  We recently showed that if a massive (or charged) body is put in a quantum
spatial superposition, the mere presence of a black hole in its vicinity will
eventually decohere the superposition. In this paper we show that, more
generally, decoherence of stationary superpositions will occur in any spacetime
with a Killing horizon. This occurs because, in effect, the long-range field of
the body is registered on the Killing horizon which, we show, necessitates a
flux of "soft horizon gravitons/photons" through the horizon. The Killing
horizon thereby harvests "which path" information of quantum superpositions and
will decohere any quantum superposition in a finite time. It is particularly
instructive to analyze the case of a uniformly accelerating body in a quantum
superposition in flat spacetime. As we show, from the Rindler perspective the
superposition is decohered by "soft gravitons/photons" that propagate through
the Rindler horizon with negligible (Rindler) energy. We show that this
decoherence effect is distinct from--and larger than--the decoherence resulting
from the presence of Unruh radiation. We further show that from the inertial
perspective, the decoherence is due to the radiation of high frequency
(inertial) gravitons/photons to null infinity. (The notion of gravitons/photons
that propagate through the Rindler horizon is the same notion as that of
gravitons/photons that propagate to null infinity.) We also analyze the
decoherence of a spatial superposition due to the presence of a cosmological
horizon in de Sitter spacetime. We provide estimates of the decoherence time
for such quantum superpositions in both the Rindler and cosmological cases.
Although we explicitly treat the case of spacetime dimension $d=4$, our
analysis applies to any dimension $d \geq 4$.
