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We describe a four-state atomic system that absorbs two photons, but does not absorb one photon. As a switch, in the ideal limit, it operates at an energy cost of one photon per switching event.Received 13 July 1998DOI:https://doi.org/10.1103/PhysRevLett.81.3611©1998 American Physical Society
Harris et al. (1998) studied this question.