 The concept of correlation between commuting projections \(A,B\) in \({\cal P} ({\cal N})\) and the notion of common cause of such correlations as a projection C commuting with both A and B and satisfying the (analogues) of the  four Reichenbachian conditions (2)–(5)  make perfect sense in quantum probability spaces. So do the concepts of common cause (in)completeness, common cause completability (with a suitably defined embedding of orthomodular lattices), measure theoretic atoms and measure theoretic non-atomicity.