  We consider an atomic beam reservoir as a source of quantum noise. The atoms
are modelled as two-state systems and interact one-at-a-time with the system.
The Floquet operators are described in terms of the Fermionic creation,
annihilation and number operators associated with the two-state atom. In the
limit where the time between interactions goes to zero and the interaction is
suitably scaled, we show that we may obtain a causal (that is, adapted) quantum
stochastic differential equation of Hudson-Parthasarathy type, driven by
creation, annihilation and conservation processes. The effect of the Floquet
operators in the continuous limit is exactly captured by the Holevo ordered
form for the stochastic evolution.
