  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.
