Electrons bent into a circular path by moving through a magnetic field (Photo: Marcin Bialek)
Photon motion in an effective magnetic field, showing the change in photon path radius as control voltage is changed (Image: Stanford University)
Left to its own ways, light will follow the same path through an optical system whether the system is being used as a camera lens or as a projector. This is called time-reversal symmetry, or reciprocity. As many new applications and methods would be enabled by access to a non-reciprocal optical system, it is unfortunate that they have been so difficult to come by. But now researchers at Stanford University have discovered how to make such non-reciprocal systems by generating an effective magnetic field for photons.
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