The dark exciton state in semiconductor quantum dots constitutes a long-lived
solid-state qubit which has the potential to play an important role in
implementations of solid-state based quantum information architectures. In this
work, we exploit deterministically fabricated QD microlenses with enhanced
photon extraction, to optically prepare and readout the dark exciton spin and
observe its coherent precession. The optical access to the dark exciton is
provided via spin-blockaded metastable biexciton states acting as heralding
state, which are identified deploying polarization-sensitive spectroscopy as
well as time-resolved photon cross-correlation experiments. Our experiments
reveal a spin-precession period of the dark exciton of $(0.82\pm0.01)\,$ns
corresponding to a fine-structure splitting of $(5.0\pm0.7)\,\mu$eV between its
eigenstates $\left|\uparrow\Uparrow\pm\downarrow\Downarrow\right\rangle$. By
exploiting microlenses deterministically fabricated above pre-selected QDs, our
work demonstrates the possibility to scale up implementations of quantum
information processing schemes using the QD-confined dark exciton spin qubit,
such as the generation of photonic cluster states or the realization of a
solid-state-based quantum memory.Comment: 5 pages, 3 figure