  A linear triple quantum dot (TQD) integrated with a quantum dot (QD) charge
sensor is realized. The TQD and the charge sensor are built from two adjacent
InAs nanowires by fine finger gate technique. The charge state configurations
of the nanowire TQD are studied by measurements of the direct transport signals
of the TQD and by detection of the charge state transitions in the TQD via the
nanowire QD sensor. Excellent agreements in the charge stability diagrams of
the TQD obtained by the direct transport measurements and by the charge-state
transition detection measurements are achieved. It is shown that the charge
stability diagrams are featured by three groups of charge state transition
lines of different slopes, corresponding to the changes in the electron
occupation numbers of the three individual QDs in the TQD. It is also shown
that the integrated nanowire QD sensor is highly sensitive and can detect the
charge state transitions in the cases where the direct transport signals of the
TQD are too weak to be measurable. Tuning to a regime, where all the three QDs
in the TQD are close to be on resonance with the Fermi level of the source and
drain reservoirs and co-existence of triple and quadruple points becomes
possible, has also been demonstrated with the help of the charge sensor in the
region where the direct transport signals of the TQD are hardly visible.
