Renyi generalizations of the conditional quantum mutual information

Renyi generalizations of the conditional quantum mutual information

Seminar author:Mark M. Wilde

Event date and time:05/07/2014 02:30:pm

Event location:IFAE seminar room

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The conditional quantum mutual information $I(A;B|C)$ of a tripartite state $\rho_{ABC}$ is an information quantity which lies at the center of many problems in quantum information theory. Three of its main properties are that it is non-negative for any tripartite state, that it decreases under local operations applied to systems $A$ and $B$, and that it obeys the duality relation $I(A;B|C)=I(A;B|D)$ for a four-party pure state on systems $ABCD$. It has been an open question to find Renyi generalizations of the conditional mutual information, that would allow for a deeper understanding of the original quantity and find applications beyond the traditional memoryless setting of quantum information theory. The present paper addresses this question, by defining different $\alpha$-Renyi generalizations $I_{\alpha}(A;B|C)$ of the conditional mutual information that all converge to the conditional mutual information in the limit $\alpha \to 1$. Furthermore, we prove that many of these generalizations satisfy the aforementioned properties. As such, the quantities defined here should find applications in quantum information theory and perhaps even in other areas of physics, but we leave this for future work. We also state a conjecture regarding the monotonicity of the Renyi conditional mutual informations defined here with respect to the Renyi parameter $\alpha$. We prove that this conjecture is true in some special cases and when $\alpha$ is in a neighborhood of one. Finally, we discuss how our approach for conditional mutual information can be extended to give Renyi generalizations of an arbitrary linear combination of von Neumann entropies, particular examples including the multipartite information and the topological entanglement entropy.