Cette thèse réalisée en collaboration avec l’entreprise Thales Alenia Space, qui étudie les protocoles de cryptographie quantique à n parties en dimension d, a un double objectif. D’une part, nous analysons la famille des inégalités de Bell homogènes introduites par par François Arnault dans [1] afin de proposer des outils théoriques pour leur compréhension et leur implémentation à l’aide d’appareils optiques appelés ditters dont une représentation mathématique est donnée par Zukowski et al. dans [2]. Avec ces outils théoriques, nous proposons de nouveaux protocoles cryptographiques en dimension d qui sont décrits dans [3] et qui utilisent ces inégalités. D’autre part, nous étudions les avantages et inconvénients de la cryptographie quantique pour la protection des communications avec un satellite LEO en environnement bruité dans différents scénarios et, pour chacun de ces scénarios, nous concluons sur l’intérêt d’utiliser des protocoles de Distribution Quantique de Clés. / This thesis in collaboration with Thales Alenia Space studies quantum cryptographic protocols for n parties in dimension d. We first analyze the family of Bell inequalities called homogeneous Bell inequalities introduces by François Arnault in [1] and we construct several theoretical tools for a better understanding of these inequalities. With these tools, we show how to implement the measurements required to test these inequalities by using optical devices calleds multiport beamsplitters and described by Zukowski et al. in [2]. We use these devices to construct new cryptographic protocols in dimension d called hdDEB which we describe in [3]. Then, we study advantages and drawbacks of the use of quantum cryptography to protect satellite links in a noisy environment. We consider several scenarios with LEO satellites and, for each of them, we conclude about the interest of using Quantum Key Distribution protocols.

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<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>

When nematic liquid crystals are placed between parallel glass plates with differing alignment directions, the bulk will twist in order to match the boundary conditions. This phenomenon of a twisted cell has been used extensively for the development of everyday liquid-crystal displays. However, there has been limited study of the twisted cell beyond the 90-degree twist case. In this thesis, I explore the behaviour of inhomogeneous liquid-crystal devices where the front and back alignment layers are uniquely and spatially patterned. This creates a non-symmetric device which can act on light differently depending on the orientation of the device and an externally applied voltage. The effect on the polarization of light is theoretically modelled using Jones matrices, and elastic continuum theory is employed to fully understand how the twist and tilt distributions of the liquid crystals change with field strength. Different pattern configurations were fabricated, tested, and characterized, revealing the complex behaviour that occurs with an applied electric field. Liquid-crystal devices provide a bespoke way of tailoring the spatial distribution of light and photons. A set of quantum key distribution experiments through underwater channels, leveraging these devices to encode information on structured photons, is also presented.

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Quantum key distribution (QKD) is an application of quantum mechanics that allowstwo parties to communicate with perfect secrecy. Traditional QKD uses polarization of individual photons, but the development of energy-time entanglement could lead to QKD protocols robust against environmental effects. The security proofs of energy-time entangled QKD rely on a violation of the Bell inequality to certify the system as secure. This thesis shows that the Bell violation can be faked in energy-time entangled QKD protocols that involve a postselection step, such as Franson-based setups. Using pulsed and phase-modulated classical light, it is possible to circumvent the Bell test which allows for a local hidden-variable model to give the same predictions as the quantum-mechanical description. We show that this attack works experimentally and also how energy-time-entangled systems can be strengthened to avoid our attack.

The security of a cryptosystem in most cases relies on the key being kept secret. Quantum key distribution (QKD) enables two authenticated parties without other prior information to share a perfectly secure key. However, repeatedly using the same key to encrypt many different messages is not perfectly secure. A trivial method to obtain a secret key is to use QKD to reestablish a new key for each message. In this thesis, we study an efficient method to update the keys. We call this method quantum key evolution (QKE). The QKE provides a new secret key in each round of the protocol. Therefore, a new secret key is established for next round of protocol execution. We study two problems to present secure schemes applying the QKE. First, we present a new quantum message transmission protocol, to transmit long secret message using less quantum bits than the methods of incorporating QKD with one-time pad, as well as some quantum secure direct communication protocols. Second, we present three-party authenticated quantum key distribution protocols which enable two communicating parties to authenticate the other's identity and establish a session key between them via a trusted center. For the security of our protocols, we give formal standard reduction proofs to the security of our protocols. We show that the security of our protocol is equivalent to the security of BB84 protocol which has been proved to be unconditionally secure. Therefore, our protocols are unconditionally secure.

