  Any evolution described by a completely positive trace-preserving linear map
can be imagined as arising from the interaction of the evolving system with an
initially uncorrelated ancilla. The interaction is given by a joint unitary
operator, acting on the system and the ancilla. Here we study the properties
such a unitary operator must have in order to force the choice of a physical-
that is, positive-state for the ancilla if the end result is to be a
physical-that is, completely positive-evolution of the system.
