  We present a microscopic theory for both equilibrium and nonequilibrium
transport properties of coupled double quantum dots (DQD). A general formula
for current tunneling through the DQD is derived by the nonequilibrium Green's
function method. Using a Hartree-Fock approach, effects of multi-level coupling
and nonequilibrium electron distributions in resonant tunneling are considered.
We find that the peak in the resonant tunneling current through two symmetric
dots will split only when the inter-dot coupling is stronger than dot-lead
coupling. We predict that population inversion can be achieved in one dot in
the nonequilibrium regime.
