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Achieving an important new capability in ultracold atomic gases, researchers at the Joint Quantum Institute, a collaboration of the National Institute of Standards and Technology and the University of Maryland, have created "synthetic" magnetic fields for ultracold gas atoms, in effect "tricking" neutral atoms into acting as if they are electrically charged particles subjected to a real magnetic field. The demonstration, described in the latest issue of the journal Nature, not only paves the way for exploring the complex natural phenomena involving charged particles in magnetic fields, but may also contribute to an exotic new form of quantum computing.
As researchers have become increasingly proficient at creating and manipulating gaseous collections of atoms near absolute zero, these ultracold gases have become ideal laboratories for studying the complex behavior of material systems. Unlike usual crystalline materials, they are free of obfuscating properties, such as impurity atoms, that exist in normal solids and liquids. However, studying the effects of magnetic fields is problematic because the gases are made of neutral atoms and so do not respond to magnetic fields in the same way as charged particles do. So how would you simulate, for example, such important exotic phenomena as the quantum Hall effect, in which electrons can "divide" into quasiparticles carrying only a fraction of the electron's electric charge?
The answer Ian Spielman and his colleagues came up with is a clever physical trick to make the neutral atoms behave in a way that is mathematically identical to how charged particles move in a magnetic field. A pair of laser beams illuminates an ultracold gas of rubidium atoms already in a collective state known as a Bose-Einstein condensate. The laser light ties the atoms' internal energy to their external (kinetic) energy, modifying the relationship between their energy and momentum. Simultaneously, the researchers expose the atoms to a real magnetic field that varies along a single direction, so that the alteration also varies along that direction.