Polarization-entangled photons produced with high-symmetry site-controlled quantum dots

Efficient sources of entangled photons 'on demand' are crucial for the development of quantum information technology(1). Such sources cannot rely on parametric down-conversion techniques because they generate entangled pairs with Poissonian statistics(2). Biexciton-exciton decays in semiconductor quantum dots have been proposed(3) and demonstrated(4-6) as a source of triggered polarization-entangled photons, but their efficiency is limited by the fine-structure splitting of exciton transitions due to low quantum dot symmetry(7). Here, we report on the generation of entangled photons from highly symmetric, site-controlled quantum dots grown in inverted pyramids(8,9). Polarization entanglement is demonstrated by measurements of the two-photon density matrix and the confirmation of several entanglement criteria. The unique symmetry and exceptional uniformity of the pyramidal quantum dots provide significant potential for producing sources of triggered entangled photons from as-grown quantum dots without resorting to any of the post-processing steps customarily used in previous studies(10).