= Quantum correlation
A <quantum correlation> is a statistical relation among observables of a composite <quantum system>. For local observables $A$ and $B$, a useful diagnostic is $\langle A\otimes B\rangle-\langle A\otimes I\rangle\langle I\otimes B\rangle$. Product states make this expression vanish for every pair of local observables.
Not every correlation in a quantum state is <quantum entanglement>. The separable mixture $[|00\rangle\langle00|+|11\rangle\langle11|]/2$ gives perfectly correlated computational-basis outcomes but has no entanglement. The pure <Bell state> $|\Phi^+\rangle$ has the same outcomes in that basis while retaining coherent relative-phase information detectable in complementary bases. The measurement choices therefore matter when characterizing the kind of correlation present.
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