"We develop a selfconsistent solution of the Schroedinger and Poisson equations in semiconductor heterostructures with arbitrary doping profiles and layer geometries. An algorithm for this nonlinear problem is presented in a multiband k.P framework for the electronic band structure using the finite element method. The discretized functional integrals associated with the Schroedinger and Poisson equations are used in a variational approach. The finite element formulation allows us to evaluate functional derivatives needed to linearize Poisson’s equation in a natural manner. Illustrative examples are presented using a number of heterostructures including single quantum wells, an asymmetric double quantum well, p-i-n-i superlattices and trilayer superlattices."

<p>Recent advancements in reactor designs offer new capabilities, not seen before. To increase flexibility and reduce operation and maintenance costs, modern reactor designs (e.g., microreactors, SMRs) embrace new technologies which would allow revolutionary operational concepts such as remote monitoring and control, semiautonomous or near-autonomous operation, and two-way communications for real-time integration with the upcoming smart electric grid. Such a continuous data transmission from and towards a reactor site could introduce vulnerabilities and necessitates the prioritization of cybersecurity. However, classical IT-based encryption schemes have been shown to be vulnerable to cyberattacks, as they rely on computational complexity. It has been shown (e.g., Shor’s algorithm) that with the advent of quantum computing practically any asymmetric encryption could be broken within hours. To address this challenge, this thesis explores the feasibility of applying Quantum Key Distribution (QKD) to nuclear reactor communications. QKD is a physical-layer security scheme relying on the laws of quantum mechanics instead of mathematical complexity. QKD promises not only unconditional security but also detection of potential intrusion, as it allows the communication parties to become aware of eavesdropping. To test the proposed hypothesis, a novel simulation tool (NuQKD) was developed to allow for real-time simulation of the BB84 QKD protocol between two remote terminals. NuQKD offers new capabilities not currently available in other simulation tools including true random numbers, modeling of equipment imperfections, and modeling of fiber optic and free space quantum channels. NuQKD was rigorously benchmarked against analytical, numerical and experimental data. Then, a reference nuclear reactor scenario is proposed that is generic enough to cover various communications links internal and external to a reactor site. Using NuQKD, the internal and external data links of the nuclear reactor reference scenario were modeled, and the receiver operating characteristics (ROC) curves were calculated for various QKD configurations. It was found that that QKD can provide adequate key rates with low false alarm rates and has the potential of addressing nuclear industry’s high standards of confidentiality up to 100 km distance using fiber optic. As a result, QKD is shown to be compatible with the existing and future point-to-point reactor communication architectures. These results motivate further study of quantum communications for nuclear reactors.</p>

The 20th century saw the first revolution of quantum mechanics, setting the rules for our understanding of light, matter, and their interaction. The 21st century is focused on using these quantum mechanical laws to develop technologies which allows us to solve challenging practical problems. One of the directions is the use quantum devices which promise to surpass the best computers and best known classical algorithms for solving certain tasks. Crucial to the design of realistic devices and technologies is to account for the open nature of quantum systems and to cope with their interactions with the environment. In the first part of this dissertation, we show how to tackle classical optimization problems of interest in the physical sciences within one of these quantum computing paradigms, known as quantum annealing (QA). We present the largest implementation of QA on a biophysical problem (six different experiments with up to 81 superconducting quantum bits). Although the cases presented here can be solved on a classical computer, we present the first implementation of lattice protein folding on a quantum device under the Miyazawa-Jernigan model. This is the first step towards studying optimization problems in biophysics and statistical mechanics using quantum devices. In the second part of this dissertation, we focus on the problem of excitonic energy transfer. We provide an intuitive platform for engineering exciton transfer dynamics and we show that careful consideration of the properties of the environment leads to opportunities to engineer the transfer of an exciton. Since excitons in nanostructures are proposed for use in quantum information processing and artificial photosynthetic designs, our approach paves the way for engineering a wide range of desired exciton dy- namics. Finally, we develop the theory for a two-dimensional electronic spectroscopic technique based on fluorescence (2DFS) and challenge previous theoretical results claiming its equivalence to the two-dimensional photon echo (2DPE) technique which is based on polarization. Experimental realization of this technique confirms our the- oretical predictions. The new technique is more sensitive than 2DPE as a tool for conformational determination of excitonically coupled chromophores and o↵ers the possibility of applying two-dimensional electronic spectroscopy to single-molecules.

