  We propose a formulation of quantum mechanics in an extended Fock space in
which a tensor product structure is applied to time. Subspaces of histories
consistent with the dynamics of a particular theory are defined by a direct
quantum generalization of the corresponding classical action. The
diagonalization of such quantum actions enables us to recover the predictions
of conventional quantum mechanics and reveals an extended unitary equivalence
between all physical theories. Quantum correlations and coherent effects across
time and between distinct theories acquire a rigorous meaning, which is encoded
in the rich temporal structure of physical states. Connections with modern
relativistic schemes and the path integral formulation also emerge.
