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arXiv:cond-mat/0501344 · Physical Review Letters

Atomic Gases at Negative Kinetic Temperature

Allard P. Mosk

Published Jul 21, 2005• 52 citations
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Abstract

We show that thermalization of the motion of atoms at negative temperature is possible in an optical lattice, for conditions that are feasible in current experiments. We present a method for reversibly inverting the temperature of a trapped gas. Moreover, a negative-temperature ensemble can be cooled (reducing |T|) by evaporation of the lowest-energy particles. This enables the attainment of the Bose-Einstein condensation phase transition at negative temperature.

DOI
10.1103/physrevlett.95.040403

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We show that thermalization of the motion of atoms at negative temperature is possible in an optical lattice, for conditions that are feasible in current experiments. We present a method for reversibly inverting the temperature of a trapped gas. Moreover, a negative-temperature ensemble can be cooled (reducing |T|) by evaporation of the lowest-energy particles. This enables the attainment of the Bose-Einstein condensation phase transition at negative temperature.
Protocols for attaining quantum degeneracy in atomic gases almost exclusively rely on evaporative cooling, a time-consuming final step associated with substantial atom loss. We demonstrate direct laser cooling of a gas of rubidium-87 (⁸⁷Rb) atoms to quantum degeneracy. The method is fast and induces little atom loss. The atoms are trapped in a two-dimensional optical lattice that enables cycles of compression to increase the density, followed by Raman sideband cooling to decrease the temperature. From a starting number of 2000 atoms, 1400 atoms reach quantum degeneracy in 300 milliseconds, as confirmed by a bimodal velocity distribution. The method should be broadly applicable to many bosonic and fermionic species and to systems where evaporative cooling is not possible.
Laser radiations are used to slow down atoms by the process o f momentum transfer.This leads to reducing the temperature to microkclvin region.Gas phase atoms are trapped by using magnetic fields.The recent advances have led to the realization of the dream of physicists o f confining the atoms and reducing their velocities to the limit imposed by quantum mechanics.A number of new experiments arc possible with the cooled and trapped atoms and ions that would be useful to solve many problems of theoretical physics. 1 unher cooling by the process o f evaporative technique has led to the observation of Bose-Hinslcin Condensation predicted by Einstein and Rose nearly seventy-five years ago.A brief review of the method of laser cooling, magnetic trapping and evaporative cooling methods used for obtaining ultracold atoms are discussed It is possible to obtain temperature in the nanokelvin region without using cryogenic methods thus simplifying the experimental methods to a great extent.
We describe in detail our recent experiments on quantum degenerate gases of ytterbium (Yb) atoms. Up to now Bose–Einstein condensates of 174 Yb and 170 Yb, and a Fermi degenerate gas of 173 Yb have been realized by efficient evaporative cooling in a crossed far-off resonant trap. Our studies showed that the efficient escape from the trap and rapid thermalization are important for efficient evaporative cooling. We also describe some recent experiments using Yb Bose–Einstein condensates such as the achievement of a scalar atom laser, the observation of an optical coherent transient, and successful loading into an optical lattice. Finally, the future prospects using Yb quantum degenerate gases are discussed in detail.
We report the creation of a Bose-Einstein condensate using buffer-gas cooling, the first realization of Bose-Einstein condensation using a broadly general method which relies neither on laser cooling nor unique atom-surface properties. Metastable helium ((4)He*) is buffer-gas cooled, magnetically trapped, and evaporatively cooled to quantum degeneracy. 10(11) atoms are initially trapped, leading to Bose-Einstein condensation at a critical temperature of 5 microK and threshold atom number of 1.1 x 10(6). This method is applicable to a wide array of paramagnetic atoms and molecules, many of which are impractical to laser cool and impossible to surface cool.
We report the creation of a Bose-Einstein condensate using buffer-gas cooling, the first realization of Bose-Einstein condensation using a broadly general method which relies neither on laser cooling nor unique atom-surface properties. Metastable helium (\\(^4\\)He*) is buffer-gas cooled, magnetically trapped, and evaporatively cooled to quantum degeneracy. 10\\(^{11}\\) atoms are initially trapped, leading to Bose-Einstein condensation at a critical temperature of \\(5 \\mu K\\) and threshold atom number of 1.1×10\\(^6\\). This method is applicable to a wide array of paramagnetic atoms and molecules, many of which are impractical to laser cool and impossible to surface cool.