  Relativistic effects affect nearly all notions of quantum information theory.
The vacuum behaves as a noisy channel, even if the detectors are perfect. The
standard definition of a reduced density matrix fails for photon polarization
because the transversality condition behaves like a superselection rule. We can
however define an effective reduced density matrix which corresponds to a
restricted class of positive operator-valued measures. There are no pure photon
qubits, and no exactly orthogonal qubit states. Reduced density matrices for
the spin of massive particles are well-defined, but are not covariant under
Lorentz transformations. The spin entropy is not a relativistic scalar and has
no invariant meaning. The distinguishability of quantum signals and their
entanglement depend on the relative motion of observers.
