  In a recent Letter, Avron et. al (math-ph/0105011) introduced a notion of
optimal quantum pumps. These are adiabatic quantum pumps which work without
dissipation. In particular, they produce neither entropy nor noise. In the
present Comment we show that in the absence of magnetic field optimal quantum
pumps always have a vanishing transmission coefficient. Such `quantum pumps' do
not make use of Quantum Mechanics since all tunneling or interference effects
are banned by vanishing of the transmission coefficient. We leave it as an
outstanding question whether genuine optimal quantum pumps with nonvanishing
transmission coefficient can be constructed by making use of the magnetic
field.
