  Bell-Kochen-Specker theorem states that a non-contextual hidden-variable
theory cannot completely reproduce the predictions of quantum mechanics. Asher
Peres gave a remarkably simple proof of quantum contextuality in a
four-dimensional Hilbert space of two spin-1/2 particles. Peres's argument is
enormously simpler than that of Kochen and Specker. Peres contextuality
demonstrates a logical contradiction between quantum mechanics and the
noncontextual hidden variable models by showing an inconsistency when assigning
noncontextual definite values to a certain set of quantum observables. In this
work, we present a proof of quantum contextuality in time using the temporal
version of Peres's contextuality argument. In analogy with the two-particle
version of Peres's contextuality argument in the context of spin measurements
at two different locations, we consider here single-particle spin measurements
at two different times t=t1 and t=t2 where the context is provided through the
time separation. Our results can be experimentally verified with the present
quantum technology.