We experimentally demonstrate a RF-assisted four-state continuous-variable quantum key distribution (CV-QKD) system in the presence of turbulence. The atmospheric turbulence channel is emulated by two spatial light modulators (SLMs) on which two randomly generated azimuthal phase patterns are recorded yielding Andrews' azimuthal phase spectrum. Frequency and phase locking are not required in our system thanks to the proposed digital phase noise cancellation (PNC) stage. Besides, the transmittance fluctuation can be monitored accurately by the DC level in this PNC stage, which is free of post-processing noise. The mean excess noise is measured to be 0.014, and the maximum secret key rate of >20Mbit/s can be obtained with the transmittance of 0.85, while employing the commercial PIN photodetectors.

This thesis investigates the phenomena of phase stability in a fiber-optical MZI (Mach-Zehnder Interferometer). The MZI is a key building block of optical systems for use in experiments with both continuous-wave light and with single photons. By splitting incoming light into two beams and allowing it to interfere with itself, an interference pattern is visible at the output, and this phenomena can be used to code information. This is the operating principle in, for example, QKD (Quantum Key Distribution) experiments. This interference requires coherence that is higher than the length difference between the beams that the incoming light is split into. Particularly the phase of the beams must be equal to achieve constructive interference. If one beam is phase-shifted (with respect to the other) due to the light having traversed a longer path, only partially constructive interference is achieved. If the phase shift also varies with time this leads to a system where experiments can no longer reliably be performed. Sources of these fluctuations are thermal, acoustic or mechanical. Fiber-optical interferometers are particularly sensitive to path length fluctuations of the waveguides as the fiber-optic medium contracts and elongates with temperature, and also has a larger surface area for circulating air to mechanically disturb the waveguides than bulk optics interferometers. In this thesis, a solution to environment-induced phase drift is presented by evaluating implementations of feedback algorithms for automatic control. The algorithms PID (Proportional-, Integral-, Derivative controller) and an ICA (IncrementalControl Algorithm) have been investigated and the performance of these controllers has been compared when used with, and without, optical enclosures. The algorithms are implemented in an FPGA (Field-Programmable Gate Array) and the controller actuates an electro-optical phase modulator that can add a phase shift to one of the light beams in the MZI. This thesis shows that significant improvement in the optical stability can be achieved with active control compared to an interferometer without active phase control. / Det här examensarbetet undersöker fenomenet fasstabilitet i en fiber-optisk MZI (Mach-Zehnder-Interferometer). MZI:n är en viktig byggsten i optiska system som används till experiment med både kontinuerligt emitterande lasrar och med enskilda fotoner. Genom att dela upp inkommande ljus i två strålar och låta det interferera med sig själv så bildas ett interferensmöster vid utgången vilket kan användas för att koda information. Det här är huvudprincipen bakom, till exempel, experiment inom QKD (kvantnyckeldistribution, eng: Quantum Key Distribution). Denna interferens förutsätter en koherens (högre än längdskillnaden mellan strålarna) mellan strålarna som det inkommande ljuset är uppdelat i. Särskilt måste fasen hos de bägge strålarna vara lika för att åstadkomma fullständig konstruktiv intereferens. Om en stråle är fasförskjuten (i förhållande till den andra) på grund av att ljuset har färdats en längre sträcka så uppnås endast delvis konstruktiv interferens. Om fasförskjutningen även varierar med tiden så leder det till ett system där experiment inte längre kan pålitligt utföras. Sådana fluktuationer är orsakade av termiskt, akustiskt samt mekaniskt varierande effekter. Fiberoptiska interferometrar är särskilt känsliga mot förändringar i vågledarnas längd. Detta på grund av att det fiberoptiska mediet dras ihop respektive sträcks ut med temperaturen, samt att fibern har en större ytarea som cirkulerande luft kan påverka mekaniskt jämfört med interferometrar konstruerade av bulkoptik. I det här examensarbetet presenteras en lösning på problemet med miljöinducerad fasskift genom att utvärdera reglertekniska återkopplande algoritmer. Algoritmerna PID (Proportionell-, Integrerande-, Deriverande regulator) samt ICA (Inkrementell Regleralgoritm, eng: Incremental Control Algorithm) har undersökts och deras prestanda har jämförts med samt utan avskärmning. Algoritmerna har implementerats i en FPGA (fältprogrammerbar grindmatris, eng: Field-Programmable Gate Array) och regulatorn styr en elektrooptisk fasmodulator som kan addera en fasförskjutning till en av ljusstrålarna i MZI:n. Resultat visar att passiv avskärmning inte är tillräckligt utan behöver användas tillsammans med aktiv reglering för att uppnå stabilitet över en längre tidsperiod. Detta examensarbete visar på att en signifikant förbättring i den optiska stabiliteten kan uppnås med aktiv reglering jämfört med en interferometer utan aktiv fasreglering.