  Dynamics of many-qubit systems, that may correspond to computational
processing with quantum systems, can be efficiently and generally approximated
by a sequence of two- and single-qubit gates. In practical applications,
however, a quantum gate prepared as a unitary transformation may appear as a
noisy channel and consequently may inhibit quantum advantages. In this work, we
apply the scheme of channel discrimination to detect if a quantum gate that is
actually realized is unitary or noisy. We show that a two-qubit unitary
transformation and its noisy counterpart can be optimally discriminated by
local resources, without the necessity of creating entanglement repeatedly. It
is also shown that the scheme can be applied to estimation of the fraction of
noise existing in quantum gates.
