  Whenever several quantum light emitters are brought in proximity with one
another, their interaction with common electromagnetic fields couples them,
giving rise to cooperative shifts in their resonance frequency. Such collective
line shifts are central to modern atomic physics, being closely related to
superradiance on one hand and the Lamb shift on the other. Although collective
shifts have been theoretically predicted more than fifty years ago, the effect
has not been observed yet in a controllable system of a few isolated emitters.
Here, we report a direct spectroscopic observation of the cooperative shift of
an optical electric dipole transition in a system of up to eight Sr ions
suspended in a Paul trap. We study collective resonance shift in the previously
unexplored regime of far-field coupling, and provide the first observation of
cooperative effects in an array of quantum emitters. These results pave the way
towards experimental exploration of cooperative emission phenomena in
mesoscopic systems.
