You can equivalently think of quantum discord as measuring how much a system is unavoidably disrupted – by ‘destroying’ superpositions or entanglement say – when information about it is gathered by measurement. It’s a measure of the ineluctable cost of measurement: how far there is to fall from the misty, elusive quantum heights to the terra firma of the classical valley. For classical systems, the discord is zero. If it is greater than zero, the system has some quantumness to it.
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