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Abstract

In an ensemble of identical atoms, cooperative effects like sub- and superradiance may alter the decay rates and the energies of specific transitions may be shifted from the single-atom value by the so-called collective Lamb shift. While such effects in ensembles of two-level systems are by now well understood, realistic multilevel systems are more difficult to handle. In this work we show that in a system of atoms under the action of an external magnetic field, the collective contribution to the level shifts can amount to sizable quantitative and qualitative deviations from the single-atom Zeeman splitting picture. We develop a formalism to describe single-photon superradiance in multilevel systems and identify three-parameter regimes, two of which present measurable deviations in the radiation spectrum compared to the case of single-atom magnetic-field-induced splitting.