  Hydrodynamic synchronization provides a general mechanism for the spontaneous
emergence of coherent beating states in independently driven mesoscopic
oscillators. A complete physical picture of those phenomena is of definite
importance to the understanding of biological cooperative motions of cilia and
flagella. Moreover, it can potentially suggest novel routes to exploit
synchronization in technological applications of soft matter. We demonstrate
that driving colloidal particles in rotating energy landscapes results in a
strong tendency towards synchronization, favouring states where all beads
rotate in phase. The resulting dynamics can be described in terms of activated
jumps with transition rates that are strongly affected by hydrodynamics leading
to an increased probability and lifetime of the synchronous states. Using
holographic optical tweezers we quantitatively verify our predictions in a
variety of spatial configurations of rotors.
