title: Cooperative Lamb shift in a mesoscopic atomic array.
abstract: According to quantum electrodynamics, the exchange of virtual photons in a system of identical quantum emitters causes a shift of its energy levels. Such shifts, known as cooperative Lamb shifts, have been studied mostly in the near-field regime. However, the resonant electromagnetic interaction persists also at large distances, providing coherent coupling between distant atoms. Here, we report a direct spectroscopic observation of the cooperative Lamb shift of an optical electric-dipole transition in an array of Sr(+) ions suspended in a Paul trap at inter-ion separations much larger than the resonance wavelength. By controlling the precise positions of the ions, we studied the far-field resonant coupling in chains of up to eight ions, extending to a length of 40  μm. This method provides a novel tool for experimental exploration of cooperative emission phenomena in extended mesoscopic atomic arrays.
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel.
authors.forename: Z
authors.initials: Z
authors.lastname: Meir
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel.
authors.forename: O
authors.initials: O
authors.lastname: Schwartz
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel and Department of Chemical Physics, Weizmann Institute of Science, Rehovot 7610001, Israel.
authors.forename: E
authors.initials: E
authors.lastname: Shahmoon
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel.
authors.forename: D
authors.initials: D
authors.lastname: Oron
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 7610001, Israel.
authors.forename: R
authors.initials: R
authors.lastname: Ozeri