  Using an approach to open quantum systems based on the effective
non-Hermitian Hamiltonian, we fully describe transport properties for a
paradigmatic model of a coherent quantum transmitter: a finite sequence of
square potential barriers. We consider the general case of asymmetric external
barriers and variable coupling strength to the environment. We demonstrate that
transport properties are very sensitive to the degree of opening of the system
and determine the parameters for maximum transmission at any given degree of
asymmetry. Analyzing the complex eigenvalues of the non-Hermitian Hamiltonian,
we show a double transition to a super-radiant regime where the transport
properties and the structure of resonances undergo a strong change. We extend
our analysis to the presence of disorder and to higher dimensions.
