  We show that magnetic fields significantly enhance a new tunneling mechanism
in quantum-field theories with photons coupling to fermionic minicharged
particles. We propose a dedicated laboratory experiment of the
light-shining-through-walls type that can explore a parameter regime comparable
to and even beyond the best model-independent cosmological bounds. With
present-day technology, such an experiment is particularly sensitive to
minicharged particles with masses in and below the meV regime.
