  Space and time are crucial twins in physics. In quantum mechanics, spatial
correlations already reveal nonclassical features, such as entanglement, and
have bred many quantum technologies. However, the nature of quantum temporal
correlations still remains in vague. In this Letter, based on the
entangled-history formalism, we prove rigorously that temporal correlations are
equivalent to spatial correlations. The effect of temporal correlations
corresponds to a quantum channel. The resulting quantifications and
classifications of quantum temporal correlations are illustrated in a natural
way. Our proposed procedures also show how to determine temporal correlations
completely.
