  In this doctoral thesis we provide one of the first theoretical expositions
on a quantum effect known as entanglement in time. It can be viewed as an
interdependence of quantum systems across time, which is stronger than could
ever exist between classical systems. We explore this temporal effect within
the study of quantum information and its foundations as well as through
relativistic quantum information. An original contribution of this thesis is
the design of one of the first applications of entanglement in time.
