  A highly tunable linear triple quantum dot (TQD) device is realized in a
single-crystalline pure-phase InAs nanowire using a local finger gate
technique. The electrical measurements show that the charge stability diagram
of the TQD can be represented by three kinds of current lines of different
slopes and a simulation performed based on a capacitance matrix model confirms
the experiment. We show that each current line observable in the charge
stability diagram is associated with a case where a QD is on resonance with the
Fermi level of the source and drain reservoirs. At a triple point where two
current lines of different slopes move together but show anti-crossing, two QDs
are on resonance with the Fermi level of the reservoirs. We demonstrate that an
energetically degenerated quadruple point, at which all three QDs are on
resonance with the Fermi level of the reservoirs, can be built by moving two
separated triple points together via sophistically tuning of energy levels in
the three QDs. We also demonstrate the achievement of direct coherent electron
transfer between the two remote QDs in the TQD, realizing a long-distance
coherent quantum bus operation. Such a long-distance coherent coupling could be
used to investigate coherent spin teleportation and super-exchange effects and
to construct a spin qubit with an improved long coherent time and with spin
state detection solely by sensing the charge states.
