For a quantum many-body system, the direct population of states of
double-excitation character is a clear indication that correlations importantly
contribute to its nonequilibrium properties. We analyze such
correlation-induced transitions by propagating the nonequilibrium Green's
functions in real-time within the second Born approximation. As crucial
benchmarks, we compute the absorption spectrum of few electrons confined in
quantum wells of different width. Our results include the full two-time
solution of the Kadanoff-Baym equations as well as of their time-diagonal limit
and are compared to Hartree-Fock and exact diagonalization data
