The mysteries of quantum correlation
Since the establishment of quantum mechanics, many fundamental problems have been stirring the hearts of physicists. Among these problems, a center issue is to understand the nature of quantum correlation. Here we reveal an essential feature of quantum correlation: its simultaneous existence in complementary bases. The classical correlation in a quantum state reaches its maximum in a certain basis and vanishes in the complementary bases, while the genuine quantum correlation does not vanish in the complementary bases. We reveal this feature of quantum correlation by defining measures based on invariance under basis change. For a bipartite quantum state, the classical correlation is the maximal correlation present in a certain optimum basis, while the quantum correlation is characterized as a series of residual correlations in mutually unbiased bases.