NOBEL PRIZE Bose-Einstein condensation revisited The award of the 2001 Nobel Prize for Physics to Eric Cornell, Wolfgang Ketterle and Carl Wieman for their "achievement of Bose-Einstein condensation in dilute gases of alkali atoms and for early fundamental studies of the properties of the condensates" is the latest chapter in a long history. Nobel condensate - how alkali metal atoms come together as the temperature is decreased. The image shows Bose-Einstein condensation at, from left to right, 400, 200, and 50 nK. One hundred years ago, some physicists began to suspect that a paper published. Bose had already sent it to the Philosophical electromagnetic radiation was packaged - or "quantized" - rather Magazine, where it had been turned down. The paper showed how than being a continuous stream.This followed Max Planck's discov­ Planck's distribution law for photons could be derived from first ery that the spectrum of light from a hot object could be explained principles. Duly impressed, Einstein translated it into German, and only if the radiators sat in discrete energy states. By 1905 Albert the paper was published in 1924 in ZeitschriftfurPhysik. Einstein concluded that the radiation itself was emitted as bursts of As a result, Einstein temporarily turned away from his dogged but energy - light quanta - later to be called photons. Einstein's key unsuccessful search for a unified theory of gravitation and electro- explanation earned him the 1921 Nobel Prize for Physics. magnetism and started work on the quantum theory of radiation. In 1924 Satyendra Nath Bose (see box) from Dacca University, in Thus was born the concept of "Bose-Einstein" statistics for quanta what was then India, wrote to Einstein asking for his help in getting ("bosons") carrying an integer value of intrinsic angular momen- > CERN Courier December 2001 21 NOBEL PRIZE Satyendra Nath Bose 1894-1974 Satyendra Nath Bose was born in Calcutta, the "The Indian Bose has given a beautiful derivation son of a railway worker. An outstanding physics of Planck's law." Soon physics history was made. student, he also had a talent for languages and Bose and Einstein met in Berlin in 1925. Bose translated milestone physics material from French returned to Dacca and in 1945 moved to and German into English for local publication. Calcutta, where he spent the remainder of his One of his efforts was a text by Einstein on career. General Relativity, the English-language rights for His name is now enshrined in physics. A which had meanwhile been acquired by a London "boson" is a particle of integer spin that obeys publisher. At Bose's request, Einstein himself Bose-Einstein statistics and is the counterpart of intervened and allowed the Bose translation to be a "fermion", which is a particle of half-integer spin used inside India. Satyendra Nath Bose - that obeys Fermi-Dirac statistics. Unlike Dirac, It was this episode that gave Bose, working in enshrined in particles. Einstein and Fermi, Bose did not achieve a Nobel Dacca, the confidence to approach Einstein again prize. However, in 1930 Venkata Raman, also of in 1924 with the new derivation of the Planck radiation law: Calcutta, earned the Nobel Prize for Physics for the light- "Respected Sir, I have ventured to send you the accompanying scattering effect that bears his name. He was the first scientist article for your perusal and opinion." Einstein was impressed: from outside Europe and the US to earn the coveted award. turn (spin). There is no limit to the number of bosons that can tem.This gave an estimate of the amount of condensed atoms in the simultaneously occupy any one quantum state. liquid - only about 8%, because strong interactions in liquid helium Einstein noted that if the number of such particles is conserved, make it deviate significantly from the ideal non-interacting gas. even totally non-interacting particles should undergo a change of Superfluid helium flows without resistance, as if no internal behaviour at low enough temperatures - Bose-Einstein condens­ frictional forces act in the liquid.This was explained by a phenomen- ation. Bose had not predicted this because he was looking at ological theory devised by L D Landau in 1941, eventually earning photons, which can simply disappear when the energy of the system him the 1962 Nobel Prize for Physics. In this theory, superfluidity is decreased. derives from the fact that when the available energy is low enough, The condensation that Einstein predicted derives from the fact only long-wavelength phonons (the vibration quanta of the medium) that the number of states available at very low energy becomes can be excited. exceedingly small. With less and less room for all of the particles when the temperature is decreased, they accumulate (condense) Electron pairs in the lowest possible (ground) energy state. Although superconductivity was first seen in 1911, reaching a full theoretical explanation took nearly 50 years. In 1957 J Bardeen, Superbehaviour and its effects LN Cooper and J R Schrieffer ("BCS") proposed a theory based on Even before this was going on, the liquefaction of helium by cryogenic phonon-mediated interactions between the electrons of the metal. pioneer H Kamerlingh Onnes (1913 Nobel Prize for Physics) opened This earned them the 1972 Nobel Prize for Physics. up new areas of physics study. Materials cooled by liquid helium to The BCS theory showed that superconductivity is due to strong within a few degrees of absolute zero showed bizarre behaviour. correlations between electrons of opposite spin.This creates a highly However, it took time for the real nature of these effects, which are coherent state that is insensitive to perturbations, hence the lack of now known as superconductivity and superfluidity, to become clear. electrical resistance. Electron pairs can be considered as bosonic Superconductivity is the virtual disappearance of electrical resis­ particles and the superconductivity transition is similar to tance at liquid-helium temperatures, and superfluidity is the virtual Bose-Einstein condensation. disappearance of viscosity as we know it. Superfluid helium flows Earlier, V L Ginsburg and Landau had suggested a phenomen- without resistance, as if no internal frictional forces act in the liquid. ological theory. Although the implications of this approach emerged (Appropriately, these properties are being exploited to the full in the only slowly, it did lead to new developments in spontaneous cryogenics for CERN's new LHC collider, the superconducting mag­ symmetry breaking, which turned out to be crucial for particle nets of which will be cooled by superfluid helium at 1.9 K). physics in what is now known as the "Higgs mechanism". | In 1938 Fritz London suggested that superfluidity could be caused Helium-3 atoms, which have half-integer spin, are not bosons and by the bosonic condensation of helium-4 atoms, which have integer should not condense like helium-4 to become superfluid. However, spin.This was supported by the fact that no similar effect had then in the same way that electron pairs make materials superconducting been seen with the rarer helium-3 isotope, the atoms of which do at low temperatures, the BCS mechanism also opens up the possi­ not have integral spin (see below). bility of superfluid helium-3. The discovery of superfluid helium-3 In the 1950s, 0 Penrose and L Onsager related superfluidity to earned the 1996 Nobel Prize for Physics for David Lee, Douglas the long-range order displayed by a highly correlated bosonic sys­ Osheroffand Robert Richardson. 22 CERN Courier December 2001 A DEFINITIVE PULL-OUT GUIDE TO RESEARCH SUPPLIERS CERN COURIER ADVERTISING SUPPLEMENT DECEMBER 2001-2002 COMPUTING, SOFTWARE AND INFORMATION TECHNOLOGY CRYOGENICS AND VACUUM RADIOFREQUENCY, POWER SUPPLIES AND ENGINEERING IMAGING, MATERIALS AND INSTRUMENTATION T ELECTRONICS AND DATA COMMUNICATION In association with JEIWI A Convergence of Excellence Beamline assemblies delivered with unmatched aualitv, speed, and price. 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