Unitarity of Entanglement and Islands in Two-Sided Janus Black Holes

TL;DR

This paper investigates entanglement dynamics and the emergence of islands in Janus black holes, demonstrating unitarity, transitional Page curve behavior, and the realization of double holography in a consistent string theory framework.

Contribution

It introduces a string theory embedding of Janus black holes, analyzes entanglement transitions, and identifies islands using a new effective lower-dimensional theory and quantum extremization.

Findings

Entanglement transition occurs around the Page time.

Islands of radiation entanglement wedge are identified and verified.

Transitional behavior of the Page curve is observed before the Page time.

Abstract

We explore the entanglement evolution of boundary intervals in eternal Janus black holes that can be embedded consistently into string theory in the low-energy limit. By studying the geodesics we show that there is a transition in the entanglement characteristic around the Page time, which manifests the unitarity of the evolution. We reproduce and reinterpret these bulk results from two different lower-dimensional perspectives: first as an interface CFT in the usual AdS/CFT correspondence and second as an effective gravity theory in one lower dimension coupled to a radiation background. In the limit where the number of interface degrees of freedom becomes large, we obtain an effective theory on appropriate branes that replace the deep interior region in the bulk, coined the shadow region. In this effective theory, we also identify the island of the radiation entanglement wedge and verify the newly proposed quantum extremization method. Our model clarifies that double holography with gravity in two higher dimensions can be realized in a concrete and consistent way and that the occurrence of islands is natural in one higher dimension. Furthermore, our model reveals that there can be a transitional behavior of the Page curve before the Page time, which is related to the emergence of new matter degrees of freedom on the branes.