Arbitrarily complete Bell-state measurement using only linear optical elements

A complete Bell-state measurement is not possible using only linear-optic elements, and most schemes achieve a success rate of no more than 50%, distinguishing, for example, two of the four Bell states but returning degenerate results for the other two. It is shown here that the introduction of a pair of ancillary entangled photons improves the success rate to 75%. More generally, the addition of 2N-2 ancillary photons yields a linear-optic Bell-state measurement with a success rate of 1-1/2N.