Negative differential conductivity and population inversion in the   double-dot system connected to three terminals
  We examine transport and microwave properties of two coupled quantum dots taken in parallel connection to the common left lead and connected to separate leads at their right side. In addition, the area between the left lead and the double-dot structure is threaded by Aharonov-Bohm magnetic flux. We determine the energies and populations of double-dot levels on the microscopic basis taking into account the interdot Coulomb interaction and show that at large lead-to-lead bias the population inversion can be achieved. For the case of strong Coulomb repulsion, this inversion leads to level crossing accompanied by the region of negative differential conductivity in the current-voltage characteristics, whereas for weaker Coulomb repulsion, the resonant microwave absorption becomes negative at high lead-to-lead voltage. 
Lev G. Murokh and Anatoly Yu. Smirnov