Limits on the observable dynamics of mixed states

It is shown that the observability of a large class of operations on mixed states is fundamentally limited. We consider trace-preserving, unital operations. This class includes unitary and perfect premeasurement operations. An upper bound on the trace distance between an untransformed state and a state transformed by one of these operations is derived. The bound is dependent only on the purity of the state. In the case of maximal mixedness, the bound implies all operations of this class are unobservable.