When two atoms scatter light as a pair

Light scattered by two atoms can carry information about whether the atoms acted together or separately. If their scattered waves keep a stable phase relationship, the light is called coherent; if atomic motion becomes entangled with the photons and reveals which atom scattered, the light is incoherent. The balance matters in experiments that use light to control or read out ultracold atoms.

In this study, Hanzhen Lin and colleagues asked what happens when two atoms are close enough that their light-induced electric dipoles interact strongly. The experiment focused on lithium-7 pairs in an optical lattice: a repeating pattern of light that confines atoms at individual sites. The researchers prepared sites containing either one atom or a pair, illuminated them with a laser, and measured scattered light. They also released the atoms after illumination and imaged their momentum distributions, which reveal how the atoms were accelerated.

The key difficulty is that tighter confinement does not simply make scattering more coherent. Localizing atoms can reduce motion-related loss of coherence, but very close pairs experience strong dipole-dipole forces. Those forces create a narrow range of separations where the laser becomes resonant with the pair. The authors call this the Franck–Condon region. Scattering from that region can excite relative motion and produce incoherent light. The result, according to the study, is a limited window of pair sizes where coherent scattering remains appreciable: pairs that are too spread out lose coherence through motion, while very tightly confined pairs are strongly affected by dipolar interactions.

To identify the coherent component, the team compared light from pairs with light from single atoms in a lattice, varying the fraction of sites made optically “dark.” In a regular array, light scattered from different sites can cancel through destructive interference. Introducing dark sites disrupts that cancellation in a controlled way, helping the researchers separate coherent scattering from other contributions. They report a collective enhancement factor of two for the coherent scattering per atom in pairs compared with single atoms. This is not a claim that all scattered light doubles: interactions can also increase incoherent scattering, and the coherent fraction depends on confinement and other conditions.

The momentum images offered another view of the interaction. Some pairs were accelerated in the resonant region and appeared as a hollow shell in momentum space, with a characteristic energy set by the laser detuning. Over time, the researchers observed that this region of the pair’s wavefunction could be depleted by ejection and dipolar forces. Fluorescence changed along with the ejection signal, supporting the interpretation that light scattering depends on how much of the pair occupies the resonant region.

The team also varied the atoms’ contact interaction, described by the scattering length. Changing it altered the pair’s probability of being in the resonant region, and therefore changed both fluorescence and the number of ejected atoms. At higher probe-light intensity, they observed a further shift in the magnetic field associated with a Feshbach resonance—a resonance that can be used to tune interactions between ultracold atoms. The authors explain this shift as an effect of the light changing the energy of the atoms’ scattering channel. Their measurements agree with a relatively simple model, but the paper notes that interactions with light in the resonant region are not perturbative, so a full treatment is more complicated.

These results provide a close-up look at processes relevant to photoassociation, in which light helps bring atoms into molecular states, and to techniques that infer atom-number parity from light scattering. They do not establish a general recipe for controlling interactions in every atomic system. The experiment uses a specific lithium setup, and some calculations simplify the atoms’ motion or neglect effects such as photon recoil. The authors also identify further regimes and tests to explore.

The source is an arXiv preprint, not evidence by itself of peer review or independent confirmation. Its reported observations and interpretations should therefore be read as research findings awaiting the scrutiny and context that publication and follow-up work can provide.