A research group has discovered significant nonreciprocal optical absorption of LiNiPO4, referred to as the optical diode effect, in which divalent nickel (Ni2+) ions are responsible for magnetism, by passing light at shortwave infrared wavelengths used in optical communications. Furthermore, they have uncovered that it is possible to switch the optical diode effect by applying a magnetic field. This is a step forward in the development of an innovative optical isolator that is more compact and can control light propagation, replacing the conventional optical isolators with complex structures.