Harvesting Entanglement with Detectors Freely Falling into a Black Hole

TL;DR

This study explores how detectors falling into a black hole can extract entanglement and information from the vacuum, revealing that correlations are mainly due to relative motion and can occur even across the event horizon.

Contribution

First investigation of entanglement harvesting protocols for free-falling detectors in a black hole spacetime, highlighting the kinematic origin of correlations and the absence of an entanglement shadow.

Findings

Correlations are largely kinematic, driven by relative velocities.

Entanglement can be harvested even when detectors are causally disconnected.

No entanglement shadow near the horizon for free-falling detectors.

Abstract

We carry out the first investigation of the entanglement and mutual information harvesting protocols for detectors freely falling into a black hole. Working in $(1+1)$-dimensional Schwarzschild black hole spacetime, we consider two pointlike Unruh-DeWitt (UDW) detectors in different combinations of free-falling and static trajectories. Employing a generalization of relative velocity suitable for curved spacetimes, we find that the amount of correlations extracted from the black hole vacuum, at least outside the near-horizon regime, is largely kinematic in origin (i.e. it is mostly due to the relative velocities of the detectors). Second, correlations can be harvested purely from the black hole vacuum even when the detectors are causally disconnected by the event horizon. Finally, we show that the previously known `entanglement shadow' near the horizon is indeed absent for the case of two free-falling-detectors, since their relative gravitational redshift remains finite as the horizon is crossed, in accordance with the equivalence principle.