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 Correlations
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.