Synthetic magnetic fields

An ultracold gas of atoms known as a Bose-Einstein condensate (BEC) is a nearly ideal system for creating new states of matter or studying many-body quantum phenomena at macroscopic scales. (For one example, see the article on Anderson localization by Alain Aspect and Massimo Inguscio in Physics Today, August 2009, page 30 . The BEC’s charge neutrality, though, hinders its use as a probe of phenomena that arise from Lorentz forces on electrons in a magnetic field; magnetic fields produce only Zeeman shifts. Researchers at the Joint Quantum Institute, a collaboration of NIST and the University of Maryland, have now removed that limitation. The researchers, led by Ian Spielman, began with a BEC of roughly 250 000 rubidium-87 atoms held at 100 nK. By illuminating the atoms with a suitable pair of laser beams close to resonance, they imprinted an effective vector potential A* on the system. In the presence of a detuning gradient, the vector potential depends on position in the trap. The spatial dependence can thus be engineered to give a nearly uniform synthetic magnetic field $B*=\nabla \times A*$ that does couple to neutral atoms. A signature of that field is the formation of vortices—the spots shown in this time-of-flight image of the BEC—that mark points about which the atoms swirl. Spielman and colleagues plan to add to their system a two-dimensional optical lattice, which may allow them to create, for example, exotic quantum Hall states of bosons. (Y.-J. Lin, R. L. Compton, K. Jiménez, J. M V. Porto, I. B. Spielman, Nature 462 , 628, 2009 http://dx.doi.org/doi:10.1038/nature08609 .)