  Friction between ordered, atomically smooth surfaces at the nanoscale
(nanofriction) is often governed by stick-slip processes. To test long-standing
atomistic models of such processes, we implement a synthetic nanofriction
interface between a laser-cooled Coulomb crystal of individually addressable
ions as the moving object, and a periodic light-field potential as the
substrate. We show that stick-slip friction can be tuned from maximal to nearly
frictionless via arrangement of the ions relative to the substrate. By varying
the ion number, we also show that this strong dependence of friction on the
structural mismatch, as predicted by many-particle models, already emerges at
the level of two or three atoms. This model system enables a microscopic and
systematic investigation of friction, potentially even into the quantum
many-body regime.
