Introduction
In standard quantum mechanics a physical system is associated with a Hilbert space . States of the system at a fixed time are represented by normalised vectors in the space and physical observables are represented by Hermitian operators on .
A physical proposition about the system at a fixed time can be represented by a projection operator on (See quantum logic). This representation links together the lattice operations in the lattice of logical propositions and the lattice of projection operators on a Hilbert space (See quantum logic).
The HPO formalism is a natural extension of these ideas to propositions about the system that are concerned with more than one time.
Read more about this topic: HPO Formalism
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