epl draft Interferometry of light propagation in pulsed fields

We investigate the use of ground-based gravitational-wave interferometers for studies of the strong-field domain of QED. Interferometric measurements of phase velocity shifts induced by quantum fluctuations in magnetic fields can become a sensitive probe for nonlinear self-interactions among macroscopic electromagnetic fields. We identify pulsed magnets as a suitable strong-field source, since their pulse frequency can be matched perfectly with the domain of highest sensitivity of gravitational-wave interferometers. If these interferometers reach their future sensitivity goals, not only strong-field QED phenomena can be discovered but also further parameter space of hypothetical hidden-sector particles will be accessible. Introduction. – Charged quantum fluctuations as predicted by quantum electrodynamics (QED) induce nonlinear self-interactions of electromagnetic fields [1]. This fundamental violation of the superposition principle of classical electrodynamics has not yet been observed on the level of macroscopic electromagnetic fields. Even though light-by-light interactions have been verified in experiments involving high-energy photons [2], an investigation of nonlinear interactions of macroscopic fields would probe QED and its vacuum structure in a large-amplitude regime which is comparatively little explored in quantum field theory. In fact, large-amplitude or strong-field experiments not only give access to unprecedented fundamental tests of QED, but also facilitate a search for hypothetical particles with light masses and weak couplings to photons (hidden-sector searches). This prospect has recently triggered a remarkable growth of experimental and theoretical activities concerned with strong-field and optical set-ups, for recent reviews see [3, 4]. A sensitive probe for vacuum nonlinearities is light propagation in strong electromagnetic fields. The lowest-order nonlinear modifications of Maxwell’s theory as induced by QED vacuum polarization are described by the (lowestorder) Heisenberg-Euler Lagrangian [1], L = 1 2 (E−B)+ 2α 2 45m4 (E−B)+7 2α 2 45m4 (E·B) , (1) where the scale of nonlinearities is set by the electron mass m, α ' 1/137 denotes the fine-structure constant, and we use ~ = c = 1. The resulting field equations predict that a plane wave in a magnetic field B in vacuum propagates at a reduced phase and group velocity [5, 6], vi = 1− ai 45 α m4 B sin θ , i = ⊥, ‖, a‖ = 14, a⊥ = 8, (2) where θ denotes the angle between the B field and the propagation direction. There are two propagation eigenmodes polarized parallel ‖ or perpendicular ⊥ to the plane spanned by B and the propagation direction. As the numerical coefficients a‖,⊥ differ for the two polarization modes, the magnetized quantum vacuum is birefringent. A number of experiments have already been carried out [7–11] or designed [12,13] to look for vacuum birefringence in terms of high-sensitivity polarimetry. The sensitivity limits achieved so far are roughly four orders of magnitude above those necessary for the QED effect. An alternative to polarimetry is given by absolute phase velocity measurements by interferometry using, e.g., gravitational-wave interferometers, as first suggested by [14], see also [15]. As gravitational-wave interferometers reach their highest sensitivity for optical-path variations at a frequency f ∼ 10Hz, the requirements for large-scale magnet systems are enormous, as has recently been discussed in detail in a concrete proposal in [16]. Indeed, using gravitational-wave interferometers for the search for nonlinear vacuum phenomena involves many parameters which need to be taken into account in an optimized fashion. In this work, we propose the use of pulsed magnets, as they are developed and used in a stap-1 ar X iv :0 90 4. 02 16 v2 [ he pph ] 2 8 Ju l 2 00 9