The primary goal is to build an elementary quantum dot network via entanglement swapping, which has been successfully demonstrated. 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 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 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. In AIP Advances 9, 085112 (2019), Optics Express 28, 19457 (2020) and AIP Advances 12, 055302 (2022), we studied QDs that emit in the telecom wavelengths. We studied the QDs that are close to surface, see publication Applied Physics Letters 118, 221107 (2021).
(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. In the publication Phys. Rev. Lett. 123, 160502 (2019), we finally demonstrated the entanglement swapping of entangled photons from quantum dots. The related PhD thesis won the first place in the BMBF Quantum Future competition. In Physical Review B 103, 075413 (2021), we have successfully increased the operating speed of QD-based entangled photon sources to 1 GHz. The related spin properties in these QDs were studied in Physical Review B 104, 075301 (2021).
(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. The related work has resulted in a prestigious BMBF quantum future grant for a team member.
