Sequential product of quantum effects

Unsharp quantum measurements can be modelled by means of the class $\mathcal{E}(\mathcal{H})$ of positive contractions on a Hilbert space $\mathcal{H}$, in brief, quantum effects. For $A,B\in\mathcal{E}(\mathcal{H})$the operation of sequential product $A\circ B=A^{1/2}BA^{1/2}$ was proposed as a model for sequential quantum measurements. We continue these investigations on sequential product and answer positively the following question: the assumption $A\circ B\geq B$implies $AB=BA=B$.

Then we propose a geometric approach of quantum effects and their sequential product by means of contractively contained Hilbert spaces and operator ranges. This framework leads us naturally to consider lattice properties of quantum effects, sums and intersections, and to prove that the sequential product is left distributive with respect to the intersection.