# [Physics] quantum discord

entropyquantum mechanicsquantum-computerquantum-entanglementquantum-information

What is quantum discord? I stumbled upon this term on Quantum Computing: The power of discord, but have never heard of it before. Can you give a bit more mathematical explanation of the term here?

It is basically a measure of the quantumness of some correlations, which is not vanishing for some separable state. It was introduced by Ollivier and Zurek (PRL/arXiv). It is the difference between two different generalizations of the classical (Shannon) conditional entropy to the quantum world, and is 0 for a pure bipartite separable state. It has been proven to be the amount of entanglement needed in the task of state-merging (PRA/arXiv and PRA/arXiv).

Definition

(PRL/arXiv) Classically the conditional entropy $$H(A|B)$$ is a measure of the uncertainty one has on the variable $$A$$ once we know the variable $$B$$. Of course, the definition of "knowing" $$B$$ becomes problematic when $$B$$ is quantum.

1. Classically, one can define $$H(A|B)$$ as the average $$H(A|B)=\sum_b {\mathcal P}(B=b)H(A|B=b)$$, each $$H(A|B=b)$$ being the entropy of $$A$$ given that the random variable has the value $$b$$. If one generalizes this to the quantum world, the $$B=b$$ part implies a quantum measurement (a POVM) which should be specified. A natural choice is the "best" measurement, the one which minimizes the entropy. The Shannon $$H$$ entropy is replaced by the Von Neumann entropy, and we define $$S(A|B_c)=\min_{\text{POVM}} \sum_{b}\mathcal{P}(\text{POVM applied to B gives } b) S(A|\text{POVM applied to B gives }b)$$.

2. The previous definition leads classically to a redefinition of the conditional entropy as an entropy difference : $$H(A|B)=H(A,B)-H(B)$$, which is always positive. Its quantum version, $$S(A|B)=S(AB)-S(B)$$ can be negative (in contrast with $$S(A|B_c)$$). Its negativity is a sufficient condition for entanglement.

The discord is defined as $$S(A|B_v)-S(A|B)$$ and is always positive. You can maybe see it as the amount of correlation between $$A$$ and $$B$$ which is destroyed by a classical measurement of $$B$$.

The state merging primitive is the following. Suppose Alice, Bob and Charly share a 3-party pure entangled state. Alice want to send her part to Bob without destroying the quantum correlations between $$AB$$ and $$C$$. Basically, she has to teleport $$A$$ to Bob, and the minimal amount of entanglement Alice and Bob need to perform this task is given by the quantum discord.