Towards quantum-dot arrays of entangled photon emitters

To make photonic quantum information a reality(1,2), a number of extraordinary challenges need to be overcome. One challenge is to achieve large arrays of reproducible 'entangled' photon generators, while maintaining compatibility for integration with optical devices and detectors(3-5). Semiconductor quantum dots are potentially ideal for this as they allow photons to be generated on demand(6,7) without relying on probabilistic processes(8,9). Nevertheless, most quantum-dot systems are limited by their intrinsic lack of symmetry, which allows only a small number (typically 1 out of 100, or worse) of good dots to be achieved per chip. The recent retraction of Mohan et al.(10) seemed to question the very possibility of simultaneously achieving site control and high symmetry. Here, we show that with a new family of (111)-grown pyramidal sitecontrolled InGaAs1-delta N delta quantum dots it is possible to overcome previous hurdles and obtain areas with up to 15% of polarization-entangled photon emitters, with fidelities as high as 0.721 +/- 0.043.