APL Photon. 2, 121303 (2017).

Matter qubits based on the spin of so-called dark excitons (DEs) are promising for applications in quantum information processing but are hard to extract. Now, Tobias Heindel and co-workers from Germany and Israel have fabricated microlenses above pre-selected InGaAs quantum dots (QDs) and demonstrated all-optical preparation and read-out of the DE spin. A layer of self-organized InGaAs QDs capped with 400 nm of GaAs was fabricated on a distributed Bragg reflector with 23 AlGaAs–GaAs mirror pairs. Microlenses were fabricated by shaping the capping layer via 3D in situ electron-beam lithography. Optical experiments were performed in a confocal micro-photoluminescence set-up at temperatures in the range of 4–20 K. The sample was excited using a wavelength-tunable continuous-wave Ti:sapphire laser at a wavelength between 850 and 880 nm. Correlations between photons emitted from different spectral lines were studied by means of polarization-sensitive intensity cross-correlation measurements.