  Reichenbach's principle asserts that if two observed variables are found to
be correlated, then there should be a causal explanation of these correlations.
Furthermore, if the explanation is in terms of a common cause, then the
conditional probability distribution over the variables given the complete
common cause should factorize. The principle is generalized by the formalism of
causal models, in which the causal relationships among variables constrain the
form of their joint probability distribution. In the quantum case, however, the
observed correlations in Bell experiments cannot be explained in the manner
Reichenbach's principle would seem to demand. Motivated by this, we introduce a
quantum counterpart to the principle. We demonstrate that under the assumption
that quantum dynamics is fundamentally unitary, if a quantum channel with input
A and outputs B and C is compatible with A being a complete common cause of B
and C, then it must factorize in a particular way. Finally, we show how to
generalize our quantum version of Reichenbach's principle to a formalism for
quantum causal models, and provide examples of how the formalism works.
