Entanglement R\'enyi Entropies from Ballistic Fluctuation Theory: the free fermionic case

Understand

The large-scale behaviour of entanglement entropy in finite-density states, in and out of equilibrium, can be understood using the physical picture of particle pairs.

• However, the full theoretical origin of this picture is not fully established yet.
• In this work, we clarify this picture by investigating entanglement entropy using its connection with the large-deviation theory for thermodynamic and hydrodynamic fluctuations.
• We apply the universal framework of Ballistic Fluctuation Theory (BFT), based the Euler hydrodynamics of the model, to correlation functions of \emph{branch-point twist fields}, the starting point for computing R\'enyi entanglement entropies within the replica approach.