The collective behaviour of physical systems is determined by the way the subsystems interact among themselves and become correlated. There are correlations in the quantum world that do not have counterpart in the classical one, such as entanglement. Entanglement arises from the inseparability between the states of the subsystems. Although entanglement has been the main kind of quantum correlation studied since the origins of Quantum Mechanics, there is another type of correlation called quantum discord. Such correlation is useful in protocols of quantum information and also to understand some physical phenomena. Quantum discord describes the incapacity of obtaining information from one interacting subsystem without perturbing it. The class of quantum correlations measure can be widening by including coherence, which describes the superposition character of states with relation to a given measurement basis. Therefore, there is a hierarchy relation between these types of quantum correlations, as shown in the picture below.
Prof. Eduardo Inácio Duzzioni