The Expectation Monad in Quantum Foundations

Bart Jacobs
(Radboud University Nijmegen)
Jorik Mandemaker
(Radboud University Nijmegen)

The expectation monad is introduced abstractly via two composable adjunctions, but concretely captures measures. It turns out to sit in between known monads: on the one hand the distribution and ultrafilter monad, and on the other hand the continuation monad. This expectation monad is used in two probabilistic analogues of fundamental results of Manes and Gelfand for the ultrafilter monad: algebras of the expectation monad are convex compact Hausdorff spaces, and are dually equivalent to so-called Banach effect algebras. These structures capture states and effects in quantum foundations, and also the duality between them. Moreover, the approach leads to a new re-formulation of Gleason's theorem, expressing that effects on a Hilbert space are free effect modules on projections, obtained via tensoring with the unit interval.

In Bart Jacobs, Peter Selinger and Bas Spitters: Proceedings 8th International Workshop on Quantum Physics and Logic (QPL 2011), Nijmegen, Netherlands, October 27-29, 2011, Electronic Proceedings in Theoretical Computer Science 95, pp. 143–182.
Published: 1st October 2012.

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