  At the first glance, the expression "transparent superconductor" may seem an
oxymoron. Still, the first principle calculations and experiments show that the
materials that behave as superconductors at low frequencies and do not absorb
in the optical domain may exist. Virtual excitation of the collective
electronic modes of such superconductors in a magnetic field appears as an
efficient way to realize the nonlinear interaction of light at the level of two
single photons. The essence of the effect is in the fact that the pondermotor
energy is proportional to the ratio of the charge squared to the mass of the
"collective particle" interacting with radiation, $e^2/m$, and therefore, for a
"particle" representing a collective motion of many electrons, it scales
linearly-, and its second-order correction quadratically with the number of the
electrons involved. This general situation is analyzed in detail in the
framework of a simple model of a fiber tube waveguide equipped with a clean
superconductor layer. It turns out that for realistic parameters, at the
micron-scale of the tube diameter and the centimeter-scale of the fiber length,
such a system is capable of performing the logic gate operation on the
polarization variables of a pair of optical photons.
