  Nonlinear oscillations and synchronisation phenomena are ubiquitous in
nature. We study the synchronization of self oscillating magneto-optically
trapped cold atoms to a weak external driving. The oscillations arise from a
dynamical instability due the competition between the screened magneto-optical
trapping force and the inter-atomic repulsion due to multiple scattering of
light. A weak modulation of the trapping force allows the oscillations of the
cloud to synchronize to the driving. The synchronization frequency range
increases with the forcing amplitude. The corresponding Arnold tongue is
experimentally measured and compared to theoretical predictions. Phase-locking
between the oscillator and drive is also observed.
