Optimal quantum pumps have a vanishing transmission coefficient
  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. 
A. Alekseev