Quantum nonlocality: How does Nature perform the trick?(1)

Since our early childhood we know in our bones that in order to interact with an object we have either to go to it or to throw something at it. Yet, contrary to all our daily experience, Nature is nonlocal: there are spatially separated systems that exhibit nonlocal correlations. In recent years this led to new experiments, deeper understanding of the tension between quantum physics and relativity and to proposals for disruptive technologies. Consider two spatially separated quantum systems, one controlled by Alice, the other by Bob, in a pure state . Alice and Bob may perform some measurements x and y on their systems and collect the results a and b, respectively. This situation is described by a conditional probability distribution p (a,bj x,y). In general this correlation doesn’t factorize: p (a,bj x,y) 6 p (aj x) � p (bj y), i.e. the two systems are correlated. At first, this is no surprise, correlations are everywhere. For example, consider two cups of the same color, either both red or both green, one in Alice’s and one in Bob’s hands. If they looks at the color of their cups, Alice and Bob’s results are correlated. In this example the origin of the correlation is obvious, Alice and Bob had only partial information: they knew that both have the same color, but they ignored which color. This differs deeply from the quantum situation, as quantum theory claims that a pure state provides a complete description of the two systems. This led EPR[2] to believe that quantum theory is incomplete in the same sense as the description ”of the same color” provides only an incomplete description of the color state of the cups.