Figure 1
Illustration: Alan Stonebraker. Concept by Arnaud Koetsier

Figure 1: Du et al. observed spin segregation in a Fermi gas: (a) The gas is prepared such that all atoms are in the same in-plane superposition of hyperfine states, which precesses due to the external magnetic trapping potential. (b) As a result of the magnetic-field inhomogeneity, atoms at different positions in the trap acquire different precession angles. (c) If two such atoms interact the exchange interaction causes precession of the individual spins of the atoms while conserving their total spin. (d) After the collision, the atoms therefore obtain an out-of-plane spin component. This out-of-plane component is opposite for the two atoms and is observed as spin segregation as the spins move apart.