We show that a quantum control procedure on a two-level system including
dissipation gives rise to a semi-group corresponding to the Lie algebra
semi-direct sum gl(3,R)+R^3. The physical evolution may be modelled by the
action of this semi-group on a 3-vector as it moves inside the Bloch sphere, in
the Bloch ball.Comment: 4 pages. Proceedings of Group 24, Paris, July, 200