Pairing effect of photons in nonlinear polar crystals

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Abstract
It is shown that photons in a polar crystal with a high nonlinearity can feel an attractive effective interaction by exchange of virtual phonons. A coherent state is unstable with respect to such an interaction, and a superguiding state is formed through the association of photons in pairs with opposite wave vectors and spins. The photon pairs travel without scattering attenuations. If the photons propagating in a wave guide enter the superguiding state, the wave guide exhibits an ultralow energy loss.