Ces dix dernières années ont vu l'essor des nanostructures GaN/Al(Ga)N en raison de leur potentiel pour le développement de technologies intersousbandes (ISB) en optoélectronique, et ce dans le spectre infrarouge complet. Ces technologies sont basées sur des transitions électroniques entre des états confinés de la bande de conduction de nanostructures, telles que les puits quantiques, les boîtes quantiques et les nanofils. Les composés III-nitrures sont prometteurs en particulier pour les domaines de la télécommunication ultra-rapide et de l'optoélectronique infrarouge rapide dans la bande 3-5 µm pour deux raisons : un large offset de bande et des temps de vie ISB inférieurs à la picoseconde. De plus, la grande énergie du phonon longitudinal optique du GaN permet d'envisager la réalisation de lasers à cascade quantique THz fonctionnant à température ambiante et de systèmes optoélectroniques ISB fonctionnant dans la bande 5-10 THz, interdite pour le GaAs. Récemment, plusieurs technologies optoélectroniques ISB basées sur le GaN ont été développées, comprenant des photodétecteurs, des switchs, et des modulateurs électro-optiques. Cependant, plusieurs défis restent à relever, en particulier concernant l'extension vers les grandes longueurs d'ondes et l'amélioration des performances des appareils pompés électriquement. Une des difficultés principales opposées à l'extension des technologies GaN ISB vers le lointain infrarouge résulte de la présence d'un champ électrique interne, qui ajoute un confinement supplémentaire, augmentant ainsi la distance énergétique entre les niveaux électroniques dans les puits quantiques. Pour pallier à ce problème, on propose une structure de puits quantique alternative, dont les multiples couches créent un potentiel pseudo-carré. On discute la robustesse de ce design quant aux variations causées par les incertitudes de croissance, et la possibilité de l'intégrer dans des structures nécessitant un transport électronique par effet tunnel résonnant. On décrit également les structures fabriquées par épitaxie par jets moléculaires et présentant de l'absorption de lumière polarisée TM dans la gamme THz. Enfin on propose un design de laser à cascade quantique basé sur ces puits quantiques pseudo-carrés. L'utilisation d'orientations non-polaires est une autre façon possible d'obtenir des potentiels carrés. Dans ce manuscrit, on compare des structures de multi puits quantiques GaN/Al(Ga)N dont les croissances ont été réalisées sur des substrats massifs de GaN orientés a et m. On montre que les meilleurs résultats en termes de propriétés structurales et optiques (interbandes et ISB) sont obtenues pour les structures plan m. On démontre des absorptions ISB à température ambiante dans la fenêtre 1.5-5.8 µm, dont la limite haute est imposée par la seconde harmonique de la bande de Reststrahlen du GaN. Le contrôle de la relaxation des porteurs dans les technologies ISB prend d'autant plus d'importance pour l'ingénierie de ces structures qu'on en augmente les efficacités. L'existence de temps de vie ISB plus longs dans les systèmes confinés latéralement a été démontrée, motivant ainsi les recherches pour intégrer les nanofils en tant qu'élément actif dans les technologies ISB. De plus, le grand rapport de la surface au volume pour les nanofils permet la relaxation élastique des tensions dues aux différences de paramètres de mailles. Cette relaxation augmente la taille de la région active efficace et améliore sa composition, dépassant les limites des systèmes planaires ou des boîtes quantiques. Dans ce manuscrit, on décrit l'observation expérimentale d'absorption de lumière infrarouge polarisée TM attribuée à la transition intrabande s-pz dans des nanodisques GaN/AlN dopés avec du Ge et insérés dans des nanofils de GaN. On compare les résultats obtenus avec les calculs théoriques, qui prennent en compte la distribution en trois dimensions de la tension, les charges de surface et les effets des corps multiples. / GaN/Al(Ga)N nanostructures have emerged during the last decade as promising materials for new intersubband (ISB) optoelectronics devices, with the potential to cover the whole infrared (IR) spectrum. These technologies rely on electron transitions between quantum-confined states in the conduction band of nanostructures –quantum wells (QWs), quantum dots (QDs), nanowires (NWs). The large conduction band offset between III-N compounds, and their sub-ps ISB recovery times make them appealing for ultrafast telecommunication devices and for fast IR optoelectronics in the 3-5 µm band. Furthermore, the large energy of GaN LO phonon (92 meV) opens prospects for room-temperature THz quantum cascade lasers and ISB devices covering the 5-10 THz band, inaccessible to GaAs. A variety of GaN-based ISB optoelectronic devices have recently been demonstrated, including photodetectors, switches and electro-optical modulators. However, a number of issues remain open, particularly concerning the extension towards longer wavelengths and the improvement of electrically pumped devices performance. One of the main challenges to extend the GaN-ISB technology towards