A symmetry analyser for non-destructive Bell state detection using EIT
  We describe a method to project photonic two-qubit states onto the symmetric and antisymmetric subspaces of their Hilbert space. This device utilizes an ancillary coherent state, together with a weak cross-Kerr non-linearity, generated, for example, by electromagnetically induced transparency. The symmetry analyzer is non-destructive, and works for small values of the cross-Kerr coupling. Furthermore, this device can be used to construct a non-destructive Bell state detector. 
S. D. Barrett, P. Kok, K. Nemoto, R. G. Beausoleil, W. J. Munro and T.   P. Spiller