Ratchets are spatially asymmetric devices in which particles can move on
average in one direction in the absence of external net forces or gradients.
This is made possible by the rectification of fluctuations, which also provide
the energy for the process. Interest in the physics of ratchets was revived in
recent years when it emerged that the ratchet principle may be a suitable
physical model for ‘molecular motors’, which are central to many
fundamental biological processes, such as intracellular transport or muscle
contraction. Most ratchets studied so far have relied on classical effects,
but recently ‘quantum ratchets’, involving quantum effects, have
also been studied. In the present article it is pointed out that semiconductor
or metal nanostructures are very suitable systems for the realisation of
experimental quantum ratchets. Recent experimental studies of a quantum
ratchet based on an asymmetric quantum dot are reviewed.
Inelastic transitions and counterflow tunneling in double-dot quantum ratchets
