Long-distance N-partite information for fermionic CFTs

TL;DR

This paper derives a general formula for the long-distance four-partite information in fermionic conformal field theories, revealing how it depends on fermionic operators and their correlation functions, with checks in a 2+1D free fermion model.

Contribution

The paper provides the first explicit formula for the long-distance four-partite information in fermionic CFTs, extending understanding of multi-region mutual information beyond scalar operators.

Findings

The formula matches lattice computations for a 2+1D free fermion.

Leading order $I_5$ vanishes in fermionic theories.

Higher odd $I_N$ vanish in free fermionic theories.

Abstract

The mutual information, $I_2$, of general spacetime regions is expected to capture the full data of any conformal field theory (CFT). For spherical regions, this data can be accessed from long-distance expansions of the mutual information of pairs of regions as well as of suitably chosen linear combinations of mutual informations involving more than two regions and their unions -- namely, the $N$-partite information, $I_N$. In particular, the leading term in the $I_2$ long-distance expansion is fully determined by the spin and conformal dimension of the lowest-dimensional primary of the theory. When the operator is a scalar, an analogous formula for the tripartite information $I_3$ contains information about the OPE coefficient controlling the fusion of such operator into its conformal family. When it is a fermionic field, the coefficient of the leading term in $I_3$ vanishes instead. In this paper we present an explicit general formula for the long-distance four-partite information $I_4$ of general CFTs whose lowest-dimensional operator is a fermion $\psi$. The result involves a combination of four-point and two-point functions of $\psi$ and $\bar{\psi}$ evaluated at the locations of the regions. We perform explicit checks of the formula for a $(2+1)$-dimensional free fermion in the lattice finding perfect agreement. The generalization of our result to the $N$-partite information (for arbitrary $N$) is also discussed. Similarly to $I_3$, we argue that $I_5$ vanishes identically at leading order for general fermionic theories, while the $I_N$ with $N=7,9, \dots$ only vanish when the theory is free.