Repulsive Casimir force in time-reversal symmetry broken media

In quantum field theory, a vacuum is not empty but full of fluctuating electromagnetic waves. If two dielectric mirrors (silicon plates) are placed facing each other in vacuum, it is well known that there exists an attractive Casimir force between them. When one of the silicon plates is replaced by a medium with time-reversal symmetry breaking (usually achieved by an external magnetic field), we show that the Casimir force can become repulsive. This opens the possibility of controlling the quantum vacuum with non-reciprocal media.