  While the standard approach to quantum systems studies length preserving
linear transformations of wave functions, the Markov picture focuses on trace
preserving operators on the space of Hermitian (self-adjoint) matrices. The
Markov approach extends the standard one and provides a refined analysis of
measurements and quantum Markov chains. In particular, Bell's inequality
becomes structurally clear. It turns out that hidden state models are natural
in the Markov context. In particular, a violation of Bell's inequality is seen
to be compatible with the existence of hidden states. The Markov model moreover
clarifies the role of the "negative probabilities" in Feynman's analysis of the
EPR paradox.
