  We address the validity of the single-mode approximation that is commonly
invoked in the analysis of entanglement in non-inertial frames and in other
relativistic quantum information scenarios. We show that the single-mode
approximation is not valid for arbitrary states, finding corrections to
previous studies beyond such approximation in the bosonic and fermionic cases.
We also exhibit a class of wave packets for which the single-mode approximation
is justified subject to the peaking constraints set by an appropriate Fourier
transform.
