Have you ever heard of the “quantum identity crisis” that an atom may experience under certain conditions? If not, you should listen to this lecture of William Phillips, which is both highly educational and entertaining – a fascinating and well understandable crash course in some basic aspects of matter, namely the formation and behavior of Bose-Einstein Condensates. Their existence had been postulated by the Indian physicist Satyendra Nath Bose and Albert Einstein in 1925, seven decades before their first realization. A few months after Phillips’ lecture, Eric Cornell, Wolfgang Ketterle and Carl Wiemann shared the Nobel Prize in Physics 2001 for this achievement. One prerequisite for their success was the “development of methods to cool and trap atoms with laser light” by Steven Chu, Claude Cohen-Tannoudji and William Phillips, which earned them the Nobel Prize in Physics 1997.

The wave nature of atoms is normally not apparent, because atoms move so fast. The sodium atoms of a street lamp, for example, have a speed of 1000 m/sec and correspondingly a de Broglie wavelength of only a fraction of their size. Cooling down the atoms makes them slower and increases their wavelength. At temperatures very close to absolute zero their wavelength becomes comparable with the spacing between the atoms. “At that point the atoms undergo kind of a quantum identity crisis - they can’t distinguish themselves from their neighbors.” In response, a large fraction of atoms converts to the lowest energy state available. “The atoms think they can hide there” – and form a Bose-Einstein-Condensate. Laser cooling, however, makes a gas of trapped atoms by far not dense and cold enough to observe such a condensation. In a gas of rubidium atoms, for example, the temperature at which almost all of the atoms have condensed into the lowest available energy state is 50 nanoKelvin! To reach this threshold, the method of evaporative cooling, which Phillips describes in detail, has to be applied.

Bose-Einstein-Condensates behave like coherent waves of atoms. “We can do optics with them similar to the optics we do with coherent light”. Within the optical cavity from which lasers arise, light of one single wavelength and color is formed. Accordingly, within the magnetic trap in which Bose-Einstein-Condensates develop, atoms in a single quantum state emerge. In analogy to the Bragg reflection of electromagnetic waves, one therefore can reflect material waves “not from physical planes of atoms in a crystal but from the planes formed by the interference of two laser beams of coherent light”. For this purpose, one first forms a condensate and then turns off the magnetic trap to obtain a cloud of extremely slow atoms in free space. Then one hits this cloud with a pair of laser pulses that causes a fraction of the atoms to separate from the cloud and move away from it. Shortly afterwards, one applies another pair of pulses and has two pieces of a wave function whose interference depends on their quantum mechanical phase. An interval of five microseconds between two pulse pairs, for example, will change the phase by 180 degrees and lead to a destructive interference, while at an interval of ten microseconds the interference would be constructive. So the atoms either cancel each other out or add up to make twice as many atoms as there would be. “We use to have this with light all the time, but when you do this with atoms somehow it makes quantum mechanics seem very real”, Phillips says. The phenomenon he describes allows physicists “to write letters onto the quantum mechanical wave function” and brings quantum mechanisms “into the realm of things that are big enough to see and almost to touch”.

Joachim Pietzsch