  Conditional distributions, as defined by the Markov category framework, are
studied in the setting of matrix algebras (quantum systems). Their construction
as linear unital maps are obtained via a categorical Bayesian inversion
procedure. Simple criteria establishing when such linear maps are positive are
obtained. Several examples are provided, including the standard EPR scenario,
where the EPR correlations are reproduced in a purely compositional
(categorical) manner. A comparison between the Bayes map, the Petz recovery
map, and the Leifer-Spekkens acausal belief propagation is provided,
illustrating some similarities and key differences.
