Two superconductors weakly coupled through an insulating potential barrier form a Josephson junction. Electrons on each side of the barrier can be described by a macroscopic wavefunction, and the phase difference between them leads to a current flowing across the junction — known as the Josephson effect. Now, Woo Jin Kwon and co-workers, as well as Niclas Luick and co-workers, have observed the same effect in charge-neutral systems made of ultracold 6Li atoms.
Like electrons, the atoms in these experiments are fermions, and can form pairs and condense in the same quantum state at sufficiently low temperature. An optical barrier inserted in the centre turned the system into a junction. A current across the barrier was induced either by imposing an initial phase difference between the two parts or by directly moving the optical barrier with respect to the atoms. Both groups observed a sinusoidal current–phase relation — a distinctive feature of the Josephson effect — by monitoring the subsequent evolution of the atom number and of the phase of the wavefunction.