An Operator Product Expansion for the Mutual Information in AdS/CFT

We investigate the behaviour of the mutual information $\mathcal{I}_{AB}$ between two "small" and wide separated spherical regions $A$ and $B$ in the $\mathcal{N}=4$ SYM gauge theory dual to Type IIB string theory in $AdS_5 \times S^5$. To this end, the mutual information is recasted in terms of correlators of surface operators $\mathcal{W}\left( Σ\right)$ defined along a surface $Σ$ within the boundary gauge theory. This construction relies on the strong analogies between the twist field operators appearing in the replica trick method used for the computation of the entanglement entropy, and the disorder-like surface operators in gauge theories. In the AdS/CFT correspondence, a surface operator $\mathcal{W}\left( Σ\right)$ corresponds to having a D3-brane in $AdS_5 \times S^5$ ending on the boundary along the prescribed surface $Σ$. Then, a long distance expansion for $\mathcal{I}_{AB}$ is provided. The coefficients of the expansion appear as a byproduct of the operator product expansion for the correlators of the operators $\mathcal{W}(Σ)$ with the chiral primaries of the theory. We find that, while undergoing a phase transition at a critical distance, the holographic mutual information, instead of strictly vanishing, decays with a power law whose leading contributions of order $\mathcal{O}(N^0)$, originate from the exchange of pairs of the lightest bulk particles between $A$ and $B$. These particles correspond to operators in the boundary field theory with the smallest scaling dimensions.

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