  In this paper we analyze the problem of "apparent" superluminal signalling
and retrocausation that can appear for particle detector models when
considering non-compactly supported field-detector interactions in quantum
field theory in curved spacetimes and in relativistic quantum information
protocols. For this purpose, we define a signalling estimator based on an
adapted version of the quantum Fisher information to perturbative regimes. This
allows us to study how the internal dynamics of the detectors (for example the
gap between the detector energy levels) have an impact on the ability of a
particle detectors to communicate with one another. Moreover, we show that,
very generally, even for detectors with infinite tails in space and time, if
the tails decay exponentially, one can define an effective lightcone, outside
of which signalling is negligible. This provides concrete evidence supporting
the use of non-compact (but exponentially decaying) detector smearings in
protocols of relativistic quantum information.
