Quantum correlation

The term quantum correlation has come to mean the expectation value of the product of the outcomes on the two sides. In other words, the expected change in physical characteristics as one quantum system passes through an interaction site. In John Bell's 1964 paper that inspired the Bell tests, it was assumed that the outcomes A and B could each only take one of two values, -1 or +1. It followed that the product, too, could only be -1 or +1, so that the average value of the product would be The term quantum correlation has come to mean the expectation value of the product of the outcomes on the two sides. In other words, the expected change in physical characteristics as one quantum system passes through an interaction site. In John Bell's 1964 paper that inspired the Bell tests, it was assumed that the outcomes A and B could each only take one of two values, -1 or +1. It followed that the product, too, could only be -1 or +1, so that the average value of the product would be where, for example, N++ is the number of simultaneous occurrences ('coincidences') of the outcome +1 on both sides of the experiment. In actual experiments though, detectors are not perfect and there are usually many null outcomes. The correlation can still be estimated using the sum of coincidences, since clearly zeros will not contribute to the average, but in practice, instead of dividing by Ntotal, it has become customary to divide by