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We study in this paper the efficiency of different two-photon states of lightto induce the simultaneous excitation of two atoms of different kinds when thesum of the energies of the two photons matches the sum of the energies of thetwo atomic transitions, while no photons are resonant with each individualtransition. We find that entangled two-photon states produced by an atomiccascade are indeed capable of enhancing by a large factor the simultaneousexcitation probability as compared to uncorrelated photons, as predicted someyears ago by Muthukrishnan et al, but that several non-entangled, separable,correlated states, produced either by an atomic cascade or parametric downconversion, or even appropriate combinations of coher- ent states, havecomparable efficiencies. We show that the key ingredient for the increase ofsimultaneous excitation probability is the presence of strong frequencyanti-correlation and not time correlation nor time-frequency entanglement.

Do entangled photons induce two-photon two-atom transitions more efficiently than other states of light ?