  A quantum dot (QD) system provides various quantum physics of nanostructures.
So far, many types of semiconductor QD structures have been fabricated and
investigated experimentally and analyzed theoretically. Presently, QD systems
have attracted considerable attention as units for the qubit system of quantum
computers. Therefore, it is vital to integrate QD systems as measurement
devices in addition to qubits. Here, we theoretically investigate the side-QD
system as a measurement apparatus for energy-levels of the target QDs. We
formulate the transport properties of both three and five QDs based on the
Green functions method. The effects of the energy-difference of two side-QDs on
the measurement current are calculated. The trade-off between the strength of
the measurement and the back-action induced by the measurement is discussed. It
is found that the medium coupling strength is appropriate for reading out the
difference of the two energy-levels.
