SMOKING GUN: As the isotropic interaction (gray sphere) lessens, keeping the magnetic interaction (arrow) constant, the BEC (central density peak) turns dipolar, evidenced by the change in shape. Image: THIERRY LAHAYE
It has long been known that—contrary to the common sense notion that like charges repel—electrons in solids tend to pair up at low temperatures to conduct electricity without heating the wire, a phenomenon known as superconductivity.
Superconductivity is utilized, among other things, to create high magnetic fields in magnetic resonance imaging (MRI) machines. The phenomenon involves the condensation of a group of bosons—particles such as the paired electrons—that dance together synchronously in the same quantum state. Known as a Bose-Einstein condensate (BEC), this unique state of matter is unlike the familiar solids, liquids or gases and can be used to make atomic lasers and more precise atomic clocks. Last week, a team of physicists led by Thierry Lahaye at the University of Stuttgart in Germany reported in Nature that they had created a BEC of chromium atoms that interact like tiny magnets over essentially infinite distances. In doing so, the atoms can form exotic states of matter impossible with traditional interactions—such as solids that flow better than liquids and atoms that arrange themselves like squares on a chessboard.
To tango in sync, the bosons need to "feel" one another through some interaction. Ever since the first BECs were produced from bosonic alkali atoms like lithium, rubidium and sodium in 1995, this interaction has been short range (effective to no more than a few nanometers) as well as isotropic: The atoms are like tiny balls, so there's no "right side up" for any of them, and they look the same from all directions. But physicists have always known that some of these atoms behave like tiny magnets and, of course, magnets do have a right side up; bring two magnets together, flip one of them around, and attraction turns to repulsion. So the question has always been: Can one form a BEC out of atoms that interact like tiny magnets?
The problem is that the dipolar (magnetic) part of any atom-atom interaction is always much smaller than the isotropic (ball-like) part, that dominates in any BEC. Fortunately, the strength of the isotropic part can be tuned, even up to the point of turning attraction into repulsion (and vice-versa), by putting the atoms next to an electromagnet and adjusting the magnetic force.
The other problem, researchers say, is that alkali atoms—prime candidates for forming BECs—are very weak magnets. This can be fixed by choosing a bosonic atom such as chromium 52 that is, for example, six times as strong a magnet as lithium.
So Lahaye's group trapped chromium 52 atoms inside a laser beam, put them in a magnetic field, and varied the field until the isotropic interaction became so weak that the dipolar part became dominant. Purely due to dipolar interaction, the chromium atoms formed a BEC that lasted several milliseconds and, when the trap was released, displayed the density peak that is the smoking gun for BECs.
Other than being direction-dependent, "dipolar interactions are much longer range than isotropic interactions, which only extend to a few nanometers," Lahaye says. Theorists have predicted that atoms in the presence of such long-range interactions form exotic quantum phases like supersolids—crystals in which some atoms flow sans resistance and so-called checkerboard phases where atoms occupy every other position in an already present lattice.
"This is something that the theorists have been discussing for a number of years and finally the experimentalists are catching up." says Rice University's Randall Hulet, himself an experimentalist, and one of the first scientists to produce a BEC.