(111)-grown InGaAs/GaAs quantum dots as ideal source for entangled photon pairs

For a number of protocols of future secure communication systems the generation of entangled photon pairs are essential. Their creation, based on the XX→X→ recombination cascade in QDs, requires necessarily a vanishing excitonic fine-structure splitting (fss). The latter is hard to achieve for QDs grown on (001)-substrate, because piezoelectricity and QD elongation induce a lateral anisotropy leading to sizable values of the fss. The structural anisotropy and the piezoelectricity are intrinsic characteristics of the (001) substrate orientation featuring different surface mobilities along [110] and [1-10] and polar axes with non-vanishing projections on the growth plane. Therefore, we propose the use of QDs grown on (111)-GaAs substrates. Using eight-band k.p theory in conjunction with an enhanced configuration interaction (CI) method we calculate the fss for (111)-grown QDs in conjunction with their exciton-biexciton separation as function of size, aspect ratio and chemical composition. Assuming an, at least, threefold rotational structural symmetry of the Qds, we show that the corresponding piezoelectric field does not lower this symmetry any further. Thus, the excitonic bright states remain degenerate; an intrinsically perfect source of entangled photon pairs is available. This result is of general character and applies to all self-organized QDs in the zinc-blende system.