spin wave
Image: JEFFREY MCGUIRK

Researchers have known for some time that atoms can perform a bizarre trick called a spin wave--the quantum equivalent of a sports audience's "wave"--by varying the directions in which their magnetic fields rotate. But evidence for this peculiar behavior has been limited to indirect detection via nuclear magnetic resonance imaging. Now scientists at Nobel laureate Eric Cornell's lab at the University of Colorado have taken the first pictures of spin waves as they undulate through space. The findings could aid efforts to develop superfast "spintronic" computers with incredibly high density memory.

"In a strange way, it's almost as if the atoms were talking with each other to behave collectively, even though one could be thousands of atomic diameters apart from another," lead researcher Jeffrey McGuirk remarks. To observe spin waves, McGuirk and physicists Heather Lewandowski and Dave Harber started with a cloud of rubidium atoms chilled to fractions of a degree above absolute zero. They then magnetically lined up the ultracold gas atoms in a sticklike formation only 400 microns long, roughly four times the width of a human hair.

Atoms act like little magnets complete with north and south poles, and can spin one way or another along their axes. "They all start pointing all in one direction, but due to quantum mechanical weirdness we start to see these little ripples," McGuirk reports. "It's like people doing the wave. You could say the magnets on one end that are pointing up are like people standing up, and the magnets on the other are like people sitting down, and in between you have magnets pointing somewhere midway." (The image on the left shows a spin wave in a rubidium gas parallel to an external magnetic field; the picture on the right shows the spin wave perpendicular to an external magnetic field. Red indicates that the wave is spinning "up"--that is, aligned with the field; blue indicates that it is spinning down, or against the field.)

The findings, which have been submitted to Physical Review Letters for publication, demonstrate that by tweaking magnetic field strength and cloud density the researchers can control how spin waves evolve and take snapshots of every stage. These results should help scientists better understand the weird, fuzzy world of quantum mechanics, which will gain importance as electronics become ever smaller.