Spin-orbit coupling in an unexpected context

In a solid-state system, spin–orbit coupling (SOC) is the interaction between the spin of an electron and the effective magnetic field that the electron feels as it moves relative to the charge field of the crystal lattice. Is there an analogous interaction for the neutral atoms in a Bose–Einstein condensate, where no charge field exists? “Yes,” say researchers at the Joint Quantum Institute of NIST in Gaithersburg, Maryland, and the University of Maryland, College Park. Defining SOC more broadly as the interaction between an atom’s spin and its momentum, the research team used a pair of laser beams to create such an interaction. In essence, the laser beams couple a spin-up state to a spin-down state that differs in momentum. The resulting coupling was equivalent to a specific type of SOC in materials. For sufficiently strong interaction, the researchers saw a separation between the up and down spins, as seen in the figure. The ability to create SOC—and other electromagnetic effects—in gases of ultracold atoms takes experimenters one step closer to a cherished goal: the simulation of condensed-matter systems. For example, interfering laser beams can create a periodically varying potential energy in which the atoms sit, reminiscent of electrons in a crystal lattice. The advantage of the atomic system is that it is defect free and allows researchers precise control over its parameters. (Y.-J. Lin, K. Jiménez-García, I. B. Spielman, Nature 471, 83, 2011 .)—Barbara Goss Levi