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Bose–Einstein condensates

BECs from the fridge

Nature Physics volume 5, pages 712–714 (2009)Cite this article

Large ensembles of atoms can be buffer-gas loaded into a magnetic trap and further evaporatively cooled all the way down to quantum degeneracy. The approach has now been shown to provide an alternative — and potentially general — route to Bose–Einstein condensation.

Rejected by the Philosophical Magazine, Satyendra Nath Bose's paper on photon statistics landed famously, in June 1924, on the desk of Albert Einstein, who had duly translated it and promptly passed it on for publication1. Led by the same dialectics that made him adopt his 1905 “heuristic viewpoint” about a gas of “light quanta”2, Einstein recognized that Bose's “viewpoint” (Einstein's parlance in quotes) can, conversely, be applied to an ideal gas of atoms or molecules3. Such a dualistic approach was consistent with Louis de Broglie's Thesis, of which Einstein had been likely aware at the time4. Bose's counting was non-Boltzmannian, as it implicitly — and fortuitously — entailed indistinguishability of the “molecules” of light or gas5. Combining Bose's counting with a constraint on the number of molecules (which is conserved) led Einstein to make the following prediction6: “I maintain that [below a critical temperature] a number of molecules steadily growing with increasing density goes over in the [ground] quantum state (which has zero kinetic energy) while the remaining molecules distribute themselves according to their [chemical potential]. A separation is effected; one part condenses, the rest remains as 'saturated ideal gas'.” However, as Fritz London put it in 1938, “in the course of time the [condensation] of the Bose–Einstein gas has rather got the reputation of having only a purely imaginary existence”7. It took the efforts of Daniel Kleppner and Thomas Greytak at MIT, and their scientific progeny, direct and removed (in particular Eric Cornell, Randy Hulet, Wolfgang Ketterle, David Pritchard and Carl Wiemann), to produce eventually, in the 1990s, Bose–Einstein condensates (BECs) in the laboratory. The latest advance, reported by Charles Doret and colleagues in Physical Review Letters8, demonstrates that a BEC can be created from a buffer-gas-cooled cloud of atoms. This approach should be a promising and general alternative to laser cooling, which so far has been the workhorse of cold-atom physics but requires tailoring to the specific atom under investigation.

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Figure 1: Key steps on the pathway across the density (n) versus absolute temperature (T) plane towards a BEC of gaseous He*.

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  1. Bretislav Friedrich is at the Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany. brich@fhi-berlin.mpg.de,

    Bretislav Friedrich

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Friedrich, B. BECs from the fridge. Nature Phys 5, 712–714 (2009). https://doi.org/10.1038/nphys1417

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