An alternative to purification in CFT

TL;DR

This paper introduces a novel method in conformal field theories to construct pure entangled states by subtracting undetectable regions, linking the von Neumann entropy to holographic duals and revealing phase transitions.

Contribution

It presents a new approach to purify bipartite states in CFTs by subtraction, connecting entanglement entropy to holographic entanglement wedge cross-sections.

Findings

von Neumann entropy equals entanglement wedge cross-section

Constructed entanglement entropy exhibits phase transition

Limits recover ordinary entanglement entropies in QFTs

Abstract

In conformal field theories, in contrast to \emph{adding} some auxiliary states into the bipartite mixed state $\rho_{AB}$ as the usual purifications do, we show a pure entangled state $\psi_{AB}$ can be constructed by \emph{subtracting} the undetectable regions. In this pure state $\psi_{AB}$, the von Neumann entropy $S_{\text{vN}}(A)$ naturally captures quantum entanglement between $A$ and $B$. We verify that $S_{\text{vN}}(A)$ is equal to the entanglement wedge cross-section $E_{W}$ in AdS spacetime, which is conjectured to be the holographic dual of the entanglement of purification. We show such constructed entanglement entropy has a phase transition. The ordinary entanglement entropies of critical and non-critical QFTs are simply limits of the two phases.