Superconductivity: pairing electrons with unbalanced populations

" In the conventional theory of superconductivity an equal number of spin up and spin down electrons are forming Cooper pairs. But how is this concept modified when the two populations differ? That problem is relevant not only in condensed matter but also in QCD where quarks are paired, as well as in experiments with ultracold fermionic atoms. The answer is pairing with finite pairing momentum.
Experimental evidence will be discussed as well as an important application of the theory, i. e., $pi$ junctions. They will play an increasing role in superconducting circuitry."