  We consider quantum computation efficiency from a new perspective. The
efficiency is reduced to its classical counterpart by imposing the
semi-classical limit. We show that this reduction is caused by the fact that
any elementary quantum logic operation (gate) suffers information loss during
transition to its classical analogue. Amount of the information lost is
estimated for any gate from the complete set. The largest loss is obtained for
non-commuting gates that allows to consider them as quantum computational
speed-up resource. Our method allows to quantify advantages of quantum
computation as compared to the classical one by direct analysis of the basic
logic involved. The obtained results are illustrated by application to quantum
discrete Fourier transform and Grover search algorithms.
