  Complete positivity is a ubiquitous assumption in the study of quantum
systems interacting with the environment, but the lack of complete positivity
of a quantum evolution (called the "negativity") can be used as a measure of
the system-bath coupling and correlation. The negativity can be computed from
the Choi representation of a channel, is always defined and bounded, and can be
used to understand environmentally induced noise in a quantum system.
