  Quantum discord is the quantitative difference between two alternative
expressions for bipartite mutual information, given respectively in terms of
two distinct definitions for the conditional entropy. By constructing a
stochastic model of shared states, classical discord can be similarly defined,
quantifying the presence of some stochasticity in the measurement process.
Therefore, discord can generally be understood as a quantification of the
system's state disturbance due to local measurements, be it quantum or
classical. We establish an operational meaning of classical discord in the
context of state merging with noisy measurement and thereby show the
quantum-classical separation in terms of a negative conditional entropy.

Quantum Physics