Physics Professor Pioneers New Method to Image Vortices in Ultracold Gases

September 3, 2010

Physics professor David Hall and his group of undergraduate researchers have invented a new technique for examining the behavior of rotating matter at the coldest temperatures in the universe.

The method—which involves an apparatus that refrigerates atoms to billionths of a degree above absolute zero—enabled them to create the first-ever movies of vortex motion in individual Bose-Einstein condensates (BECs). And they developed the technique in Hall’s very own campus laboratory.

A paper about the research appears in the Sept. 3 issue of the prestigious journal Science (the abstract of the work is online as well).

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One of the first movies produced by Hall’s team. The upper row shows the pictures taken by the camera, where brighter regions correspond to higher atomic density in the condensate; the lower is a sequence of surface fits to the data, where the approximate locations of the vortices are indicated in blue and red. Each frame is approximately 100 microns on a side, or about the width of a human hair.

“Our technique lets us visualize how these vortices behave, whether alone or together with other vortices,” Hall explained, adding that he hopes it may eventually provide new perspectives on the properties of superconductors and superfluids. “Scientists have been able to observe vortices in BECs for about a decade now, but this is the first study that examines their behavior in real-time.”

Hall has long studied Bose-Einstein condensation, the peculiar process that occurs when a gas of noninteracting particles is made extremely cold: a substantial portion of the gas condenses into the lowest-energy state of the system, and its atoms lose their individuality and behave in a collective fashion. One manifestation of this collective behavior is the phenomenon of superfluidity (a zero-viscosity phase of matter where “fluid” can flow without friction), which has also been an interest of Hall’s for many years.

Since 1995, when the scientific community began creating BECs in dilute gases, researchers have been intrigued by what happens when a superfluid rotates. “A gas or liquid often forms a vortex when it rotates—think of the characteristic whirlpool pattern in a hurricane or in water going down the drain,” Hall said. “A rotating superfluid also can form one or many (miniature) whirlpools—they’re called quantized vortices, in this case—that interact with one another as they move about the condensate. One of the most exciting aspects of our research is that we can now observe those interactions as they take place.”

Superconductors, which are materials that conduct electricity without resistance below certain temperatures, develop quantized vortices as well. But in their case, the vortices appear in the electrical current density, said Hall. And it’s the application of a magnetic field to the superconductor that causes the phenomenon, rather than rotation.

Previously, scientists had only been able to generate movies of quantized vortices in helium and some superconductors; doing so in BECs was a challenge because the radii of the vortex cores are ordinarily several times smaller than even the wavelength of light that permits imaging. (In other words, they are simply too small to see without destroying the BEC.) The situation has been frustrating for many physicists, said Hall, since BECs are highly controllable and are otherwise considered a model system to study, both theoretically and experimentally.

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But Hall and his students Daniel Freilich ’10, Dylan Bianchi ’09, Adam Kaufman ’09 and Thomas Langin ’11 changed that. Beginning in July 2008, they developed and implemented a complex, minimally destructive imaging technique to visualize vortex motion, using an apparatus built in Hall’s Amherst lab that has been refrigerating atoms to billionths of a degree above absolute zero since 2002. The Science paper reflects both this preliminary investigation and work presented in Freilich’s senior thesis.

“Quantized vortices are among the most fundamental features of a superfluid,” said Hall of the research. “I’m pretty thrilled that we are able to study them here at Amherst, as well as have our work recognized for publication in Science.”

He said he’s also excited for the broader implications of the study. “Understanding the behavior of these quantized vortices is essential to gaining insight into superfluids and superconductors. With better understanding comes the possibility of improving materials design; for example, creating superconductors that remain superconducting at higher current densities.”

A bonus was thatAmherststudents could contribute to such groundbreaking research, noted Hall, adding that he continued his work this summer with Langin, Emine Altuntas ’11, Aftaab Dewan ’12 and Shenglan Qiao ’13.

“I’m always pleased when I consider that this research is largely done by undergraduates,” he said. “I’m quite proud of what they’ve accomplished, under considerable pressure, to arrive at this point—and I suspect that they can’t help but be caught up in the swirl of things.”