The classical-quantum boundary for correlations: discord and related measures

Understand

One of the best signatures of nonclassicality in a quantum system is the existence of correlations that have no classical counterpart.

• Different methods for quantifying the quantum and classical parts of correlations are amongst the more actively-studied topics of quantum information theory over the past decade.
• Entanglement is the most prominent of these correlations, but in many cases unentangled states exhibit nonclassical behavior too.
• Thus distinguishing quantum correlations other than entanglement provides a better division between the quantum and classical worlds, especially when considering mixed states.