  For a machine to be useful in practice, it preferably has to meet two
requirements: namely, (i) to be able to perform work under a load and (ii) its
operational regime should ideally not depend on the time at which the machine
is switched-on. We devise a minimal setup, consisting of two atoms only, for an
ac-driven quantum motor which fulfills both these conditions. Explicitly, the
motor consists of two different interacting atoms placed into a ring-shaped
periodic optical potential -- an optical "bracelet" --, resulting from the
interference of two counter-propagating Laguerre-Gauss laser beams. This
bracelet is additionally threaded by a pulsating magnetic flux. While the first
atom plays a role of a quantum "carrier", the second serves as a quantum
"starter", which sets off the "carrier" into a steady rotational motion. For
fixed zero-momentum initial conditions the asymptotic carrier velocity
saturates to a unique, nonzero value which becomes increasingly independent on
the starting time with increasing "bracelet"-size. We identify the quantum
mechanisms of rectification and demonstrate that our quantum motor is able to
perform useful work.
