Extremal black hole

Routing protocol

Entropy (arrow of time)

Quantum entanglement

Physics

Unitarity

Paleontology

Context (archaeology)

Computer network

Theoretical physics

Computer science

String theory

Black hole (networking)

Quantum mechanics

Brane

Routing (electronic design automation)

Quantum

Biology

Black brane

Link-state routing protocol

Journal of High Energy Physics

We compute holographic entanglement entropy for subregions of a BCFT thermal state living on a nongravitating black hole background. The system we consider is doubly holographic and dual to an eternal black string with an embedded Karch-Randall brane that is parameterized by its angle. Entanglement islands are conventionally expected to emerge at late times to preserve unitarity at finite temperature, but recent calculations at zero temperature have shown such islands do not exist when the brane lies below a critical angle. When working at finite temperature in the context of a black string, we find that islands exist even when the brane lies below the critical angle. We note that although these islands exist when they are needed to preserve unitarity, they are restricted to a finite connected region on the brane which we call the atoll. Depending on two parameters -- the size of the subregion and the brane angle -- the entanglement entropy either remains constant in time or follows a Page curve. We discuss this rich phase structure in the context of bulk reconstruction.

Entanglement phase structure of a holographic BCFT in a black hole background