The incompleteness of relativistic Spekkens' contextuality

We show that the operational definition of contextuality introduced by Spekkens is, in general, not Lorentz invariant. Specifically, we consider particle states with both spin and momentum, apply a Lorentz transformation to obtain the states in a new inertial frame, and then trace out the momentum degrees of freedom in both frames. We find that spin states that are contextual in one frame can become non-contextual in the other. Hence, the Spekkens' notion of contextuality, when restricted to spin states only, is a frame-dependent concept. We apply our results to predict a novel relativistic effect concerning the task of discriminating between two quantum states. We show that the probability of success for a moving observer exceeds that of an observer at rest.