Nonequilibrium Green's functions approach to the pair distribution function of quantum many-body systems

The pair distribution function (PDF) is a key quantity for analyzing correlation effects of quantum systems both in and far from equilibrium. We derive an expression for the PDF in terms of single particle Green's functions - the solutions of the Keldysh-Kadanoff-Baym equations in the two-time plane. The result includes initial correlations and generalizes previous density matrix expressions from single-time quantum kinetic theory. As an illustration we present numerical results for the PDF of electrons and holes in a strongly correlated electron-hole bilayer. These are calculated in different many-body approximations and then compared with Path integral Monte Carlo results.