  Quantum dots (QDs) made from semiconductors are among the most promising
platforms for the developments of quantum computing and simulation chips, and
have advantages over other platforms in high density integration and in
compatibility to the standard semiconductor chip fabrication technology.
However, development of a highly tunable semiconductor multiple QD system still
remains as a major challenge. Here, we demonstrate realization of a highly
tunable linear quadruple QD (QQD) in a narrow bandgap semiconductor InAs
nanowire with fine finger gate technique. The QQD is studied by electron
transport measurements in the linear response regime. Characteristic
two-dimensional charge stability diagrams containing four groups of resonant
current lines of different slopes are found for the QQD. It is shown that these
current lines can be individually assigned as arising from resonant electron
transport through the energy levels of different QDs. Benefited from the
excellent gate tunability, we also demonstrate tuning of the QQD to regimes
where the energy levels of two QDs, three QDs and all the four QDs are
energetically on resonance, respectively, with the fermi level of source and
drain contacts. A capacitance network model is developed for the linear QQD and
the simulated charge stability diagrams based on the model show good agreements
with the experiments. Our work presents a solid experimental evidence that
narrow bandgap semiconductor nanowires multiple QDs could be used as a
versatile platform to achieve integrated qubits for quantum computing and to
perform quantum simulations for complex many-body systems.
