  Instantons lead to strong correlations between up and down quarks with spin
zero and anti-symmetric color wave functions. In cold and dense matter,
$n_b>n_c\simeq 1 fm^{-3}$ and $T<T_c\sim$ 50 MeV, these pairs Bose-condense,
replacing the usual $< \bar qq >$ condensate and restoring chiral symmetry. At
high density, the ground state is a color superconductor in which diquarks play
the role of Cooper pairs. An interesting toy model is provided by QCD with two
colors: it has a particle-anti-particle symmetry which relates $<\bar qq>$ and
$< qq>$ condensates.
