Ephemeral Islands, Plunging Quantum Extremal Surfaces and BCFT channels

TL;DR

This paper studies entanglement entropy dynamics in black hole and BCFT models, revealing non-monotonic behavior, island phenomena, and connections to quasiparticle descriptions, with implications for understanding quantum extremal surfaces.

Contribution

It demonstrates the matching of entropy evolution between black hole JT gravity and free fermion BCFT, highlighting island and OPE channel phenomena in a high-temperature regime.

Findings

Non-monotonic entropy evolution matches between models.

Quantum extremal surfaces can emerge and plunge back inside the horizon.

Entropy reaches equilibrium with a no-island saddle.

Abstract

We consider entanglement entropies of finite spatial intervals in Minkowski radiation baths coupled to the eternal black hole in JT gravity, and the related problem involving free fermion BCFT in the thermofield double state. We show that the non-monotonic entropy evolution in the black hole problem precisely matches that of the free fermion theory in a high temperature limit, and the results have the form expected for CFTs with quasiparticle description. Both exhibit rich behaviour that involves at intermediate times, an entropy saddle with an island in the former case, and in the latter a special class of disconnected OPE channels. The quantum extremal surfaces start inside the horizon, but can emerge from and plunge back inside as time evolves, accompanied by a characteristic dip in the entropy also seen in the free fermion BCFT. Finally an entropy equilibrium is reached with a no-island saddle.