Abstract

A four photon experiment which prepares two independent photons in pure nonmaximal singlet- and triplet-like states is proposed. Two photons from two input photon singlets (generated by nonlinear type-II crystals) interfere in the fourth order at an asymmetrical beam splitter. Coincidental detections of those of them which emerge from different sides of the beam splitter—let us call them preselector photons—serve as a gate for a preselection of the other two unpolarized companion photons from the pairs—let us call them Bell photons—into a nonmaximal polarization correlated state. Input photon singlets are generated either by means of beam splitters fed from nonlinear crystals of type-II with tangent signal and idler cones or by means of single nonlinear crystals of type-II with intersecting signal and idler cones. Such setups can be used for closing all the remaining loopholes in the Bell theorem with detection efficiency and visibility of preselector photons being arbitrarily low and with detection efficiency of Bell photons being as low as 67%. The setups can also be used for preparing nonmaximal entangled states without losing counts as opposed to the attenuation method. On the other hand, the setups with a symmetric beam splitter and without polarizers behind it can be used for a preparation of spin-correlated states of photons in the spin space by means of controlling other than spin observables.

© 1996 IEEE