Large-scale integration of MoS2 in electronic devices requires the development of reliable and cost-effective deposition processes, leading to uniform MoS2 layers on a wafer scale. Here we report on the detailed study of the heterogeneous vapor-solid reaction between a pre-deposited molybdenum solid film and sulfur vapor, thus resulting in a controlled growth of MoS2 films onto SiO2/Si substrates with a tunable thickness and cm(2)-scale uniformity. Based on Raman spectroscopy and photoluminescence, we show that the degree of crystallinity in the MoS2 layers is dictated by the deposition temperature and thickness. In particular, the MoS2 structural disorder observed at low temperature (<750 °C) and low thickness (two layers) evolves to a more ordered crystalline structure at high temperature (1000 °C) and high thickness (four layers). From an atomic force microscopy investigation prior to and after sulfurization, this parametrical dependence is associated with the inherent granularity of the MoS2 nanosheet that is inherited by the pristine morphology of the pre-deposited Mo film. This work paves the way to a closer control of the synthesis of wafer-scale and atomically thin MoS2, potentially extendable to other transition metal dichalcogenides and hence targeting massive and high-volume production for electronic device manufacturing.

Photonic entanglement swapping, the procedure of entangling photons without any direct interaction, is a fundamental test of quantum mechanics 1 and an essential resource to the realization of quantum networks 2 . Probabilistic sources of non-classical light can be used for entanglement swapping, but quantum communication technologies with device-independent functionalities demand for push-button operation 3 that, in principle, can be implemented using single quantum emitters 4 . This, however, turned out to be an extraordinary challenge due to the stringent requirements on the efficiency and purity of generation of entangled states. Here we tackle this challenge and show that pairs of polarization-entangled photons generated on-demand by a GaAs quantum dot can be used to successfully demonstrate allphotonic entanglement swapping. Moreover, we develop a theoretical model that provides quantitative insight on the critical figures of merit for the performance of the swapping

Quantum key distribution—exchanging a random secret key relying on a quantum mechanical resource—is the core feature of secure quantum networks. Entanglement-based protocols offer additional layers of security and scale favorably with quantum repeaters, but the stringent requirements set on the photon source have made their use situational so far. Semiconductor-based quantum emitters are a promising solution in this scenario, ensuring on-demand generation of near-unity-fidelity entangled photons with record-low multiphoton emission, the latter feature countering some of the best eavesdropping attacks. Here, we use a coherently driven quantum dot to experimentally demonstrate a modified Ekert quantum key distribution protocol with two quantum channel approaches: both a 250-m-long single-mode fiber and in free space, connecting two buildings within the campus of Sapienza University in Rome. Our field study highlights that quantum-dot entangled photon sources are ready to go beyond laboratory experiments, thus opening the way to real-life quantum communication.

Several semiconductor quantum dot technologies have been investigated for the generation of entangled photon pairs. Among the others, droplet epitaxy enables control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of entanglement-ready quantum dots that can be fabricated with this method is still limited to values around 5%, and matching the energy of the entangled photons to atomic transitionsa promising route towards quantum networking -remains an outstanding challenge.Here, we overcome these hurdles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared to previous reports. Our method improves drastically the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% thanks to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations.Keywords: Quantum dots, entanglement, droplet epitaxy, fine structure splitting, rubidium, resonant two-photon excitation Under the ongoing effort to develop practical quantum technologies, the search for a suitable entangled photon source is an active research direction, as it plays a role in key quantum communication protocols and some approaches to quantum computation. 1,2 Above all, it is a fundamental requirement for the realization of repeaters capable to transfer quantum entanglement over long distances.Epitaxial quantum dots (QDs) are a promising alternative to parametric down-converters, given their ability to generate photons ondemand with high efficiency and their compatibility with semiconductor foundries. 3,4 In order to use QD entanglement resources in reallife technologies, two main roadblocks have to be overcome. The first is related to the difficulty of consistently finding emitters capable 1 arXiv:1710.03483v1 [cond-mat.mes-hall]

On-demand generation of background-free single photons from a solid-state source Applied Physics Letters 112, 093106 (2018);

We introduce a high-temperature droplet epitaxy procedure, based on the control of the arsenization dynamics of nanoscale droplets of liquid Ga on GaAs(111)A surfaces. The use of high temperatures for the self-assembly of droplet epitaxy quantum dots solves major issues related to material defects, introduced during the droplet epitaxy fabrication process, which limited its use for single and entangled photon sources for quantum photonics applications. We identify the region in the parameter space which allows quantum dots to self-assemble with the desired emission wavelength and highly symmetric shape while maintaining a high optical quality. The role of the growth parameters during the droplet arsenization is discussed and modelled.

Semiconductor quantum dots are currently emerging as one of the most promising sources of non-classical light on which to base future quantum networks. They can generate single photons as well as pairs of entangled photons with unprecedented brightness, indistinguishability, and degree of entanglement. These features have very recently opened up the possibility to perform advanced quantum optics protocols that were previously inaccessible to single quantum emitters. In this work, we report on two experiments that use the non-local properties of entanglement to teleport quantum states: three-photon state teleportation and four-photon entanglement teleportation. We discuss all the experimental results in light of a theoretical model that we develop to account for the nonidealities of the quantum source. The excellent agreement between theory and experiment enables a deep understanding of how each parameter of the source affects the teleportation fidelities and it pinpoints the requirements needed to overcome the classical limits. Finally, our model suggests how to further improve quantum-dot entangled-photon sources for practical quantum networks.

In this paper we present the exceptional thermal strain release provided by micrometric Si pillar arrays to Ge epitaxial patches suspended on them, for different pillar aspect ratios and patch sizes. By combining 3D and 2D Finite Element Method simulations, low-energy plasma-enhanced chemical vapor deposition on patterned Si substrates, μ-Raman, μ-photoluminescence and XRD measurements, we provide a quantitative and consistent picture of this effect with the patch sizes. Strain relaxation up to 85% of the value for the corresponding planar films can be obtained for a squared patch 100 μm in size. Finally, the enhanced thermal strain relaxation is analytically explained in terms of the Si pillar lateral tilting, critically dependent on the pillar aspect ratio, very similarly to the well-known case of a deflected beam. Our results are transferable to any material deposited, or wafer bonded at high temperature, on any patterned substrate: wafer bowing can be controlled by micrometric patterned features well within the present capabilities of deep reactive ion etching