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arXiv · 2604.05815

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

Raúl Arias·Agustín Tamis
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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.

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Raúl Arias, Agustín Tamis. 2026-04-07. Probing the Factorized Island Branch with the Capacity of Entanglement in JT Gravity. https://doi.org/10.1103/rw5x-4t3d

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