Physicists in the US are the first to segregate a Fermi gas of ultracold atoms according to their spin — with “spin-up” and “spin-down” atoms moving to opposite sides of the optical trap in which they were contained.
John Thomas and colleagues at Duke University found that about 60% of the lithium-6 atoms became segregated and that the spin-up and spin-down atoms remained apart for several seconds. However, they are puzzled as to why the segregation lasts much longer, and is more intense, than predicted by theory.
Thomas's team used a Fermi gas of ultracold lithium-6 atoms that are trapped in the centre of a vacuum chamber by a laser beam. The team begin their experiment with all the atoms in the spin-up state. The gas is then hit with a radiofrequency pulse that puts each atom into a coherent superposition of spin-up and spin-down states. This means that until, a measurement is made on the spin of an atom, it is both spin-up and spin-down.
The team then switch on a magnetic field that varies in strength across the trap and causes the atoms to migrate to the centre. Occasionally, two atoms collide leaving one atom “spin-up” with respect to the magnetic field and the other “spin-down”. The atoms then move away from the centre with velocities that are correlated to their spins — spin-up electrons moving in one direction and spin-down electrons in the other.
The team found that this process took about 200 ms to reach a maximum spin segregation of 60% — and that this segregation endured for about 5 s. By contrast Cornell’s bosons remained segregated for a mere 200 ms — something that has been successfully described by a theory that assumes that the atoms interact strongly with each other.
However, Thomas says that this “theory would predict that our spins would segregate in 7 ms and relax back to equilibrium in about 7 ms”.