Quantum Transport in Disorder

Permanent Staff: Vincent Josse, Alain Aspect

PhD Student: Yukun Guo (funded by SIMONS Wave project), Xudong Yu 

Postdoc: Niranjan Myneni (funded by SIMONS Wave project)

Ex members: Baptiste Lecoutre (now in SAFRAN), Azer Trimeche (SIMONS Wave project), Vasiliki Angelopoulou (now PhD at Copenhaguen) , Adrien Signoles (Now at PASQAL), Mukhtar Mussawadah (now at CAILABS), Vincent Denechaud (CIFRE/DGA -SAFRAN grant - now professor), Valentin Volchkov (Marie Sklodowska-Curie Grant No. 655933, now permanent position at MPQ Germany), Jérémie Richard (now at IXBLUE), Kilian Müller, Fred Jendrzejewski (now professor at Heidelberg), Alain Bernard (now professor), Patrick Cheinet (now CR at Paris Saclay), Juliette Billy (now Assistant Professor at Toulouse), William Guerin (now CR at INLN, Nice), Jean Felix Riou, Marie Fauquembergue

Our team study the transport properties of matter waves in well controlled disordered potentials, focusing especially on the celebrated Anderson localization. Landmarks results has been obtained in the team, such as the direct observation of 1D and 3D Anderson Localization or Coherent Backscattering (which is a direct signature of phase coherent effect in disorder media). Using a novel method (based on spectroscopy) our current goal is to investigate in detail the Anderson transition that occurs in 3D, which constitutes an utmost challenge in the field. This work is supported by the SIMONS foundation via the multidisplinary WAVE project that gathers physicists and mathematiciansto study this celebrated phenomenon in a renewed perspective. 

Bichromatic state-dependent disordered potential for Anderson localization of ultracold atoms (B. Lecoutre et al. Eur. Phys. J. D, 76, 218 (2022). The ability to load ultracold atoms at a well-defined energy in a disordered potential is a crucial tool to study quantum transport, and in particular Anderson localization. In this paper, we present a new method for achieving that goal by rf transfer of atoms in an atomic Bose-Einstein condensate from a disorder insensitive state to a disorder sensitive state. It is based on a bichromatic laser speckle pattern, produced by two lasers whose frequencies are chosen so that their light-shifts cancel each other in the first state and add-up in the second state. Moreover, the spontaneous scattering rate in the disorder-sensitive state is low enough to allow for long observation times of quantum transport in that state. We theoretically and experimentally study the characteristics of the resulting potential.

Spectral functions and localization-landscape theory in speckle potentials, P. Pelletier et al. Phys. Rev. A, 105, 023314 (2022). Spectral function is a key tool for understanding the behavior of Bose-Einstein condensates of cold atoms in random potentials generated by a laser speckle. In this paper we introduce a method for computing the spectral functions in disordered potentials. Using a combination of the Wigner-Weyl approach with the localization-landscape theory, we build an approximation for the Wigner distributions of the eigenstates in the phase space and show its accuracy in all regimes, from the deep quantum regime to the intermediate and semiclassical. Based on this approximation, we devise a method to compute the spectral functions using only the landscape-based effective potential. The paper demonstrates the efficiency of the proposed approach for disordered potentials with various statistical properties without requiring any adjustable parameters.

Ultracold atoms in disordered potentials: elastic scattering time in the strong scattering regime (A. Signoles et al., New. J. Phys. 21, 105002, 2019). We study the elastic scattering time $\tauS$ of ultracold atoms propagating in optical disordered potentials in the strong scattering regime, going beyond the recent work of J. Richard \emph{et al.} \textit{Phys. Rev. Lett.} \textbf{122} 100403 (2019). There, we identified the crossover between the weak and the strong scattering regimes by comparing direct measurements and numerical simulations to the first order Born approximation. Altogether our study characterizes the validity range of usual theoretical methods to predict the elastic scattering time of matter waves, which is essential for future close comparison between theory and experiments, for instance regarding the ongoing studies on Anderson localization.   

Elastic Scattering Time of Matter-Waves in Disordered Potentials (Phys. Rev. Lett. 122, 100403, 2019). We report on an extensive study of the elastic scattering time τ_S of matter-waves in optical disordered potentials. Using direct experimental measurements, numerical simulations and comparison with first-order Born approximation based on the knowledge of the disorder properties, we explore the behavior of τ_S over more than three orders of magnitude, spanning from the weak to the strong scattering regime. We study in detail the location of the crossover and, as a main result, we reveal the strong influence of the disorder statistics, especially on the relevance of the widely used Ioffe-Regel-like criterion kl_S~~1. While it is found to be relevant for Gaussian-distributed disordered potentials, we observe significant deviations for laser speckle disorders that are commonly used with ultracold atoms. Our results are crucial for connecting experimental investigation of complex transport phenomena, such as Anderson localization, to microscopic theories.

Measurement of Spectral Functions of Ultracold atoms in Disordered Potentials (Phys. Rev. Lett. 120, 060404, 2018). We report on the measurement of the spectral functions of non-interacting ultra-cold atoms in a three-dimensional disordered potential resulting from an optical speckle field. Varying the disorder strength by two orders of magnitude, we observe the crossover from the "quantum" perturbative regime of low disorder to the "classical" regime at higher disorder strength, and find an excellent agreement with numerical simulations. The method relies on the use of state-dependent disorder and the controlled transfer of atoms to create well-defined energy states. This opens new avenues for experimental investigations of three-dimensional Anderson localization.

Suppression and Revival of Weak Localization Through Control of Time Reversal Symmetry (Phys. Rev. Lett. 114, 205310, 2015). We report on the observation of suppression and revival of coherent backscattering of ultra-cold atoms launched in an optical disorder in a quasi-2D geometry and submitted to a short dephasing pulse, as proposed in T. Micklitz et al., Phys. Rev. B 91, 064203 (2015). This observation demonstrates a novel and general method to study weak localization by manipulating time reversal symmetry in disordered systems.In future experiments, this scheme could be extended to investigate higher order localization processes at the heart of Anderson (strong) localization.

Coherent Backscattering of Ultracold atoms (Phys. Rev. Lett. 109, 195302, 2012). We report on the direct observation of coherent backscattering (CBS) of ultra-cold 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. 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.

First evidence of 3D Anderson localization with ultracold atoms (Jendrzejewski et al. Nature Physics 8, 392 2012, arXiv:1108.0137). A BEC, suspended against gravity by a magnetic levitation, is allowed to expand in a 3D laser speckle disorder. This disorder is created by crossing two coherent speckle fields at 90°, resulting in short correlation lengths in all directions in space. A phenomenological analysis of our data distinguishes a localized component of the resulting density profile from a diffusive component. The observed localization cannot be interpreted as the classical trapping of particles with energy below the classical percolation threshold in the disorder, nor can it be understood as quantum trapping in local potential minima. Instead, our data are compatible with the self-consistent theory of Anderson localization tailored to our system. This experiment paves the way towards an utmost challenge : the precise inspection of metal-insulator phase transition (between localized and diffusive states) in 3D.

Direct observation of 1D Anderson localization of matter wave (Billy et al. Nature 493, 891 (2008)). A very dilute BEC is released in a 1D waveguide in presence of laser speckle disorder. We observed that the propagation is stopped by a very weak amount of disorder (i.e. without any classical trapping). The inspection of the localized density profiles reveal an exponential decay in the wings, i.e. the emblematic signatures of Anderson localization. The localization lengths have been measured and were found to be in good agreement with theoretical predictions (L. Sanchez-Palencia et al. Phys. Rev. Lett. 98, 210401 (2007)).