  We introduce a notion of universal preparability for a state of a system,
more precisely: for a normal state on a von Neumann algebra. It describes a
situation where from an arbitrary initial state it is possible to prepare a
target state with arbitrary precision by a repeated interaction with a sequence
of copies of another system. For $\mathcal{B}(\mathcal{H})$ we give criteria
sufficient to ensure that all normal states are universally preparable which
can be verified for a class of non-commutative birth and death processes
realized, in particular, by the interaction of a micromaser with a stream of
atoms. As a tool the theory of tight sequences of states and of stationary
states is further developed and we show that in the presence of stationary
faithful normal states universal preparability of all normal states is
equivalent to asymptotic completeness, a notion studied earlier in connection
with the scattering theory of non-commutative Markov processes.
