  Physical implementations of large-scale quantum processors based on
solid-state platforms benefit from realizations of quantum bits positioned in
regular arrays. Self-assembled quantum dots are well-established as promising
candidates for quantum optics and quantum information processing, but they are
randomly positioned. Site-controlled quantum dots, on the other hand, are grown
in pre-defined locations, but have not yet been sufficiently developed to be
used as a platform for quantum information processing. In this letter we
demonstrate all-optical ultrafast complete coherent control of a qubit formed
by the single-spin/trion states of a charged site-controlled nanowire quantum
dot. Our results show that site-controlled quantum dots in nanowires are
promising hosts of charged-exciton qubits, and that these qubits can be cleanly
manipulated in the same fashion as has been demonstrated in randomly-positioned
quantum dot samples. Our findings suggest that many of the related excitonic
qubit experiments that have been performed over the past 15 years may work well
in the more scalable site-controlled systems, making them very promising for
the realization of quantum hardware.
