Direct weak localization signature with ultracold atoms: the CBS revival

Kilian Müller, Fred Jendrzejewski, Jérémie Richard +5 more

Fred Jendrzejewski, Kilian Müller, Alain Bernard +3 more

Vincent Josse

Fred Jendrzejewski, Kilian Müller, Jérémie Richard +5 more

We report on the direct observation of coherent backscattering (CBS) of ultracold atoms in a quasi-two-dimensional configuration. Launching atoms with a well-defined momentum in a laser speckle disordered potential, we follow the progressive build up of the momentum scattering pattern, consisting of a ring associated with multiple elastic scattering, and the CBS peak in the backward direction. Monitoring the depletion of the initial momentum component and the formation of the angular ring profile allows us to determine microscopic transport quantities. We also study the time evolution of the CBS peak and find it in fair agreement with predictions, at long times as well as at short times. The observation of CBS can be considered a direct signature of coherence in quantum transport of particles in disordered media. It is responsible for the so called weak localization phenomenon, which is the precursor of Anderson localization.

Floriane Arrouas, Julien Hébraud, Nicolas Ombredane +9 more

The Coherent Backscattering (CBS) peak is a well-known interferential signature of weak localization in disordered or chaotic systems. More recently, a second interference feature—the Coherent Forward Scattering (CFS) peak—was predicted to emerge in the regime of strong localization. However, it has never been directly observed. Here we report the direct observation of the CFS peak and demonstrate its dual role as a signature of non-ergodicity and as a probe of symmetries in quantum chaotic systems. Using a shaken rotor model realized with a Bose-Einstein condensate (BEC) of ultracold atoms in a modulated optical lattice, we investigate dynamical localization in momentum space. The CFS peak emerges in the position distribution as a consequence of non-ergodic dynamics, while its growth timescale and width reveal the underlying localization length. By finely tuning the modulation, we control time-reversal and parity symmetries and measure their distinct effects on both CBS and CFS peaks in position space. Our results highlight the strong link of both temporal growth and contrast of the coherent scattering peaks with symmetry and localization. This work opens several perspectives for investigating coherent scattering signatures in a many-body context. Ergodicity (or lack thereof) plays a major role in the relaxation of quantum systems to equilibrium, yet the search of experimentally accessible quantities acting as its signatures is still ongoing. Here, the authors observe the sought-after features of the coherent forward scattering peak from a Bose-Einstein condensate within an optical lattice.

Nicolas Cherroret, Tomasz Karpiuk, Cord Axel Müller +2 more

Using analytical and numerical methods, it is shown that the momentum distribution of a matter wave packet launched in a random potential exhibits a pronounced coherent backscattering (CBS) peak. By analyzing the momentum distribution, key transport times can be directly measured. The CBS peak can be used to prove that transport occurs in the phase-coherent regime, and measuring its time dependence permits monitoring the transition from classical diffusion to Anderson localization.