Quantum bipartite systems can be correlated in various ways. For pure
states, the total correlation can be quantified using mutual
information while the entanglement entropy can be used to quantify the 'quantum correlation'. For mixed states the situation is not as clear
cut. First, there is no unique measure of entanglement. Second, there
are some tasks involving bipartite separable state that have no
classical analogue. The 'quantum part' of the correlation can therefore
be quantified in different ways depending on the task, these can be
entanglement monotones or more general quantum correlation measures that
do not vanish for separable states, e.g. discord.
In this talk I will begin with a brief overview of the mathematical properties of quantum discord and similar quantities. I will then give some physical examples of where these quantities can be useful and some intuition on where they are probably useless.