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Posted: December 20, 2007
How atoms change places
(Nanowerk News) Scientists at the Johannes Gutenberg University in Mainz, in collaboration with scientists from Harvard and Boston University, have achieved direct control and observation of a fundamental mechanism of quantum magnetism in an experiment with ultracold atoms. "We were able to realize a control-lable magnetic interaction in a system of two coupled atoms as found in many topical magnetic materials such as the parent substances of high-temperature superconductors", Stefan Trotzky from the QUANTUM group explains. The group, led by Prof. Immanuel Bloch, studies the basic interactions of particles with the help of ultracold atoms trapped in a lattice formed by laser-light.
The work on quantum mechanical exchange was published online in Science Express on December 20, 2007 ("Time-Resolved Observation and Control of Superexchange Interactions with Ultracold Atoms in Optical Lattices"). The processes studied are relevant not only for the understanding of novel materials with special magnetic and electronic properties. They also form the basis of chemical bonds in molecules. Furthermore, they can be seen as an important conceptual part of a quantum computer with ultracold atoms which would exceed the performance of a conventional computer by far.
The physicists cool atoms in their experiments to temperatures near absolute zero, approximately -273 degrees Celsius. They hold them in place by means of an optical lattice, which can be seen as a "crystal of light". Within the lattice, the atoms sit like eggs in an egg carton. The novel aspect of the Mainz experiment lies in the fact that an atom initially located on a certain site of the light-crystal is only allowed to move to and from a single neighboring site. The resulting system consists of pairwise-coupled lattice sites in which the dynamics of a few interacting atoms can be studied in detail.
"In these double wells, we were able to directly observe how two neighboring atoms with different spins change places" says Trotzky. The spin of a particle can be understood as an internal rotation and forms the basis of magnetism in solids. The scientists placed one atom with its spin directed upwards and another one with its spin directed downwards on the two sides of the double well. In classical physics, they would not be allowed to pass through the barrier between them. However, the quantum mechanical nature of the particles enables this crossing through a hopping, or "tunnelling" process. Therefore, the atoms can in fact exchange their places. This can be observed directly, since one can distinguish the two by their spin.
For a strong repulsion between the particles, one will never find the two together on one side of the double well. This state is forbidden due to the law of energy conservation. However, one observes the exchange of the two atoms: if the one carrying the spin up is initially located on the left side of the double well, it can be found on the right after a certain time, whereas the other atom with spin down which started from the right well will then be located on the left. The basis of this effect is a synchronous hopping event, or more formally a "second order" tunnelling process. Already this summer, the physicists in Mainz succeeded in observing a similar process with ultracold atoms, where two particles initially placed on the same side of a barrier crossed it simultaneously even though they were strongly repelling each other. In the present work, the atoms also cross the barrier simultaneously, but in opposite directions. The rate of this exchange thereby depends on the height of the barrier and the strength of the atom-atom repulsion. Both parameters can be set over a wide range with high accuracy in the experiment.
The exchange of two particles with spin can be described as an interaction between the spins themselves. One remarkable aspect is that no direct contact of the atoms is needed for this. The shape of the double well alone determines the nature of the spin-spin interaction, and therefore the magnetic properties of this miniature model of a solid. In the future, the scientists want to use these properties of the system to create exotic many-particle states. Some of these states might serve as starting points for novel schemes of quantum computation.