medium. An external field can thus polarize the vacuum, altering its dielectric properties and inducing a multitude of dispersive and absorptive phenomena. Some of them, like the famous Lamb shift of atomic energy levels, are well-established and can be measured with high accuracy in experiment. Others, like vacuum birefringence and the spontaneous production of real electron-positron pairs from the vacuum, have so far eluded observation - in spite of strong efforts towards their experimental detections. This is due to the technical limitations of reaching field strengths near the critical scale characterized by the electron mass.In this theoretical project, we plan to study vacuum scenarios resembling the electron-positron quantum vacuum, whose phenomenologies are characterized by much lower energy and field scales. Quantum vacua of this kind arise in various areas of physics. They range from special condensed-matter systems (graphene) where the electrons in the valence band behave like low-mass particles, to advanced theories of particle physics which predict the existence of very light, weakly interacting particles (such as axions and minicharges). The scenarios to be considered can be classified in three categories: (a) quantum vacua of low-dimensional systems; (b) quantum vacua involving fluctuations of hypothetical light particles; (c) quantum vacua in the presence of external fields. The corresponding polarization effects occur on energy scales that can be probed experimentally with currently available high-precision techniques such as atomic spectroscopy, polarimetry and voltage metering.Our aims are twofold. (1) We want to make use of experimentally verified vacuum polarization effects, such as the Lamb shift and photon splitting, to probe new particle candidates. From the sensitivities achieved in these experiments as well as in Cavendish-like setups, stringent bounds on the parameter spaces of axion-like particles and minicharges will be infered. (2) With regard to hitherto unobserved effects (such as vacuum birefringence or vacuum decay into pairs), we shall study analogies in systems with closely related properties, and explore novel ways to amplify these effects. A broad range of techniques shall be used, including dimensional compactification, renormalization, quantum kinetic equations and perturbation theory. Our studies will reveal interrelations between different classes of the quantum vacuum and significantly advance our understanding of various yet unobserved vacuum polarization phenomena.