Figure 1. Sample structure and spectral properties of the used quantum dots.

(a) Cross-sectional 3D view of an atomic force microscopy (AFM) image of a nanohole in an AlGaAs layer. The colour scale reflects the local surface inclination (white for flat areas and black for inclinations >16°). The height-to-width ratio is amplified 17 times to highlight the nanohole shape. A GaAs quantum dot (QD) with a height of about 7 nm is obtained after its filling with GaAs and overgrowth with AlGaAs. (b) Sketch of the sample structure. (c) Top view of the AFM measurement with colour scale reflecting the local height (0 corresponds to the average height of the flat areas surrounding the nanohole). The lateral scale of the AFM is as in a −250 × 250 nm². The highly symmetric shape is crucial to obtain the high degree of entanglement observed here. (d) Microphotoluminescence spectrum of a representative QD under non-resonant excitation (laser photon energy E_L=2.54 eV). The neutral exciton emission line is labelled as X. The biexciton line (XX) is not visible in these excitation conditions. (e) Spectrum of the same QD as in d under resonant two photon excitation (E_L=1.5775, eV). The X and XX are visible and have very similar intensity. Two weaker charged states appear (C₁ and C₂). Inset: For resonant two-photon excitation, the 9 ps laser pulses are tuned to half of the energy difference between |XX> and ground state |0>.