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The physical vacuum of a relativistic quantum field theory amounts to a non-trivial quantum state. It encodes information about the full particle content of the underlying microscopic theory in the form of virtual processes, also referred to as vacuum fluctuations. If the theory features charged particles, fluctuations of the latter give rise to nonlinear effective couplings between electromagnetic fields that vanish in the formal limit of a vanishing Planck constant, but persist for a nonzero physical value. In turn, they inherently modify Maxwell’s linear theory of classical electrodynamics.