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Physical Review Letters

Quantum Enhancement of the Index of Refraction in a Bose-Einstein Condensate

Pieter C. Bons, R. de Haas, Damaz de Jong, Adrien E.D. Groot, P. van der Straten

Published Apr 28, 2016• 46 citations
Publisher

Abstract

We study the index of refraction of an ultracold bosonic gas in the dilute regime. Using phase-contrast imaging with light detuned from resonance by several tens of linewidths, we image a single cloud of ultracold atoms for 100 consecutive shots, which enables the study of the scattering rate as a function of temperature and density using only a single cloud. We observe that the scattering rate is increased below the critical temperature for Bose-Einstein condensation by a factor of 3 compared to the single-atom scattering rate. We show that current atom-light interaction models to second order of the density show a similar increase, where the magnitude of the effect depends on the model that is used to calculate the pair-correlation function. This confirms that the effect of quantum statistics on the index of refraction is dominant in this regime.

DOI
10.1103/physrevlett.116.173602

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Abstract The tendency of identical bosons to bunch, seen in the Hanbury Brown–Twiss effect and Bose–Einstein condensation, is a hallmark of quantum statistics. This bunching can enhance the rates of fundamental processes such as atom–atom and atom–light scattering when atoms scatter into already occupied states. For non-interacting bosons, the enhancement of light scattering follows directly from the occupation of the atom’s final momentum state. Here we study scattering between off-resonant light and atoms in a quasi-homogeneous Bose gas with tunable interactions and show that even weak interactions, which essentially do not alter the momentum distribution, strongly affect atom–light scattering. Changes in local atomic correlations suppress the bosonic enhancement under weak repulsive interactions and increase the scattering rate under attractive ones. Moreover, if the interactions are rapidly tuned, light scattering reveals correlation dynamics that are orders of magnitude faster than the collisional dynamics of the momentum-space populations. Its extreme sensitivity to correlation effects makes off-resonant light scattering a powerful probe of many-body physics in ultracold atomic gases.
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Phase contrast imaging is used to observe Bose-Einstein condensates at finite temperature in situ. The imaging technique is used to accurately derive the absolute phase shift of a probe laser beam due to both the condensate and the thermal cloud. The accuracy of the method is enhanced by using the periodicity of the intensity signal as a function of the accumulated phase. The measured density profiles can be described using a two-relevant-parameter fit, in which only the chemical potential and the temperature are to be determined. This allows us to directly compare the measured density profiles to different mean-field models in which the interaction between the condensed and the thermal atoms is taken into account to various degrees.