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The generation and long-haul transmission of highly entangled photon pairs is a cornerstone of emerging photonic quantum technologies with key applications such as quantum key distribution and distributed quantum computing. However, a natural limit for the maximum transmission distance is inevitably set by attenuation in the medium. A network of quantum repeaters containing multiple sources of entangled photons would allow overcoming this limit. For this purpose, the requirements on the source's brightness and the photon pairs' degree of entanglement and indistinguishability are stringent. Despite the impressive progress made so far, a definitive scalable photon source fulfilling such requirements is still being sought after. Semiconductor quantum dots excel in this context as sub-Poissonian sources of polarization entangled photon pairs. In this work, we present the state-of-the-art set by GaAs based quantum dots and use them as a benchmark to discuss the challenges toward the realization of practical quantum networks.

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.

Quantum teleportation and entanglement swapping represent pivot concepts in quantum information science. Here, we show that entangled photon pairs generated on-demand by quantum-dots can be used to implement successfully quantum teleportation and entanglement swapping protocols., Part of ISBN 9781943580569QC 20201229

Several semiconductor quantum dot techniques have been investigated for the generation of entangled photon pairs. Among the other techniques, droplet epitaxy enables the control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of the entanglement-ready quantum dots that can be fabricated with this method is still limited to around 5%, and matching the energy of the entangled photons to atomic transitions (a promising route toward quantum networking) remains an outstanding challenge. Here, we overcome these obstacles 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 with previous reports. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due 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.

I punti quantici (quantum dots, QDs) epitassiali possono generare fotoni in uno stato di polarizzazione entangled tramite la cascata radiativa bieccitone-eccitone. Le potenzialità di funzionamento deterministico e di scalabilità dei dispositivi sono vantaggi unici per le applicazioni in reti quantistiche. Tuttavia, una alta simmetria strutturale e una scelta ponderata dei materiali sono cruciali per affrontare le principali cause di degradazione dell'entanglement, ossia la separazione in energia (fine structure splitting, FSS) tra i due stati eccitonici ed i campi magnetici nucleari oscillanti legati all'interazione iperfine. Questa tesi si concentra su QDs di GaAs/AlGaAs cresciuti su un substrato di GaAs (111)A con un nuovo approccio alla epitassia da goccia, in cui la fase fondamentale di cristallizzazione è eseguita ad una temperatura significativamente più alta rispetto ai precedenti tentativi. La scelta specifica di orientazione del substrato, denotata da un basso coefficiente di adsorbimento per l'As, favorisce l'incorporazione di As nella goccia piuttosto che con adatomi di Ga sulla superficie. Diversamente dall'epitassia da goccia convenzionale, limitata a temperature del substrato sotto i 250°C, la formazione di nanostrutture è osservata fino a 520°C. Ciò porta ad una maggiore qualità cristallina dei QDs e della barriera circostante e ad un ridotto impatto dell'interdiffusione, evidenziato dal confronto tra macro-fotoluminescenza e simulazioni dei livelli energetici basate sulle geometrie misurate tramite microscopia a forza atomica su campioni senza copertura. Il controllo sulla dinamica di crescita porta alla fabbricazione di QDs con diversi rapporti di forma e, quindi, alla progettazione riproducibile della lunghezza d'onda d'emissione. Così è dimostrato il funzionamento attorno a 780 nm, che permette l'integrazione con memorie ottiche al Rb, un importante obiettivo per la realizzazione di ripetitori quantistici. La forma di piramide troncata co, Epitaxial semiconductor quantum dots (QDs) can generate polarization-entangled photons through the biexciton-exciton radiative cascade. The potential for on demand operation and device scalability are unique assets for future applications in quantum networking. However, a high QD structural symmetry and a proper choice of materials are crucial to tackle the main sources of entanglement degradation, namely the presence of a fine structure energy splitting (FSS) between the two bright exciton states and fluctuating nuclear magnetic fields due to the hyperfine interaction. This thesis focuses on GaAs/AlGaAs QDs grown on a (111)A GaAs substrate by a novel approach based on droplet epitaxy, where the fundamental crystallization step is performed at a temperature which is significantly higher than in previous reports. The specific choice of substrate orientation, characterized by a very low As sticking coefficient, favors As incorporation inside the droplet rather than with Ga adatoms on the surface. In contrast to standard droplet epitaxy, which is restricted to substrate temperatures below 250°C, quantum dot formation is observed up to 520°C. The increased growth temperature improves the crystalline quality of the QDs and their surrounding barrier and strongly reduces the impact of interdiffusion. This is confirmed by comparing ensemble photoluminescence with energy level simulations based on the geometrical features probed by atomic force microscopy on uncapped samples. The control over the growth dynamics leads to the fabrication of QDs with different aspect ratios and, therefore, to the reproducible design of the emission wavelength. Thus, operation in the 780 nm range is demonstrated, which allows the frequency-matching of these QDs with Rb-based optical quantum memories, an important target for the realization of quantum repeaters. At the same time, a truncated pyramid shape with regular hexagonal base is achieved, also fulfilling the requirements on high in-plane

