  Classical sliding friction is dominated by the slip-stick mechanism, where
contacts between two bodies are alternately formed and sheared as the bodies
move past each other. When the interface between two bodies is perfectly
smooth, classical friction goes to zero, a state called superlubricity. In this
limit, much weaker mechanisms, called quantum friction are predicted. These
mechanisms are based on an exchange of elementary excitations between two
bodies moving relatively to each other. For the friction to be called quantum,
the excitations must arise from the quantum mechanical description of the
bodies. Photons and phonons are such excitations. Friction results from an
irreversible momentum transfer from these excitations to the bodies, affecting
their motion. We measured the friction force between crystallites of solid 4He
moving relative to each other at low temperatures. Our data are in excellent
agreement with the concept of quantum phonon friction.
