  Dissipative processes in physics are usually associated with non-unitary
actions. However, the important resource of entanglement is not invariant under
general unitary transformations, and is thus susceptible to unitary
"dissipation". In this note we discuss both unitary and non-unitary dissipative
processes, showing that the former is ultimately of value, since reversible,
and enables the production of entanglement; while even in the presence of the
latter, more conventional non-unitary and non-reversible, process there exist
nonetheless invariant entangled states.