The search for a suitable entangled photon source is an active research direction because such sources play an important role in key quantum communication protocols and different quantum computation approaches. We present the fabrication and the characterization of an entangled photon source based on GaAs/AlGaAs QDs grown on a ( l l l )A substrate by a novel approach based on droplet epitaxy. DE is still quite far from meeting the fundamental requirements for the practical realization of a hybrid semiconductor−atomic quantum network. The average fine structure splitting value is still too high and gives rise to a minor fraction of only approximately 5% of entanglement-ready emitters. Additionally, we want to match the QDs emission with Rb—based quantum memories (around 780 nm). Finally, a long-standing drawback is the low substrate temperature during the formation of the nanostructures and the surrounding barrier, which places a limit on the crystalline and optical quality of the material, issue that can be only partially overcome with an annealing process. Thanks to the specific choice of substrate orientation, the use of low Al concentration in the barrier and finely tuned growth conditions, we overcome this limitations. Our method drastically improves the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% due 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. Fidelity measurements under two-photon resonant excitation yielded a value of 0.8. which already reaches the state-of the art for the more studied InGaAs/GaAs QDs in absence of postselection or external FSS tilting, thus confirming the intriguing potentiality of this technique.

A hybrid metal-semiconductor nanosystem for the generation of THz radiation, based on the fabrication of GaAs quantum molecules-Ga metal nanoparticles complexes through a self assembly approach, is proposed. The role of the growth parameters, the substrate temperature, the Ga and As flux during the quantum dot molecule (QDM) fabrication and the metal nanoparticle alignment are discussed. The tuning of the relative positioning of QDMs and metal nanoparticles is obtained through the careful control of Ga droplet nucleation sites via Ga surface diffusion. The electronic structure of a typical QDM was evaluated on the base of the morphological characterizations performed by atomic force microscopy and cross sectional scanning electron microscopy, and the predicted results confirmed by micro-photoluminescence experiments, showing that the Ga metal nanoparticle-GaAs quantum molecule complexes are suitable for terahertz generation from intraband transition.

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 cm2-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.

We investigate the effect of surface Ga accumulation on the growth of In0.5Ga0.5N by PAMBE at low temperatures (T= 440 °C). We find that the control of the surface metal condition is very important because the crystallization process strongly depends on the metal flux impinging on the surface. The growth rate rapidly decreases when the supply of metal flux exceeds certain amount. Such phenomenon can be explained by the model taking into account droplet effects on the incorporation of metals adatom into the crystal.

We present a fully self‐assembly technique, based on Droplet Epitaxy, to fabricate semiconductor surfaces functionalized with asymmetric planar quantum dot molecules –metal nanoparticles hybrid systems for efficient THz generation. The role of growth parameters on molecule fabrication and metal nanoparticle alignment for the matching of the grown system with the design requirements is discussed.

