Abstract: Nitroprusside single crystals having the formula M.sub.n [Fe(CN).sub.5 ]nH.sub.2 O, where M is a metal that forms a nitroprusside is found to have the property, when cooled to a temperature in the range between 160.degree. K. and 250.degree. K., of convertability into a long-duration metastable state by irradiation with coherent blue-green light and to return to the original state when irradiated with red light. Barium nitroprusside Ba[Fe(CN).sub.5 ]2H.sub.2 O can be switched rapidly from one state to the other without requiring cooling below about 200.degree. K. and lends itself particularly well to making single crystal memories for writing with a blue-green laser beam and reading with a red laser beam, with the readout being picked up by a photodetector diode array, on which the radiation from the nitroprusside crystal is focused. The incident laser beams are pulsed and the photodetector is turned off during pulsing.