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arXiv:1212.4453 · Physical Review Letters

Bose-Einstein Condensation of Atoms in a Uniform Potential

Alexander L. Gaunt, Tobias F. Schmidutz, Igor Gotlibovych, Robert P. Smith, Zoran Hadzibabic

Published May 16, 2013• 780 citations
arXivHTMLPDF

Abstract

We have observed the Bose-Einstein condensation of an atomic gas in the (quasi)uniform three-dimensional potential of an optical box trap. Condensation is seen in the bimodal momentum distribution and the anisotropic time-of-flight expansion of the condensate. The critical temperature agrees with the theoretical prediction for a uniform Bose gas. The momentum distribution of a noncondensed quantum-degenerate gas is also clearly distinct from the conventional case of a harmonically trapped sample and close to the expected distribution in a uniform system. We confirm the coherence of our condensate in a matter-wave interference experiment. Our experiments open many new possibilities for fundamental studies of many-body physics.

DOI
10.1103/physrevlett.110.200406

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We have observed the Bose-Einstein condensation of an atomic gas in the (quasi)uniform three-dimensional potential of an optical box trap. Condensation is seen in the bimodal momentum distribution and the anisotropic time-of-flight expansion of the condensate. The critical temperature agrees with the theoretical prediction for a uniform Bose gas. The momentum distribution of a noncondensed quantum-degenerate gas is also clearly distinct from the conventional case of a harmonically trapped sample and close to the expected distribution in a uniform system. We confirm the coherence of our condensate in a matter-wave interference experiment. Our experiments open many new possibilities for fundamental studies of many-body physics.
The ideal Bose gas in a highly anisotropic harmonic potential is studied. It is found that Bose-Einstein condensation occurs in two distinct steps as the temperature is lowered. In the first step the specific heat shows a sharp feature, but the system still occupies many one-dimensional quantum states. In the second step, at a significantly lower temperature, the ground state becomes macroscopically occupied. It should be possible to verify these predictions using present-day atom traps. The two-step behavior can occur in a rather general class of anisotropic traps, including the box potential.
The Bose-Einstein condensation of the Bose gas is studied in a d-dimensional random box.When the length of the box L satisfies the uniform and the Gauss distributions,the critical temperature of the Bose-Einstein condensation TC is obtained respectively.Compared with that of the rigid box TRC,we find that TC is not more than TRC,and their specific relations are depended on the given distribution functions.In the same way,the condensate problem is discussed for the Bose gas confined in a potential well of harmonic-oscillator,and harmonic trap frequency is a random variable.The similar results are obtained.
The study of BEC in weakly interacting system holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles, and may advance present understanding of superconductivity and superfluidity. By employing several new experimental techniques, the dark SPOT trap, rf induced evaporation, and the optically plugged magnetic trap, the authors were able to observe BEC in a gas of sodium atoms. The striking signature of Bose condensation was the sudden appearance of a bimodal velocity distribution below the critical temperature of {approximately} 2 {mu}K. The distribution consisted of an isotropic thermal distribution and an elliptical core attributed to the expansion of a dense condensate. Condensates contained up to 5 x 10{sup 5} atoms at densities exceeding 10{sup 14} cm{sup {minus}3}. Such high densities are unprecedented in cold atomic gases and open up new possibilities for studying a weakly interacting Bose gas over a broad range of densities and therefore strengths of interaction.
We discuss the experimental signal for the Bose-Einstein condensation of cold atoms in an optical lattice at finite temperature. Instead of using the visibility of the interference pattern via time-of-flight imaging, we show that the momentum-space density profile in the first Brillouin zone, in particular its bimodal distribution, provides an unambiguous signal for the Bose-Einstein condensation. We confirm this point with detailed calculations of the change in the atomic momentum distribution across the condensation phase transition, taking into account both the global trapping potential and the atomic interaction effects.
Abstract This chapter describes the main properties of the ideal Bose gas confined by a harmonic potential. It calculates the temperature dependence of the condensate density and identify the value of the critical temperature in terms of the oscillator frequency and the number of atoms. This identification provides an important temperature scale for experiments with ultracold atomic gases. The chapter also discusses the behaviour of the expanding cloud after the release of the trap and how the anisotropy of the gas evolves in time, reflecting the initial anisotropy of the momentum distribution. It finally discusses the possibility of getting Bose–Einstein condensation through adiabatic transformations, working with non-harmonic trapping potentials or with mixtures of two atomic species trapped by different harmonic potentials.
We have observed Bose-Einstein condensation of sodium atoms. The atoms were trapped in a novel trap that employed both magnetic and optical forces. Evaporative cooling increased the phase-space density by 6 orders of magnitude within seven seconds. Condensates contained up to 5\ifmmode\times\else\texttimes\fi{}105{10}^{5}105 atoms at densities exceeding 1014{10}^{14}1014 ${\mathrm{cm}}^{\ensuremath{-}3}$. The striking signature of Bose condensation was the sudden appearance of a bimodal velocity distribution below the critical temperature of \ensuremath{\sim}2\ensuremath{\mu}K. The distribution consisted of an isotropic thermal distribution and an elliptical core attributed to the expansion of a dense condensate.