Bose-Einstein-condensates making waves: a many-particle phenomenon
(January 26, 2016) Quantum objects cannot just be understood as the sum of their parts. This is what makes quantum calculations so complicated. Scientists at TU Wien (Vienna) have now calculated Bose-Einstein-condensates, revealing the secrets of the particles’ collective behaviour.
Quantum systems are extremely hard to analyse if they consist of more than just a few parts. It is not difficult to calculate a single hydrogen atom, but in order to describe an atom cloud of several thousand atoms, it is usually necessary to use rough approximations. The reason for this is that quantum particles are connected to each other and cannot be described separately. Kaspar Sakmann (TU Wien, Vienna) and Mark Kasevich (Stanford, USA) have now shown in an article published in “Nature Physics” that this problem can be overcome. They succeeded in calculating effects in ultra-cold atom clouds which can only be explained in terms of the quantum correlations between many atoms. Such atom clouds are known as Bose-Einstein condensates and are an active field of research.
Quantum physics is a game of luck and randomness. Initially, the atoms in a cold atom cloud do not have a predetermined position. Much like a die whirling through the air, where the number is yet to be determined, the atoms are located at all possible positions at the same time. Only when they are measured, their positions are fixed. “We shine light on the atom cloud, which is then absorbed by the atoms”, says Kaspar Sakmann. “The atoms are photographed, and this is what determines their position. The result is completely random.”
There is, however, an important difference between quantum randomness and a game of dice: if different dice are thrown at the same time, they can be seen as independent from each other. Whether or not we roll a six with die number one does not influence the result of die number seven. The atoms in the atom cloud on the other hand are quantum physically connected. It does not make sense to analyse them individually, they are one big quantum object. Therefore, the result of every position measurement of any atom depends on the positions of all the other atoms in a mathematically complicated way.