Spatial Markov Semigroups Admit Hudson-Parthasarathy Dilations

This is the updated version of a preprint from September 2008. It presents, for the first time, the result that (normal, strongly continuous) Markov semigroups on $B(G)$ ($G$ a separable Hilbert space) admit a {\it Hudson-Parthasarathy dilation} (that is, a dilation to a cocycle perturbation of a {\it noise}) if and only the Markov semigroup is {\it spatial} (that is, if it dominates an elementary CP-semigroup). The proof is by {\it general abstract nonsense} (taken from Arveson's classification of $E_0$--semigroups on $B(H)$ by Arveson systems up to cocycle conjugacy) and not, as usual, by constructing the cocycle as a solution of a quantum stochastic differential equation in the sense of Hudson and Parthasarathy. All other results that make similar statements (especially, Skeide [Ske16]) for more general $C^*$--algebras) have been proved later by suitable adaptations of the methods exposed here. (They use Hilbert module techniques, which we carefully avoid here in order to make the result available without any appeal to Hilbert modules.