Observation of the Gordon-Ashkin momentum diffusion with ultracold atoms scattered by a standing wave

In 1980, Gordon and Ashkin predicted that the momentum diffusion of an atom in a light field is maximized where the spontaneous emission noise is minimized—specifically at the nodes of a standing wave where the light intensity vanishes. We investigate this counterintuitive phenomenon by scattering a periodic array of Dysprosium atoms with a comensurate optical standing wave. Utilizing the narrow-line transition at 626 nm, a one-dimensional array of atoms was prepared with a 626 nm optical lattice detuned by 10^6 linewidth, then scattered by a resonant standing wave. We reveal that atoms initially localized at the intensity nodes undergo rapid momentum diffusion driven by the large gradients of the electric field amplitude. Our simulations confirm that the initial momentum spread is dominated by the coherent evolution of the wavepacket in the dark regions of the potential, directly verifying the Gordon-Ashkin prediction that the strongest mechanical effects of light can occur in the absence of photon scattering. These results provide a microscopic validation of the fundamental mechanisms governing laser cooling and trapping and highlight the complex role of measurement backaction in open quantum systems.