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Technical Physics

Coherent amplification of beams of atoms inelastically scattered by the Bose-Einstein condensate

E. V. Orlenko, Boris G. Matisov

Published Jan 1, 2006• 5 citations
Publisher

Abstract

The subject of consideration is coherent amplification of a beam of impurity atoms inelastically scattered by a system of ultracold atoms with collectively excited modes. A kinematic model of weak localization of a new type is used that provides a straightforward explanation for this phenomenon and also makes it possible to derive the angular dependences of the scattering intensity. Based on this model, angular and energy criteria for the existence of the new-type weak localization in an ultracold system of atoms (the Bose-Einstein condensate) are determined. Particular emphasis is on how the principle of indistinguishability influences the weak localization of a beam of atoms inelastically scattered by the Bose-Einstein condensate when scattered and condensed atoms are identical.

DOI
10.1134/s1063784206010014

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The subject of consideration is coherent amplification of a beam of impurity atoms inelastically scattered by a system of ultracold atoms with collectively excited modes. A kinematic model of weak localization of a new type is used that provides a straightforward explanation for this phenomenon and also makes it possible to derive the angular dependences of the scattering intensity. Based on this model, angular and energy criteria for the existence of the new-type weak localization in an ultracold system of atoms (the Bose-Einstein condensate) are determined. Particular emphasis is on how the principle of indistinguishability influences the weak localization of a beam of atoms inelastically scattered by the Bose-Einstein condensate when scattered and condensed atoms are identical.
A non-Anderson weak localization of an electron beam scattered from disordered matter is considered with respect to the principle of electron indistinguishability. A weak localization of electrons of a new type is essentially associated with inelastic processing. The origin of inelasticity is not essential. We take into account the identity principle for electron beam and electrons of the atom of the scatterer with an open shell. In spite of isotropic scattering by each individual scatterer, the electron exchange contribution has a hidden parameters effect on the resulting angular dependence of the scattering cross-section. In this case, the electrons of the open shell of an atomic scatterer can be in the s-state, that is, the atomic shell remains spherically symmetric. The methods of an invariant time-dependent exchange perturbation theory and a Green functions with exchange were applied. An additional angular dependence of the scattering cross-section appears during the coherent scattering process. It is shown exactly for the helium scatterer that the role of exchange effects in the case of a singlet is negligible, while for the triplet state, it is decisive, especially for those values of the energy of incident electrons when de Broglie’s waves are commensurate with the atomic.
Summary form only given. Bose-Einstein condensed atomic gases are a new class of quantum fluids with many novel properties. We have studied collective modes and dissipation by perturbing the condensate with focused laser beams. The scattering of photons imparts momentum to the condensate and creates excitations. Consequently, the coherence and collective behavior of the condensate can strongly affect optical properties. We used light scattering to demonstrate that the coherence length of a condensate is equal to its physical size, and obtained dramatic evidence for the presence of correlated momentum excitations in the many-body condensate wavefunction. At higher laser intensity, a new form of superradiance was found which provided a gain mechanism for the generation of directed beams of atoms and light. This process amplified matter waves in a phase coherent way.
Using a semiclassical theory we give a unified approach to a class of coherent atom field interactions in a Bose–Einstein condensate, including superradiant light scattering, coherent amplification of atoms, and coherent light amplification. The populations of the excited states are explicitly taken into account, and the role of the recoil kinetic energies of the atoms with different momenta is discussed. In addition to the known features of superradiant emission, we obtain a shift in the spectrum of the scattered light as a consequence of the atomic recoil. The same theoretical method yields results that are in agreement with the experimental findings for atomic beam amplification, as well as for light amplification. Considering the atomic beam amplification process, we obtain a single outgoing amplified atomic beam. In treating the coherent light amplification, the unexpectedly small group velocity seen in the experiments is reproduced, and connected to the double Λ configuration of the levels involved.
We numerically study time-dependent quantum transport of an interacting Bose-Einstein condensate based on the Gross-Pitaevskii equation in a two-dimensional correlated disorder potential. The scattering- and transport mean-free-path is numerically calculated and compared with a diagrammatic approach. Furthermore we introduce the phenomenon of coherent backscattering in the context of Bose-Einstein condensates. In the limit of a vanishing atom-atom interaction, a sharp cone in the angle-resolved density of the scattered matter wave is observed, arising from constructive interference between amplitudes propagating along reversed scattering paths. Weak interaction transforms this coherent backscattering peak into a pronounced dip, indicating destructive instead of constructive interference. This agrees with results obtained from a diagrammatic theory of weak localization in the presence of nonlinearity. For intermediate interaction strength we observe intrinsic time-dependent dynamic, which leads to a suppression of coherent backscattering.
Atom loss resonances in ultracold trapped atoms have been observed at scattering lengths near atom-dimer resonances, at which Efimov trimers cross the atom-dimer threshold, and near two-dimer resonances, at which universal tetramers cross the dimer-dimer threshold. We propose a new mechanism for these loss resonances in a Bose-Einstein condensate of atoms. As the scattering length is ramped to the large final value at which the atom loss rate is measured, the time-dependent scattering length generates a small condensate of shallow dimers coherently from the atom condensate. The coexisting atom and dimer condensates can be described by a low-energy effective field theory with universal coefficients that are determined by matching exact results from few-body physics. The classical field equations for the atom and dimer condensates predict narrow enhancements in the atom loss rate near atom-dimer resonances and near two-dimer resonances due to inelastic dimer collisions.