An introduction to quantum discord and non-classical correlations beyond   entanglement
  In this didactic article we explore the concept of quantum correlations beyond entanglement. We begin by introducing and motivating the classically correlated states and then showing how to quantify the quantum correlations using an entropic approach, arriving at a well known measure called the quantum discord. Quantum correlations and discord are then operationally linked with the task of local broadcasting. We conclude by providing some alternative perspectives on quantum correlations and how to measure them. 
Gerardo Adesso, Marco Cianciaruso, Thomas R. Bromley
quant-ph cond-mat.stat-mech math-ph math.MP physics.data-an