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Science

Bose-Einstein Condensation of Potassium Atoms by Sympathetic Cooling

Giovanni Carlo Modugno, G. Ferrari, G. Roati, Robert J. Brecha, Andrea Simoni, M. Inguscio

Published Nov 9, 2001• 485 citations
Publisher

Abstract

We report on the Bose-Einstein condensation of potassium atoms, whereby quantum degeneracy is achieved by sympathetic cooling with evaporatively cooled rubidium. Because of the rapid thermalization of the two different atoms, the efficiency of the cooling process is high. The ability to achieve condensation by sympathetic cooling with a different species may provide a route to the production of degenerate systems with a larger choice of components.

DOI
10.1126/science.1066687

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We report on the Bose-Einstein condensation of potassium atoms, whereby quantum degeneracy is achieved by sympathetic cooling with evaporatively cooled rubidium. Because of the rapid thermalization of the two different atoms, the efficiency of the cooling process is high. The ability to achieve condensation by sympathetic cooling with a different species may provide a route to the production of degenerate systems with a larger choice of components.
We show, both experimentally and theoretically, that sympathetic cooling of 87Rb{}^{87}\mathrm{Rb}87Rb atoms in the ∣F=2,mF=2〉|{F=2,m}_{F}=2〉∣F=2,mF​=2〉 state by evaporatively cooled atoms in the $|{F=1,m}_{F}=\ensuremath{-}1〉$ state can be precisely controlled so as to produce a condensate in either or both states. We also study the thermalization rate between two species. Our model renders a quantitative account of the observed role of the overlap between the two clouds and verifies that sympathetic cooling becomes inefficient when the masses are very different. Our calculation also yields an analytical expression of the thermalization rate for a single species.
Evaporative cooling of an atom gas of bosons, whereby the hottest atoms are progressively removed, is an effective and widely used method for forming Bose-Einstein condensates. For Fermi gases, however, collisions between identical fermions are forbidden, and new tricks, such as mixing two Fermi gases with different spin states, have been used to form Fermi condensates. Truscott et al. (p. 2570; see the Perspective by O'Hara and Thomas) now create quantum-degenerate Bose and Fermi condensates simultaneously from a boson-fermion mixture. Heat is removed from the fermions through elastic collisions with the bosons, a process called sympathetic cooling. They show that different statistics associated with the two types of gases can actually be visualized.
We report on the generation of a quantum degenerate Fermi-Fermi mixture of two different atomic species. The quantum degenerate mixture is realized employing sympathetic cooling of fermionic 6Li and 40K gases by an evaporatively cooled bosonic 87Rb gas. We describe the combination of trapping and cooling methods that proved crucial to successfully cool the mixture. In particular, we study the last part of the cooling process and show that the efficiency of sympathetic cooling of the 6Li gas by 87Rb is increased by the presence of 40K through catalytic cooling. Because of the differing physical properties of the two components, the quantum degenerate 6Li-40K Fermi-Fermi mixture is an excellent candidate for a stable, heteronuclear system allowing the study of several so far unexplored types of quantum matter.
We report on the realization of quantum degenerate gas mixtures of the alkaline-earth-metal element strontium with the alkali-metal element rubidium. A key ingredient of our scheme is sympathetic cooling of Rb by Sr atoms that are continuously laser cooled on a narrow-linewidth transition. This versatile technique allows us to produce ultracold gas mixtures with a phase-space density of up to 0.06 for both elements. By further evaporative cooling we create double Bose-Einstein condensates of 87{}^{87}87Rb with either 88{}^{88}88Sr or 84{}^{84}84Sr, reaching more than $1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ condensed atoms per element for the 84{}^{84}84Sr-87{}^{87}87Rb mixture. These quantum gas mixtures constitute an important step towards the production of a quantum gas of polar, open-shell RbSr molecules.
Experimental methods of laser and evaporative cooling, used in the production of atomic Bose-Einstein condensates have recently been extended to realize quantum degeneracy in trapped Fermi gases. Fermi gases are a new rich system to explore the implications of Pauli exclusion on scattering properties of the system, and ultimately fermionic superfluidity. We have produced a new macroscopic quantum system, in which a degenerate Li-6 Fermi gas coexists with a large and stable Na-23 BEC. This was accomplished using inter-species sympathetic cooling of fermionic 6Li in a thermal bath of bosonic Na-23. We have achieved high numbers of both fermions (less than 10(exp 5) and bosons (less than 10(exp 6), and Li-6 quantum degeneracy corresponding to one half of the Fermi temperature. This is the first time that a Fermi sea was produced with a condensate as a "refrigerator".
We report the realization of a Bose-Einstein condensate of 39{}^{39}39K atoms without the aid of an additional atomic coolant. Our route to Bose-Einstein condensation comprises sub-Doppler laser cooling of large atomic clouds with more than 1010{10}^{10}1010 atoms and evaporative cooling in an optical dipole trap where the collisional cross section can be increased using magnetic Feshbach resonances. Large condensates with almost 106{10}^{6}106 atoms can be produced in less than 15 s. Our achievements eliminate the need for sympathetic cooling with Rb atoms, which was the usual route implemented until now due to the unfavorable collisional property of 39{}^{39}39K. Our findings simplify the experimental setup for producing Bose-Einstein condensates of 39{}^{39}39K atoms with tunable interactions, which have a wide variety of promising applications, including atom interferometry to studies on the interplay of disorder and interactions in quantum gases.