  We revisit the properties of qubits under Lorentz transformations and, by
considering Lorentz invariant quantum states in the Heisenberg formulation,
clarify some misleading notation that has appeared in the literature on
relativistic quantum information theory. We then use this formulation to
consider the transformation properties of qubits and density matrices under
space-time and gauge transformations. Finally we use our results to understand
the behaviour of entanglement between different partitions of quantum systems.
Our approach not only clarifies the notation, but provides a more intuitive and
simple way of gaining insight into the behaviour of relativistic qubits. In
particular, it allows us to greatly generalize the results in the current
literature as well as substantially simplifying the calculations that are
needed.
