Horizons Decohere Quantum Superpositions

We show that if a massive (or charged) body is put in a quantum superposition of spatially separated states in the vicinity of a black hole or cosmological horizon, the mere presence of the horizon will eventually destroy the coherence of the superposition.

This occurs because, in effect, the long-range fields sourced by the superposition register on the horizon which forces the emission of entangling “soft gravitons / photons” through the horizon. This enables the horizon to harvest “which path” information about the superposition. We estimate the decoherence time for such quantum superpositions in the presence of (Killing) horizons.

Finally, we compute the quantum information content (viz. fidelity) of the superposed graviton / photon field in the black hole interior, and give a direct relationship between the information accessible by any optimal observer(s) in the interior and the decoherence of the superposition in the exterior.