the far-IR comes from the polarization-induced internal electric field, which imposes an additional confinement that increases the energetic distance between the electronic levels in the QWs. In order to surmount this constraint, I propose alternative multi-layer QW designs that create a pseudo-square potential profile. The robustness of the designs in terms of variations due to growth uncertainties, and the feasibility of their integration in devices architectures requiring resonant tunneling transport are discussed. Experimental realizations by molecular-beam epitaxy displaying TM-polarized THz absorption are presented. A quantum cascade laser design incorporating pseudo-square QWs is introduced. An alternative approach to obtain square potential profiles is the use of nonpolar orientations. In this thesis, I compare GaN/Al(Ga)N multi-quantum wells grown on a and m nonpolar bulk GaN showing that the best results in terms of structural and optical (interband and ISB) performance are obtained for m-plane structures. Room-temperature ISB absorption in the range of 1.5–5.8 µm is demonstrated, the longer wavelength limit being established by the second order of the Reststrahlen band in GaN. As ISB devices are pushed towards higher efficiencies, the control of carrier relaxation becomes a key aspect for device engineering. Longer intraband lifetimes have been proven to exist in laterally confined systems, which motivates studies to incorporate NWs as active elements in ISB devices. Furthermore, the large NW surface-to-volume ratio allows misfit strain to be elastically released, extending the viable active region size and composition beyond the limits of planar systems or QDs. In this thesis, I report the experimental observation of TM-polarized IR absorption assigned to the s-pz intraband transition in Ge-doped GaN/AlN nanodisks inserted in self-assembled GaN NWs. Results are compared with theoretical calculations accounting for the 3D strain distribution, surface charges and many-body effects.STAR

<p dir="ltr">InSbAs is a promising platform for exploring topological superconductivity and spin-based device applications, thanks to its strong spin-orbit coupling (SOC) and high effective <i>g</i>-factor. This thesis investigates low-temperature transport of electrons confined in InSb_1-xAs_x quantum wells. Specifically, we study the properties of electrons confined in 2D and 0D by fabricating gated Hall bars and gate-defined quantum dots. Theoretical considerations suggest that InSbAs will have stronger SOC and a larger effective <i>g</i>-factor compared to InAs and InSb. Both the SOC and effective <i>g</i>-factor change as a function of arsenic mole fraction, but much remains to be understood in real material systems. Here, we study the dominant scattering mechanisms, effective mass, spin-orbit coupling strength, and the <i>g</i>-factor in InSb_1-xAs_x quantum wells grown by molecular beam epitaxy. </p><p dir="ltr">We explore 30 nm InSb_1-xAs_x quantum wells with arsenic mole fractions of <i>x</i> = 0.05, 0.13, and 0.19. The 2DEG properties were studied by fabricating gated Hall bars and placing them in a perpendicular magnetic field at low temperatures (T = 10 - 300 mK). All samples showed high-quality transport with mobility greater than 100,000 cm²/Vs. For the <i>x</i> = 0.05 sample, the 2DEG displays a peak mobility μ = 2.4 x 10⁵ cm²/Vs at a density of <i>n</i> = 2.5 x 10¹¹ cm^-². We investigated the evolution of mobility as a function of arsenic mole fraction and 2DEG density for all samples. As the arsenic mole fraction increases, peak mobility decreases, and the dependence of mobility on density becomes weaker, suggesting that short-range scattering becomes the dominant scattering mechanism. We extracted an alloy scattering rate of τ_alloy = 45 ns^-1 per % As, an important parameter for understanding the impact of disorder on induced superconductivity. The high mobility, strong spin-orbit coupling, and low effective mass in this material system resulted in a beating pattern in the Shubnikov de Haas oscillations, allowing for the extraction of the Rashba parameter as a function of density and arsenic mole fraction. We observed a gate tunable spin-orbit coupling and, as predicted by theory, an increase in spin-orbit coupling with increasing arsenic mole fraction. For the sample with x = 0.19, the highest Rashba parameter is α_R ~ 300 meVÅ, which is significantly higher than in InSb. </p><p dir="ltr">In addition, we explored 0D confinement by fabricating a gate-defined quantum dot in an InSb₀_.87As_0.13 quantum well. By studying the evolution of Coulomb blockade peaks and differential conductance peaks as a function of magnetic field, a nearly isotropic in-plane effective <i>g</i>-factor in the [1-10] and [110] crystallographic directions was extracted, ranging from 49-58. The values extracted are 1.8 times higher than in a quantum dot fabricated in pure InSb. Furthermore, this study produced the first demonstration of a tunable spin-orbit coupling in this material system. This was achieved by measuring the avoided crossing gap, mediated by spin-orbit coupling, between the ground state and excited state in a magnetic field. The avoided crossing gap indicates the strength of the spin-orbit coupling; the maximum energy separation extracted is Δ_SO ~100 μeV. </p><p dir="ltr">Our work should stimulate further investigation of InSbAs quantum wells as a promising platform for applications requiring strong spin-orbit coupling, such as topological superconductivity or spin-based devices.</p>

