Seventy Years Later: The Creation of a Bose-Einstein Condensate in an Ultracold Gas

Carl Wieman

Eric Cornell, Carl Wieman

Wolfgang Rohringer, D. Fischer, Michael Trupke +2 more

In 1924/25, Satyendranath Bose and Albert Einstein predicted that particles of integer spin (today called bosons) should undergo a statistical phase transition when cooled to temperatures very close to absolute zero (Bose, 1924; Einstein, 1925). This phenomenon, nowadays called Bose-Einstein condensation, has been experimentally observed seventy years later using (nanokelvin temperatures) of bosonic neutral atoms (Anderson et al., 1995; Bradley et al., 1995; Davis et al., 1995c). This spectacular experimental achievement was awarded the Nobel price in 1995 and has triggered a new research direction ultracold gases with more than 200 groups worldwide today. 1 Bose-Einstein condensates (BEC) of atomic represent a fascinating exotic state of matter with properties entirely determined by the laws of mechanics. Although they consisting of several thousands up to millions of atoms, the state can in most cases be described by a single collective wave function with a common phase. This wave function usually has a spatial extent ranging between 1-100 microns, allowing us to observe mechanics essentially by eye using basic magnification optics. The Bose-Einstein condensate can be described as a coherent matter wave in close analogy to the optical field emitted by (or inside) a laser. This analogy has brought about the field of quantum-atom-optics which aims to implement standard elements and experiments known from laser optics using matter waves. As heavy rest mass particles, such as atoms, are very sensitive to gravity or acceleration/rotations, matter-wave interferometers promise orders of magnitude in sensitivity gain compared to their photonic counterparts (Berman, 1996). Over the last decade, experimental tools for the creation and manipulation of matter have reached an impressive degree of sophistication. This allows the construction of tailored Hamiltonians for the system and the quantum simulation of more general physical situations, not connected to atomic physics alone. Optical lattice potentials allow to mimic solid state physics with a high degree of control over essentially all experimental parameters. Particle interactions can be tuned via Feshbach resonances, allowing the study of superconductivity, superfluidity and the formation of

S. Stringari

S. Brandt

Abstract Einstein used double quotes on two words, because in the concept of an ideal gas there is no interaction between its constituents, whereas condensation and saturation are properties of a real gas of interacting atoms or molecules. The ‘interaction’ bringing about Bose–Einstein condensation is the symmetry requirement for multi-particle wave functions of bosons, sometimes called exchange force (see page 176). For this to become effective, the wave functions of individual bosons have to overlap, i.e., the average (or thermal ) de Broglie wavelength of the bosons has to be on the order of or larger than the average distance between them. In other words, a Bose–Einstein condensate is expected to form if the ratio of wavelength and distance exceeds a certain value. (The third power of this ratio is called phase-space density, see margin.) At very low temperature the average distance can be quite large; interactions between bosons become unimportant and the situation of an ideal gas is well approximated. We saw in Episode 97 how such temperatures for samples of neutral atoms were reached. These methods, greatly simplified and extended, were the basis for the achievement of Bose–Einstein condensates by the groups ofWieman and Cornell [2] and of Ketterle [3] in 1995, seventy years after Einstein’s prediction.

Sudhir R. Jain