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

TL;DR

This paper compares the efficiency of entangled and non-entangled two-photon states in inducing simultaneous two-atom excitations, highlighting the importance of frequency anti-correlation over entanglement.

Contribution

It demonstrates that non-entangled correlated states can match entangled states in excitation efficiency, emphasizing frequency anti-correlation as the key factor.

Findings

Entangled two-photon states can significantly enhance excitation probability.

Non-entangled correlated states have comparable efficiencies to entangled states.

Frequency anti-correlation, not entanglement, is crucial for excitation enhancement.

Abstract

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