Quantum Transport of Matter Waves
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.
In close relation to the fundamental transport properties of electrons in condensed matter physics, we aim to study the quantum propagation of ultracold bosonic matter-waves through tailored optical potentials. Such a system can be considered a quantum simulator. We focus on two main topics:
Disorder lies at the heart of many fundamental phenomena in condensed matter systems, such as metal-insulator transition in amorphous electronic conductors, superfluidity in porous media, and possibly high-Tc superconductivity.
The celebrated Anderson localization (P. W. Anderson Phys. Rev. 109, 1492, 1958) is one of the most emblematic effect of the disorder. Indeed, it predicts that the disorder can completely freeze the motion of quantum particles, leading to a genuine metal-insulator transition. This intriguing effect results from the destructive quantum interferences between many scattering paths and is ubiquituous to wave physics. To date, Anderson localization has been observed with differents systems, for electronic or classical waves ( Lagendijk et al. Phys. Today August 2009, for a recent review). However, despite extensive theoretical and experimental efforts over the past 50 years, the precise understanding of this localization effect remains an exciting but formidable task, both for experiment and theory (see e.g. the webpage of the theory team in our group).
Ultracold atomic systems offers new approaches to these issues. In particular, their great promises have been demonstrated in our group by two landmarks experiments: the first demonstration of Anderson localization with matter waves in 1D, and recently 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)).
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.
The Bose-Einstein condensation of atoms in the lowest level of a trap represents the matter-wave analogue to the accumulation of photons in a single mode of a laser cavity. In analogy to photonic lasers, coherent atom lasers can then be obtained by outcoupling a part of the atoms from a trapped Bose-Einstein condensate (which acts as a reservoir) to free space. Such atomic outcouplers (for instance realized by applying a radio frequency field that spin flips the atoms from an initial trapped state to a final free state) play the role of the partially transmitting mirror in optics. For the first realization of atom lasers, the atoms were however simply falling under gravity, leading to the rapid increase in energy, i.e. an important decrease of the de Broglie wavelength.
In the Atom Optics group, we develop a new type of atom laser: the guided atom laser (see Fig. 3). In this configuration the atoms are directly outcoupled in a horizontal waveguide, in close analogy with ‘pig-tailed’ photonic lasers. This scheme permits (i) a better control of the atomic motion and, in particular, to create a large and constant de Broglie wavelength along the propagation and (ii) to control the intra-laser interactions (analogous to photonic Kerr non-linearities) by tuning the atom laser flux.
Thus, this guided atom laser opens new prospects to study quantum transport phenomena past obstacles, both in the linear (with low interactions) and the nonlinear regime (with strong interactions).
First demonstration of a quasi-continuous horizontal guided atom laser (W. Guerin et al., Phys. Rev. Lett. 97, 200402 (2006)). A BEC is produced in a hybrid opto-magnetic trap composed of a horizontal optical waveguide for the transverse confinement and a shallow magnetic trap that provides the longitudinal one. The atom laser is directly outcoupled by a rf field into the waveguide. Cancelling the gravity allows us to create large and constant de Broglie (around 1µm) along the propagation.
Characterization of the flux properties of a guided atom laser (A. Bernard et al., New. J. Phys. 13, 065015 (2011)). We investigated here both the outcoupling spectrum (flux of the atom laser versus rf frequency of the outcoupler) and the flux limitations imposed to obtain a quasicontinuous emission. This work has allowed us to identify the operating range of the guided atom laser and to confirm its promises for studying quantum transport phenomena.
Beam quality of an atom laser (J. F. Riou et al. Phys. Rev. Lett. 96, 070404 (2006)). In analogy to photon lasers, we generalized here the M2 factor to characterize the quality of nonideal atom-laser beams. Indeed the atom laser wavefront can be distorted by the strong lensing effect of the Bose-Einstein condensate (BEC) from which it is outcoupled, In this work the M2 factor is predicted theorically for different lensing effects. These predictions are found in good agreement with the observations.
If you are interested in our research, please do not hesitate to contact Vincent Josse. We have open positions for PhD or post-docs. We also usually offer to motivated undergraduate students at various levels to work with us.