  A theoretical model of a quantum device which can factorize any number N in
two steps i.e. by preparing an input state and performing a measurement is
discussed. The analysis reveals that the duration of state preparation and
measurement is proportional to N while the energy consumption grows like log N.
These results suggest the existence of Heisenberg-type relation putting limits
on the efficiency of a quantum computer in terms of a total computation time, a
total energy consumption and a classical complexity of the problem.
