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We report on a quantum counter which converts far-infrared radiation to visible light. The 891-Ghz radiation of an HCN laser, absorbed in optically pumped ruby by the transition Ē (2E) →2Ā (2E), was detected by an increased R2 fluorescence. By Zeeman splitting of the 2E levels the narrow-band detector was tuned to the laser line.
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Lengfellner et al. (1976) studied this question.
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