Clouds of atoms act like bouncing balls

To study the transport of spin in an ultracold gas, Martin Zwierlein and colleagues at MIT separated a cloud of spin-up atoms from a cloud of spin-down atoms and sent them traveling toward one another. Surprisingly, the two puffs, each a million times thinner than air, ricocheted off one another like billiard balls. The figure shows the spin-up (red) and spin-down (blue) clouds at 1-ms intervals during the first bounce. Such extreme behavior occurs when the atoms have the strongest possible interactions. The researchers cooled a trapped gas of fermionic atoms to nanokelvin temperatures, where the gas was degenerate, with atoms filling most of the lowest energy levels. Then they tuned an applied magnetic field to the so-called Feshbach resonance, where the atoms have their strongest possible interactions. The strong scattering between atoms with opposite spins, combined with the absence of interactions between those of the same spin, explains how the repulsion can be large enough for the clouds to rebound. After about six increasingly damped bounces, the clouds penetrated one another and the spins became homogeneously mixed. It took nearly a second for the spins to equalize. The diffusion time determines the spin diffusivity, which reached a minimum quantum value of ℏ /m, where m is the atomic mass. Zwierlein and his colleagues hope experiments like this can lend insight into the hydrodynamic properties of other strongly interacting systems, such as quark–gluon plasmas or neutron stars. (A. Sommer et al., Nature 472, 201, 2011 .)—Barbara Goss Levi