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Recent work by Danielson, Satishchandran, and Wald (DSW) has shown that blackholes -- and, in fact, Killing horizons more generally -- impart a fundamentalrate of decoherence on all nearby quantum superpositions. The effect can beunderstood from measurement and causality: An observer (Bob) in the black holeshould be able to disturb outside quantum superpositions by measuring theirsuperposed gravitational fields, but since his actions cannot (by causality)have this effect, the superpositions must automatically disturb themselves. DSWcalculated the rate of decoherence up to an unknown numerical factor fordistant observers in Schwarzschild spacetime, Rindler observers in flatspacetime, and static observers in de Sitter spacetime. Working inelectromagnetic and Klein-Gordon analogs, we flesh out and generalize theircalculation to derive a general formula for the precise decoherence rate forKilling observers near bifurcate Killing horizons. We evaluate the rate inclosed form for an observer at an arbitrary location on the symmetry axis of aKerr black hole. This fixes the numerical factor in the distant-observerSchwarzschild result, while allowing new exploration of near-horizon and/ornear-extremal behavior. In the electromagnetic case we find that thedecoherence vanishes entirely in the extremal limit, due to the "Black holeMeissner effect" screening the Coulomb field from entering the black hole. Thissupports the causality picture: Since Bob is unable to measure the field of theoutside superposition, no decoherence is necessary -- and indeed none occurs.

Decoherence from Horizons: General Formulation and Rotating Black Holes