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The conditional quantum entropy is an entropy measure used in quantum information theory. It is a generalization of the conditional entropy of classical information theory. For a bipartite state ${\rho }^{AB}$, the conditional entropy is written $S(A|B{)}_{\rho }$, or $H(A|B{)}_{\rho }$, depending on the notation being used for the von Neumann entropy.

For the remainder of the article, we use the notation $S(\cdot )$ for the von Neumann entropy, which we simply call "entropy".

Definition

Given a bipartite quantum state ${\rho }^{AB}$, the entropy of the entire system is $S(AB{)}_{\rho }\stackrel{\mathrm{d}\mathrm{e}\mathrm{f}}{=}S({\rho }^{AB})$, and the entropies of the subsystems are $S(A{)}_{\rho }\stackrel{\mathrm{d}\mathrm{e}\mathrm{f}}{=}S({\rho }^A)=S({\mathrm{t}\mathrm{r}}_B{\rho }^{AB})$ and $S(B{)}_{\rho }$. The von Neumann entropy measures how uncertain we are about the value of the state; how much the state is a mixed state.

By analogy with the classical conditional entropy, one defines the conditional quantum entropy as $S(A|B{)}_{\rho }\stackrel{\mathrm{d}\mathrm{e}\mathrm{f}}{=}S(AB{)}_{\rho }-S(B{)}_{\rho }$.

An equivalent (and more intuitive) operational definition of the quantum conditional entropy (as a measure of the quantum communication cost or surplus when performing quantum state merging) was given by Michał Horodecki, Jonathan Oppenheim, and Andreas Winter in their paper "Quantum Information can be negative" [1].

Properties

Unlike the classical conditional entropy, the conditional quantum entropy can be negative. This is true even though the (quantum) von Neumann entropy of single variable is never negative. The negative conditional entropy is also known as the coherent information, and gives the additional number of bits above the classical limit that can be transmitted in a quantum dense coding protocol. Positive conditional entropy of a state thus means the state cannot reach even the classical limit, while the negative conditional entropy provides for additional information.

References

Nielsen, Michael A. and Isaac L. Chuang (2000). Quantum Computation and Quantum Information. Cambridge University Press, ISBN 0-521-63503-9.

• Mark M. Wilde, "From Classical to Quantum Shannon Theory", arXiv:1106.1445.