Number 54 (Story #1), November 4, 1991 by Phillip F. Schewe and Ben Stein TWO-PHOTON BOUND STATES might occur in extremely nonlinear optical materials, materials in which the amount of output light is not linearly proportional to the amount of input light. Ze Cheng, a physicist at Huazhong University in China, suggests that photons can pair off in nonlinear crystals via the exchange of virtual phonons and that the resultant "superlight" would propagate without scattering attenuations (Physical Review Letters, 11 November 1991). Cheng compares this "superguiding" effect to low-temperature superconductivity, in which electron pairs (Cooper pairs formed by the exchange of phonons) link up to constitute a supercurrent. According to Richard Slusher of AT&T Bell Labs (908-582-4094), an expert on the properties of light in nonlinear media (including squeezed light), the degree of nonlinearity---and hence the intensity of the light entering the crystal---needed to produce the exotic pairing effect is probably beyond present experimental techniques. Nevertheless, Raymond Y. Chiao of the University of California at Berkeley (510-642-4965) and his group are also seeking two-photon states. They feel that such states exist theoretically and should be detectable experimentally. (Physical Review Letters, 9 September 1991.)