Boundary duals of bulk detectors

This paper investigates the boundary dual of a local Unruh-DeWitt detector in anti-de Sitter (AdS) space. By leveraging the HKLL (Hamilton-Kabat-Lifschytz-Lowe) reconstruction program, the authors represent a local bulk detector as a smeared operator in the dual boundary conformal field theory (CFT). The study applies this formalism to analyze entanglement harvesting—a protocol where two initially uncorrelated detectors become entangled through interaction with a quantum field—in the Poincaré patch of AdS3. The authors evaluate the two-detector density matrix to quadratic order in the coupling constant, comparing scenarios where a bulk detector is paired with either a boundary detector or a second bulk detector.

The authors successfully construct boundary representations for bulk detectors, which they term HKLL detectors. They find that these detectors possess a nontrivial spacetime smearing that depends on the HKLL kernel. In the limit where the bulk detector approaches the conformal boundary, the HKLL detector reproduces standard CFT harvesting results, confirming the consistency of the dictionary. When both detectors are in the bulk, the authors observe two distinct regimes: one dominated by harvesting correlations and another where causal signalling becomes significant. They also address a potential tension between bulk microcausality and the overlapping support of dual HKLL detectors, showing that bulk locality is recovered through precise cancellations within the boundary integrals.

This work provides an operational dictionary for probing bulk physics using boundary-based protocols. By mapping bulk detectors to boundary operators, the authors enable the use of relativistic quantum information tools to study the emergence of bulk geometry and entanglement structure in holographic theories. This approach offers a new way to probe the interior of AdS spacetimes without requiring the replica trick or subregion entanglement measures, providing a complementary perspective on the holographic dictionary.