  Polarisable atoms and molecules experience the Casimir-Polder force near
magnetoelectric bodies, a force that is induced by quantum fluctuations of the
electromagnetic field and the matter. Atoms and molecules in relative motion to
a magnetoelectric surface experience an additional, velocity-dependent force.
We present a full quantum-mechanical treatment of this force and identify a
generalised Doppler effect, the time delay between photon emission and
reabsorption, and the Roentgen interaction as its three sources. For
ground-state atoms, the force is very small and always decelerating, hence
commonly known as quantum friction. For atom and molecules in electronically
excited states, on the contrary, both decelerating and accelerating forces can
occur depending on the magnitude of the atomic transition frequency relative to
the surface plasmon frequency.
