R\'enyi mutual information in quantum field theory, tensor networks, and gravity

TL;DR

This paper introduces a new class of correlation measures called $oldsymbol{ extit{ extalpha}-z}$ R extit{é}nyi mutual informations, which are positive, monotonic, and can be computed in various quantum many-body systems, providing bounds on correlations.

Contribution

It develops the $ extalpha-z$ R extit{é}nyi mutual informations, proving their properties and applicability across multiple quantum systems, including conformal field theories and holography.

Findings

$ extalpha-z$ RMIs are positive and monotonic under quantum operations.

They upper bound and, for certain parameters, lower bound connected correlation functions.

An implementable replica trick for computing RMIs in many-body systems.

Abstract

We explore a large class of correlation measures called the $\alpha-z$ R\'enyi mutual informations (RMIs). Unlike the commonly used notion of RMI involving linear combinations of R\'enyi entropies, the $\alpha-z$ RMIs are positive semi-definite and monotonically decreasing under quantum operations, making them sensible measures of total (quantum and classical) correlations. This follows from their descendance from R\'enyi relative entropies. In addition to upper bounding connected correlation functions between subsystems, we prove the much stronger statement that for certain values of $\alpha$ and $z$, the $\alpha-z$ RMIs also lower bound connected correlation functions. We develop an easily implementable replica trick which enables us to compute the $\alpha-z$ RMIs in a variety of many-body systems including conformal field theories, free fermions, random tensor networks, and holography.