Foreword

“Love at first sight!” was how Nicolas Gisin described his emotion when he learned about Bell’s theorem. When I heard this, I relived an autumn day of 1974 when I was immersed in study of John Bell’s paper, little known at the time, and understood that it was possible to render an experimental verdict on the debate between Bohr and Einstein on the interpretation of quantum mechanics. Even though a few physicists knew of the problem raised by Einstein, Podolsky, and Rosen (EPR), not many had heard of Bell’s inequalities, and few were those who considered questions relating to the fundamental concepts of quantum mechanics worthy of serious attention. The EPR paper, published in 1935 in Physical Review, was readily available in university libraries, but the same could not be said for the paper by Bell, published in an obscure new journal that was destined to disappear after only four issues. In those pre-internet days, papers not published in the major journals had to rely on photocopies for their dissemination. I had got my own copy from a file put together by Christian Imbert, a young professor at the Institut d’Optique, on the occasion of a visit by Abner Shimony, invited to Orsay by Bernard d’Espagnat. But once under the spell of Bell’s ideas, I decided that my doctoral thesis would deal with experimental tests of Bell’s inequalities, and Imbert accepted to take me under his wing. In Bell’s (impressively clear) paper, I was able to identify the crucial challenge for experimentalists: altering the orientations of the polarization detectors while entangled particles were still propagating from their source into the measurement regions. The point was to preclude influence of the polarizer orientations either on the emission mechanism or on the measurement, by application of the principle of relativistic causality, which forbids physical effects from propagating faster than the speed of light. Such an experiment would be able to scrutinise the essence of the conflict between quantum mechanics on the one hand and the world view held by Einstein on the other. Einstein defended local realism, which combines two principles. First, that there exists a physical reality of a system. Second, that a system cannot be influenced (the locality assumption) by anything that happens to another system separated from the first by a spacelike interval of spacetime, since those two systems would have