Mysterious Motion-Damping Explained
Theorists have discovered an explanation for experimental findings that have puzzled researchers for years, opening new possibilities for studying and manipulating ultracold atoms in lattices.
Charles Clark and Ippei Danshita created a mathematical simulation, derived from quantum-mechanical first principles, of an experiment conducted in 2005 (see illustration above). The researchers had cooled rubidium atoms into a Bose-Einstein condensate (BEC), one of the hallmarks of which is superfluidity. The BEC atoms were confined in a 2-D lattice formed by two pairs of counter-propagating laser beams (X and Y), forcing the atoms into thin columns. The ensemble was exposed to a weak magnetic field gradient, causing the atoms to oscillate vertically about 60 times per second. Adding a third pair of beams (Z) along the atoms’ axis caused remarkable damping of the motion that was highly dependent on the Z beam’s wave amplitude. The researchers noted at the time that there was no theoretical framework that would fully explain the cause of the transition to strong damping.
Clark and Danshita’s simulation agrees quantitatively with all the measurements in the 2005 paper. It attributes the damping effect to quantum fluctuations unique to 1D systems that cause the gradual decay of superfluid motion. As an increasingly large fraction of the atoms goes from the superfluid to energy-dissipating (non-BEC) state, atomic motion decreases accordingly.