In other words, it is the expected change in physical characteristics as one quantum system passes through an interaction site.
The legitimacy of this method relies on the assumption that the observed coincidences constitute a fair sample of the emitted pairs.
Following local realist assumptions as in Bell's paper, the estimated quantum correlation converges after a sufficient number of trials to where a and b are detector settings and λ is the hidden variable, drawn from a distribution ρ(λ).
The quantum correlation is the key statistic in the CHSH inequality and some of the other Bell inequalities, tests that open the way for experimental discrimination between quantum mechanics and local realism or local hidden-variable theory.
Quantum correlations give rise to various phenomena, including interference of particles separated in time.