  Collective phenomena emerging from non-linear interactions between multiple
oscillators, such as synchronization and frequency locking, find applications
in a wide variety of fields. Optomechanical resonators, which are intrinsically
non-linear, combine the scientific assets of mechanical devices with the
possibility of long distance controlled interactions enabled by travelling
light. Here we demonstrate light-mediated frequency locking of three distant
nano-optomechanical oscillators positioned in a cascaded configuration. The
oscillators, integrated on a chip along a coupling waveguide, are optically
driven with a single laser and oscillate at gigahertz frequency. Despite an
initial frequency disorder of hundreds of kilohertz, the guided light locks
them all with a clear transition in the optical output. The experimental
results are described by Langevin equations, paving the way to scalable
cascaded optomechanical configurations.
