Periodic Reporting for period 3 - QD-NOMS (Elementary quantum dot networks enabled by on-chip nano-optomechanical systems) Reporting period: 2020-01-01 to 2021-06-30 Summary of the context and overall objectives of the project We are at the dawn of quantum information processing, and its future appears bright in spite of the many remaining challenges. A most outstanding one is scalability, which ensures the interconnection between individual microscopic quantum systems and then the construction of a macroscopic quantum system for practical applications. The project QD-NOMS will demonstrate the downscaling of single quantum dot (QD) systems with full tunabilities, and as well as the upscaling of the complexity of an elementary QD network. The longstanding challenge in scalability of QDs is solved with an innovative on-chip NOMS and several other important techniques. The ambition of this project is to bring the research on semiconductor QD systems to a level comparable to state-of-the-art researches with atoms and spontaneous parametric down conversion sources. With the disruptive technologies developed here, scalable, deterministic, on-chip integrated and semiconductor-compatible single and entangled photon sources will be realized. They can offer a fresh opportunity in future quantum networks and bring optical quantum information processing into a new regime. The success of QD-NOMS will also bring the semiconductor QD based platforms, after a decade of development, to the attention of practical applications. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far There are two main objectives in this project: 1, Building an elementary quantum dot network via multiphoton entanglement; 2, Performing quantum optics experiments with on-chip integrated quantum dot sources. Three workpackges (WP) were designed to achieve the objectives. (1) In WP1, our task is to grow high quality QD samples with high homogeneity and low initial fine structures splitting (FSS). In the publication Nature Communications 8, 15501 (2017), we show that a large ensemble of as-grown polarization-entangled photon emitters can be obtained, using an emerging family of GaAs/AlGaAs QDs grown by droplet etching and nanohole infilling. These QDs exhibit very small FSS, very high inhomogeneity (wavelength distribution<2nm, one of the best values so far) and short radiative lifetime. All measured QDs can emit single pairs of entangled photons with ultra-high purity and high degree of entanglement. In the publication Appl. Phys. Lett. 110, 151102 (2017), we further showed that an independent tuning of emission energy and decay time of neutral excitons confined in single QDs can be achieved by simultaneously employing vertical electric fields and lateral biaxial strain fields. This offers a promising route to engineer the indistinguishability of photons emitted from spatially separated single photon sources. (2) In WP2, we have done preliminary experiments for realizing the multiphoton interference between QDs. In the publication Nature Communications 9, 2994 (2018), we fulfilled the task of achieving a high source brightness of 10 million counts/s. We build a broadband optical antenna with an extraction efficiency of 65% (i.e. more than 40 million counts/s on the detector and 4 times better than our best estimation previously) and demonstrate a highly-efficient entangled-photon source by collecting strongly entangled photons (fidelity of 0.9) at a pair efficiency of 0.372 ± 0.002 per pulse. This is to date among the brightest entangled photon sources. In the publication Physical Review B 98, 161302 (2018), we successfully fulfilled the task of building a FPGA-based active feedback system. It is used to stabilize the frequency of single photons emitted by two separate QDs to an atomic standard. These results, together with the achievements in WP1, removes a major road blocker for multiphoton quantum interference experiments based on QDs.(3) In WP3, we have invented a monolithically integrated Microelectromechanical systems (MEMS) device with great potential for on-chip quantum photonic applications. High-quality epitaxial PMN–PT thin films have been grown on SrTiO3 buffered Si and show excellent piezoelectric responses. Dense arrays of MEMS with small footprints are then fabricated based on these films, forming an on-chip strain tuning platform. Currently we are designing new MEMS structures, and investigating the possibility of coupling semiconductor quantum light sources with waveguides. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) Thanks to the rapid development in our group (and, as well as in the whole community in recent years), we are able to achieve several exceptional results which are beyond the state of the art. For example, our goal in WP1 is to grow high quality QD samples with high homogeneity and low initial fine structures splitting (FSS). In the publication Nature Communications 8, 15501 (2017), we show that all measured QDs in our newly developed samples can emit single pairs of entangled photons with ultra-high purity and high degree of entanglement. One goal in WP2 is to achieve a bright QD sample that can emit 10 million counts per second into the first collection lens. This was a good estimation by considering the developments in the community in year 2016 (when I applied for the ERC grant). In 2018 we built a broadband optical antenna with an extraction efficiency of 65% (i.e. more than 40 million counts/s on the detector and 4 times better than our best estimation previously) and demonstrate a highly-efficient entangled-photon source by collecting strongly entangled photons (fidelity of 0.9) at a pair efficiency of 0.372 ± 0.002 per pulse. This is to date among the brightest entangled photon sources.We are now proceeding to the multiphoton interference experiments with QDs. This is the key objective of our project. We expect that the goals (although, quite ambitious) in our proposal will be accomplished at the end of the project.