  Crowd synchrony, which corresponds to the synchronization of different and
independent oscillators that interact with each other via a common
intermediate, is ubiquitous in many fields. Here, we experimentally demonstrate
the effect of crowd synchrony, analogous to that of the Millennium Bridge, by
resorting to coupled lasers. When the number of lasers is below a critical
number, there is no synchronization, but after reaching the critical number,
the lasers instantaneously synchronize. We show that the synchronization of the
lasers as a function of their number follows a first-order-like transition, and
that our experimental results are in good agreement with those predicted by
theoretical models.
