  We present a photonic wave packet construction which is immune against the
decoherence effects induced by the action of the Lorentz group. The amplitudes
of a pure quantum state representing the wave packet remain invariant
irrespective of the reference frame into which the wave packet has been
transformed. Transmitted information is encoded in the helicity degrees of
freedom of two correlated momentum modes. The helicity encoding is considered
to be particularly suitable for free-space communication. The integral part of
the story is information retrieval on the receiver's side. We employed probably
the simplest possible helicity (polarization) projection measurement originally
studied by Peres and Terno. Remarkably, the same conditions ensuring the
invariance of the wave packet also guarantee perfect distinguishability in the
process of measuring the helicity.
