The authors propose a general approach that can produce a silica fiber with large and thermodynamically stable second order nonlinearity. They estimate that it is possible to achieve second harmonic generation efficiency of more than 10% in such a nonlinear fiber with a length of a few hundred microns. By maintaining a high degree of spatial symmetry, the proposed nonlinear fiber can naturally generate polarization entangled photon pairs, a physical object of enormous significance for quantum information technology. (c) 2007 American Institute of Physics.