Collision-Induced Internal-Motional Entanglement in Two Flying Atoms

In ultracold-atom experiments using optical lattices or optical tweezer arrays, atomic motion is often treated as frozen, allowing internal-state coherence can be studied independently [1,2]. At the same time, there is growing interest in regimes where atomic motion itself influences internal-state dynamics through state-dependent forces or interactions [3,4]. Here, we access such a regime by launching two ⁸⁷Rb atoms toward each other with controlled velocities of order ~1 m/s [5]. During the collision, relative motion must be treated as a dynamical degree of freedom. We perform two-atom Ramsey interferometry while varying the interatomic separation in the range b = 6-10 μm. We observe that the Ramsey fringe visibility is reduced during the collision. Importantly, this reduction does not occur as a simple monotonic decay: instead, the visibility exhibits a damped oscillatory behavior, reflecting the interplay between internal-state correlations and internal–motional entanglement. By modeling the system in an extended Hilbert space that explicitly includes both internal and motional degrees of freedom [6], we analyze this behavior and identify internal–motional entanglement as the dominant mechanism underlying the observed visibility loss, rather than irreversible decoherence. Our results highlight the essential role of atomic motion in determining internal-state coherence during atomic collisions. More generally, flying atoms provide a natural platform for probing nonadiabatic quantum dynamics driven by motion-induced time-dependent interactions.

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