Entanglement plays a crucial role in many protocols for quantum cryptography and in various approaches for quantum computation. Semiconductor quantum dots (QDs) have been proposed as a source of polarization-entangled photons which can be integrated in an electrically driven solid state device [1]. The generation process relies on the biexciton-exciton radiative cascade. However, e-h exchange interaction often induces a fine structure splitting (FSS) between the bright exciton states and destroys quantum correlation. In the case of QDs grown on commonly used (100) substrates, this degeneracy lifting is caused by the C2v symmetry stemming from asymmetric interfaces, strain anisotropy, piezoelectric fields and shape elongation [2,3]. A viable alternative relies on growing QDs on the higher symmetry (111) substrate. Theoretical investigations have shown that QDs with C3v symmetry should exhibit zero FSS [4,5]. The most studied III-V materials do not grow in the Stranski-Krastanov mode on a (111) surface, however different techniques such as droplet epitaxy [6] or the use of patterned substrates [7] have demonstrated to be able to overcome this limitation. In our work we focus on GaAs/AlGaAs (111) QDs grown by droplet epitaxy. Polarization resolved single dot photoluminescence measurements on hexagonal QDs are presented. Charged and bi-excitonic complexes are consistently identified by means of power and polarization dependence analyses. A broad FSS energy distribution is observed, with an average value smaller than the one reported for QDs grown on (100) substrates using the same technique and emitting at similar wavelengths. The phase distribution of the polarization axis evidences the absence of systematic anisotropies. These results are in agreement with previous studies on similar samples [8]. Recent advances in fabrication have proven the ability to obtain atomically flat substrates and to gradually tune the shape from hexagonal to triangular by changing the growt

Our work, focused on the heteroepitaxial Ge/Si (001) system shows that patterned Si substrates in appropriate pillar arrays, featuring micrometric dimensions within the common deep-etching capabilities and array size suitable for several applications (up to few hundreds microns), do provide exceptional compliance to the thermal strain of a continuous Ge film deposited on top. Micro-Raman and XRD measurements on the Ge suspended layer, as obtained by Low Energy Plasma Enhanced Chemical Vapor Deposition, confirm the theoretical predictions. Actually, the sharp onset with pillar aspect ratio of such an effect is demonstrated to be triggered by the free rotation of the pillars, as induced by the thermal contraction of the film. The strain relaxation is deeply analyzed both by FEM simulations and experimental XRD results, showing a very good quantitative agreement. Our findings have far reaching consequences beyond the crystalline heteroepitaxial systems, whenever a film of any microstructure, or composition, is deposited in suspended patches on suitable pillar arrays.

Two-dimensional (2D) materials represents nowadays the new frontiers of semiconductor technology [1]. At one side, great effort has been done in research applied to graphene producing new technology that will rebuild many industrial sectors. On the other side non- graphene 2D materials like transition metal dichalcogenides (TMDs) are highly attractive because they offer complementary properties to graphene [2] but still lack a large-scale production method for high quality and well controlled layers. MoS2, one of the most studied TMDs materials, has been produced by using many techniques, but the deposition through chemical methods, i.e. based on the use of materials such as Mo or MoO3 and then the reaction with sulfur, already demonstrates to fit better the stringent requirements of lateral uniformity on the centimeter scale, vertical scalability and structural optimization as function of the growth parameters [3]. However, many details about the chemistry of the reaction between Mo precursor and sulfur needs further clarification in view of a large-area production [4]: in particular, it is not clear the role of the materials used as precursors, of the substrate type and of the deposition temperature, which drives Mo-S reaction, but also other undesired ones. This work is focused on the study of some of these fundamental parameters in the growth of MoS2 on SiO2/Si substrates by vapor-solid chemical reaction between Mo pre-deposited thin film precursor (TFP) and sulfur (figure 1(a)). We demonstrate that the control of the Mo film thickness allows us to have accurate control of the MoS2 thickness. This methodology leads to MoS2 nanosheets with a tunable number of layers and uniformly extended throughout the whole area of the support substrates (figure 1(b)). In this type of process, the role of the growth temperature and of the substrate on the structural properties of the layers becomes dramatically important. Various techniques are employed to characterize the MoS2