Abstract
Nowadays entangled photons are often produced by spontaneous parametric down conversion in nonlinear birefringent crystals. Entangled photon pairs can show a very convincing break of the Bell inequality, usually tested through the CHSH (Clauser, Horne, Shimony, Holt) procedure. However, a "compensating crystal", placed after the non-linear crystal, is necessary for the traditional setup. The purpose of this crystal has been explained using the concept of indistinguishability. In this paper we argue that indistinguishability is an unsatisfactory explanation. We would prefer an explanation based on physical mechanisms and not on an abstract and not easily testable concept like indistinguishability. Experimental details seems to reveal that there may be some interesting physics going on that we have not appreciated so far. Such details may be of great help in order to find a more satisfactory explanation of the process leading to entanglement.
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