  Classical random walks and Markov processes are easily described by Hopf
algebras. It is also known that groups and Hopf algebras (quantum groups) lead
to classical and quantum diffusions. We study here the more primitive notion of
a quantum random walk associated to a general Hopf algebra and show that it has
a simple physical interpretation in quantum mechanics. This is by means of a
representation theorem motivated from the theory of Kac algebras: If $H$ is any
Hopf algebra, it may be realised in $\Lin(H)$ in such a way that $\Delta
h=W(h\tens 1)W^{-1}$ for an operator $W$. This $W$ is interpreted as the time
evolution operator for the system at time $t$ coupled quantum-mechanically to
the system at time $t+\delta$. Finally, for every Hopf algebra there is a dual
one, leading us to a duality operation for quantum random walks and quantum
diffusions and a notion of the coentropy of an observable. The dual system has
its time reversed with respect to the original system, leading us to a CTP-type
theorem.
