21 January 2008 ICFO in Science

A landmark advance in ultracold atoms held in
optical lattices discussed
in Science by Profs.
Lewenstein and Sanpera.
Trapping ultracold atoms like eggs in an egg carton by means of optical lattices has a wide range of potential applications, many of them related to magnetism. These include experimental settings reproducing realistically spin models, low temperature quantum magnets, High-Tc Superconductivity and Quantum Information. These applications are reviewed in a Perspective published in Science by Maciej Lewenstein and Anna Sanpera (ICREA Professors at ICFO and at the Universitat Autònoma de Barcelona, respectively). The Perspective is inspired by a landmark experimental paper by German and USA researchers appearing in the same issue.

An optical lattice is a spatially ordered array of potential wells produced by the interference pattern of counterpropagating laser beams. “In simpler terms”, the authors of the Perspective write, “the optical lattice looks effectively like an egg carton, where the atoms, like eggs, can be arranged to form crystals of quantum matter”. Quantum mechanics allows the atoms to tunnel through the barriers. Thus, their spins can interact through the so called “superexchange interactions”.

The paper published in Science analyzes the simplest matter crystal consisting of a pair of wells, where each well is loaded with an individual rubidium atom. By imposing an additional magnetic field, it is possible to tune the hopping and test the accuracy of the underlying theoretical spin model (the Bose-Hubbard model). Moreover, depending on the value of the applied field, the superexchange interactions can either be antiferromagnetic or ferromagnetic.

Although spin models have been traditionally constructed as ideal approximations of real magnetic materials, ultracold atoms in optical lattices allow for an almost perfect experimental realization of such models. Testing these models will help to understand the most fundamental physics associated with quantum magnetic ordering and quantum phase transitions. Moreover, they provide a direct path toward the realization of low-temperature quantum magnets. Further developments could lead to the realization of polymerized lattices, made of complexes of a few close sites, forming dimers, trimers, etc. A trimerized kagome lattice could provide a solid basis for studying resonating valence bond states, that have been proposed to be the explanation of high-temperature superconductivity. A quadruply polymerized square lattice is expected to have quantum information processing applications.