<p dir="ltr">The two-dimensional electron gas (2DEG) in shallow InAs quantum wells, combined with epitaxial aluminum, is commonly used to study topological superconductivity. Key features include strong spin-orbit coupling, a high effective g-factor, and the ability to manage proximity-induced superconductivity. My thesis discusses two aspects of this unique material. In the first section, I report on the transport characteristics of shallow InGaAs/InAs/InGaAs quantum wells and evaluate the effect of modulation doping on these shallow InAs quantum well structures. We systematically investigate the magnetotransport properties in relation to doping density and spacer thickness. Optimized samples show peak mobilities exceeding 100,000 cm²/Vs at n_2DEG < 10¹²cm^-2 in gated Hall bar, marking the highest mobility observed in this type of heterostructure. Our findings suggest that the doping layer moves the electron wave function away from the surface, minimizing surface scattering and enhancing mobility. This mobility improvement does not compromise Rashba spin-orbit coupling or induced superconductivity. In the second section, motivated by a theoretical study by Peng et al., we explore tunneling spectroscopy measurements on DC current biased planar Josephson junctions made on an undoped hybrid epitaxial Al-InAs 2DEG heterostructure. We observe four tunneling conductance peaks in the spectroscopy that can be adjusted by DC current bias. Our analysis indicates that these results come from strong coupling between the tunneling probe and the superconducting leads, rather than from Floquet engineering. We also touch on potential improvements to the device's design and materials. This work lays the groundwork for further investigation of Floquet physics in planar Josephson junctions. This thesis ends with a discussion of other unusual physics that could be explored in these novel shallow InAs quantum wells coupled with epitaxial aluminum.</p>

Cette thèse porte sur la génération d'états quantiques mésoscopiques arbitraires de la lumière par l'utilisation itérée d'un protocole élémentaire simple, basé sur un procédé d'adjonction cohérente de photons. Ce protocole tire parti des deux descriptions ondulatoires et corpusculaires complémentaires de la lumière, en impliquant des états davantage propices à une description discrète (les photons, corpuscules élémentaires de la lumière) avec des mesures continues sur le champ électromagnétique (les mesures de quadratures). Nous démontrons la validité du protocole expérimentalement en montrant que celui-ci permet la génération d'états chats de Schrödinger optiques (superposition cohérente d'états quasi-classiques) comprimés en quadrature. Ces états présentent un grand intérêt étant donné qu'ils peuvent constituer la brique élémentaire du calcul quantique à états cohérents, le "qubit" (ou bit quantique). Nous étudions également la mise en oeuvre théorique du protocole dans la production d'états plus complexes : les états "Peignes en quadrature" dont la fonction d'onde sur la quadrature x est constituée d'une succession de fins pics gaussiens modulés par une large enveloppe gaussienne. Outre l'intérêt que ceux-ci présentent dans la mise en oeuvre de codes correcteurs d'erreurs quantiques, nous montrons qu'ils permettent de réaliser des violations des inégalités de Bell par mesures homodynes seulement, ouvrant la voie à des tests sans échappatoires / In this thesis, we propose a new scheme for the generation of mesoscopic optical quantum states of light, by the iterated use of an elementary protocol, based on a coherent adjunction of photons. This protocol takes advantage of the two complementary wave and particle descriptions of light, by involving intrinsically discrete states (photons, elementary particles of light) and continuous measurements on the electromagnetic field (quadrature measurements). We experimentally prove the validity of the protocol by showing that it enables the generation of squeezed optical Schrödinger cat states (coherent superposition of quasi-classical states). These states are of great interest as they can be used as the elementary piece in the coherent state quantum calculation, the "qubit" (or quantum bit). We also study the theoretical implementation of the protocol for the production of more complex states : the "quadrature Comb states", whose wavefunction along the quadrature x consists in a succession of thin gaussian peaks modulated by a larger gaussian envelope. Amongst other applications like quantum error correcting codes, we show that they allow for a violation of Bell's inequalities with homodyne measurements only, opening the path towards loophole-free violations