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Is It a Gas, Fluid, Solid, or All of the Above?

Because rubidium is magnetic, however, Stamper-Kurn and his Berkeley colleagues thought that the magnetic interactions between rubidium atoms in the gas might nudge them to adopt a type of regular spacing like atoms in a solid.

To look for this ordering, Stamper-Kurn’s team used a conventional laser trapping technique to confine a gas of millions of rubidium atoms in an oblong, surfboardlike trap. They then cooled the sample to below 500 nanokelvin. Lastly, they hit their collection of rubidium atoms with a beam of circularly polarized light, which is reflected differently by atoms with a different magnetic orientation and can, therefore, reveal the magnetic orientation of the atoms in the sample. What they saw was that within their optical trap, the rubidium atoms ordered themselves into an array of 5-micrometer-square domains, inside which all of the atoms adopted a similar magnetic orientation. What’s more, these domains adopted a crystalline-like ordering, with alternating domains with different magnetic directions. This ordering wasn’t perfect like the regular lattice of sodium and chlorine atoms in table salt. But it’s not random either (see picture). “There is some emergent order which shows up in this system,” Stamper-Kurn says.

Once the Berkeley researchers spotted the ordered makeup of the atoms, they decided to check whether the gas was coherent as well. Using another laser, they nudged two groups of rubidium atoms already in their trap. They found that the atoms interfered with each other in the same way that two coherent light beams create an interference pattern of light and dark stripes, an unmistakable sign of their wavelike quantum nature.

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