Evolution Operators for Linearly Polarized Two-Killing Cosmological   Models
  We give a general procedure to obtain non perturbative evolution operators in closed form for quantized linearly polarized two Killing vector reductions of general relativity with a cosmological interpretation. We study the representation of these operators in Fock spaces and discuss in detail the conditions leading to unitary evolutions. 
J. Fernando Barbero G., Daniel G\'omez Vergel, Eduardo J. S.   Villase\~nor