Probing the Factorized Island Branch with the Capacity of Entanglement in JT Gravity

TL;DR

This paper demonstrates that the capacity of entanglement reveals additional structure in JT gravity's island saddle, beyond what von Neumann entropy captures, especially in the replica geometry.

Contribution

It shows that the capacity of entanglement detects finite-$n$ information in the factorized island saddle of JT gravity, beyond the entropy.

Findings

Capacity acquires a correction at late times and high temperature.

Entropy remains unchanged at the first nontrivial order.

Replica data contains physically meaningful information beyond the entropy.

Abstract

Black hole islands are usually diagnosed through the von Neumann entropy, but the full replica saddle contains more information than survives in the limit $n \to 1$. In this paper we show that the capacity of entanglement can detect that extra structure already within the controlled factorized island branch of JT gravity coupled to a large-$c$ bath. In the late-time high-temperature regime, the entropy plateau remains unchanged at the first nontrivial order, while the capacity acquires a definite correction. This provides a sharp semiclassical example in which nearby replica data are physically meaningful even when the entropy itself appears rigid. Our result shows that the factorized island saddle already carries finite-$n$ information beyond the entropy, and that the capacity is a natural observable for exposing it. More broadly, it highlights that the physics of island saddles is not exhausted by the $n=1$ limit: the surrounding replica geometry can contain additional, and observable, information about how the semiclassical saddle is assembled.