  We show that a quantum particle subjected to a positive force in one path of
a Mach-Zehnder interferometer and a null force in the other path may receive a
negative average momentum transfer when it leaves the interferometer by a
particular exit. In this scenario, an ensemble of particles may receive an
average momentum in the opposite direction of the applied force due to quantum
interference, a behavior with no classical analogue. We discuss some
experimental schemes that could verify the effect with current technology, with
electrons or neutrons in Mach-Zehnder interferometers in free space and with
atoms from a Bose-Einstein condensate.
