  A distance measure is presented between two unitary propagators of quantum
systems of differing dimensions along with a corresponding method of
computation. A typical application is to compare the propagator of the actual
(real) process with the propagator of the desired (ideal) process; the former
being of a higher dimension then the latter. The proposed measure has the
advantage of dealing with possibly correlated inputs, but at the expense of
working on the whole space and not just the information bearing part as is
usually the case, i.e., no partial trace operation is explicitly involved. It
is also shown that the distance measure and an average measure of channel
fidelity both depend on the size of the same matrix: as the matrix size
increases, distance decreases and fidelity increases.
