  The spin is the prime example of a qubit. Encoding and decoding information
in the spin qubit is operationally well defined through the Stern-Gerlach
set-up in the non-relativistic (i.e., low velocity) limit. However, an
operational definition of the spin in the relativistic regime is missing. The
origin of this difficulty lies in the fact that, on the one hand, the spin gets
entangled with the momentum in Lorentz-boosted reference frames, and on the
other hand, for a particle moving in a superposition of velocities, it is
impossible to "jump" to its rest frame, where spin is unambiguously defined.
Here, we find a quantum reference frame transformation corresponding to a
"superposition of Lorentz boosts," allowing us to transform to the rest frame
of a particle that is in a superposition of relativistic momenta with respect
to the laboratory frame. This enables us to first move to the particle's rest
frame, define the spin measurements there (via the Stern-Gerlach experimental
procedure), and then move back to the laboratory frame. In this way, we find a
set of "relativistic Stern-Gerlach measurements" in the laboratory frame, and a
set of observables satisfying the spin $\mathfrak{su}(2)$ algebra. This
operational procedure offers a concrete way of testing the relativistic
features of the spin, and opens up the possibility of devising quantum
information protocols for spin in the special-relativistic regime.
