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arXiv:cond-mat/9906035 · Physical Review Letters

Excitation of Phonons in a Bose-Einstein Condensate by Light Scattering

Dan Stamper-Kurn, A. P. Chikkatur, Axel Görlitz, S. Inouye, Subhadeep Gupta, David E. Pritchard, Wolfgang Ketterle

Published Oct 11, 1999• 451 citations
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Abstract

Stimulated small-angle light scattering was used to measure the structure factor of a Bose-Einstein condensate in the phonon regime. The excitation strength for phonons was found to be significantly reduced from that of free particles, revealing the presence of correlated pair excitations and quantum depletion in the condensate. The Bragg resonance line strength and line shift agreed with predictions for the homogeneous Bose gas using a local density approximation.

DOI
10.1103/physrevlett.83.2876

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Stimulated small-angle light scattering was used to measure the structure factor of a Bose-Einstein condensate in the phonon regime. The excitation strength for phonons was found to be significantly reduced from that of free particles, revealing the presence of correlated pair excitations and quantum depletion in the condensate. The Bragg resonance line strength and line shift agreed with predictions for the homogeneous Bose gas using a local density approximation.
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We report a measurement of the excitation spectrum omega(k) and the static structure factor S(k) of a Bose-Einstein condensate. The excitation spectrum displays a linear phonon regime, as well as a parabolic single-particle regime. The linear regime provides an upper limit for the superfluid critical velocity, by the Landau criterion. The excitation spectrum agrees well with the Bogoliubov spectrum in the local density approximation, even close to the long-wavelength limit of the region of applicability. Feynman's relation between omega(k) and S(k) is verified, within an overall constant.
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We present a single-scattering formalism for incoherent resonant light scattering by dilute quantum gas systems such as the atomic-trap Bose-Einstein condensates. We show that resonant scattering gives access to more information than the dynamical structure factor, familiar from non-resonant scattering. In particular, we show that the detuning dependence of the incoherent scattering cross-section allows the direct determination of the BEC pairing density <ψψ>< ψψ><ψψ>, which is a broken symmetry and provides evidence that the condensate is not in a good number state.