  The problem of distinguishing two unitary transformations, or quantum gates,
is analyzed and a function reflecting their statistical distinguishability is
found. Given two unitary operations, $U_1$ and $U_2$, it is proved that there
always exists a finite number $N$ such that $U_1^{\otimes N}$ and $U_2^{\otimes
N}$ are perfectly distinguishable, although they were not in the single-copy
case. This result can be extended to any finite set of unitary transformations.
Finally, a fidelity for one-qubit gates, which satisfies many useful properties
from the point of view of quantum information theory, is presented.
