Ephemeral islands, plunging quantum extremal surfaces and BCFT channels

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