A METHOD FOR REACHING DETECTION EFFICIENCIES NECESSARY FOR OPTICAL LOOPHOLE-FREE BELL EXPERIMENTS

A method for preparing loophole-free four-photon Bell experiments whic h requires a detection efficiency of 67% is proposed. It enables reali stic detection efficiencies of 75% at a visibility of 85%. Each of the two type-II crystals down converts one correlated photon pair and we entangle one photon from one pair with one photon from the other pair on a highly transparent beam splitter. The entanglement selects two ot her conjugate photons into a Bell state. Wide solid angles for the con jugate photons then enable us to collect close to 100% of them. The ca ses when both photon pairs come from only one of the two crystals are successfully taken into account. Hardy's equalities are discussed. (C) 1997 Published by Elsevier Science B.V.