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With increasing parts of the society being digital and quantum computers developing which can break current encryption methods, there is a stronger need for new methods of encryption to ensure safe communication. The Vernam cipher, which is based on key distribution, is an encryption method which can withstand quantum computers. However, for the Vernam cipher to work, the key to decode the messages has to be distributed safely between the two people in contact. Quantum key distribution (QKD) is a possible way to distribute this key and therefore to ensure secure communication based on fundamental quantum mechanical principles. This thesis studies the implementation of an optical setup node for an entanglement-based QKD network. The report includes a brief overview of the relevant theory of quantum mechanics and quantum information for understanding the subject of quantum communication. Furthermore, QKD is defined and the two different protocols BB84, which is a prepare-and-measure QKD protocol together with E91, an entanglement based QKD protocol, is described. The node is implemented using polarizers, mirrors, beamsplitters, polarizing beamsplitters, waveplates, single-mode fibers and detectors. The purpose is to take a photon with an arbitrary polarization state and divide it into four different photons, each going into detectors measuring the intensity of H, V, D respectively A polarization. The result shows that the implemented setup has to be modified to be able to be used in a real single-photon QKD network. This mainly depends on large intensity losses in the beamsplitters but also that the fiber coupling needs to be increased. Two examples of how this can be done is realigning the components in the setup or increasing the degrees of freedom for the light entering the single-mode fibers connected to the detectors.

Quantum physics allows us a vision of Nature's forces that bind the world, all its seeds and sources. After decades of primarily scientific research, we've arrived at a stage in time where quantum technology can be applied to practical problems and add value outside the field. Four pillars of quantum technologies are commonly identified: quantum computing, quantum simulation, quantum communication, and quantum sensing. For example, quantum computers will allow us to model quantum systems beyond our current capabilities, and quantum communication allows us to protect information unconditionally based on physics. Quantum sensing will enable us to measure our reality beyond classical limits. Within all of these areas, optical photons play a unique role. In quantum computer implementations (e.g. photonic, trapped ion, or superconducting) photons can serve as a computational resource, for system read-out, or for linking distant hardware nodes. Quantum communication can only be realized via photons, utilizing the low-loss propagation of photons in optical fibers, on photonic devices as well as in free space. In quantum sensing and metrology, squeezed light can be used to go beyond the current limits of sensing methods. Therefore, the quantum technology field crucially relies on precise and efficient methods to generate, steer, manipulate and detect photons. This dissertation discusses work in integrated photonic circuits, self-assembled semiconductor quantum dot devices, and superconducting nanowire single--photon detectors. We integrate multiple materials on a silicon nitride platform, including Cu2O as a platform for solid-state Rydberg physics, WS2 to improve non-linear light-generation within Si3N4, and hBN as an excellent single-photon emitter.We demonstrate optically active quantum dots as single-photon emitters in the telecom C-band and their compatibility with commercial telecom equipment.We strain-control the fine-structure splitting of these devices, which is re, Kvantfysiken ger oss en möjlighet att skåda naturens krafter som binder världen, alla dess frön och källor. Efter decennier av främst vetenskaplig forskning har vi nått det stadie i tiden där kvantteknologi kan tillämpas på praktiska problem och tillföra värde utanför akademin. Vanligtvis identifieras fyra pelare av kvantteknologier: kvantberäkning, kvantsimulering, kvantkommunikation och kvantsensorer. Till exempel kommer kvantdatorer att tillåta oss att modellera kvantsystem utöver våra nuvarande möjligheter, och kvantkommunikation tillåter oss att skydda information villkorslöst baserat på fysikens lagar samtidigt som kvantavkänning kommer att göra det möjligt för oss att mäta vår verklighet bortom klassiska gränser. Inom alla dessa områden spelar optiska fotoner en unik roll. I kvantdatorimplementationer (t.ex. fotoniska, fångade joner eller supraledande) kan fotoner fungera som en beräkningsresurs, för systemavläsning eller för att länka avlägsna hårdvaru-noder. Kvantkommunikation kan endast förverkligas via fotoner, på grund av den låga förlusten av fotoner i optiska fibrer, på fotoniska enheter såväl som i fri luft. Inom kvantavkänning och metrologi kan klämt ljus användas för att överskrida de nuvarande gränserna för avkänningsmetoder. Därför förlitar sig kvantteknikområdet på exakta och effektiva metoder för att generera, styra, manipulera och detektera fotoner. Den här avhandlingen diskuterar arbete i integrerade fotoniska kretsar, självmonterade halvledarkvantpricksenheter och supraledande nanotrådsdetektorer för enstaka fotoner. Vi integrerar flera material på en kiselnitridplattform, inklusive Cu2O som en plattform för rydbergs fysik i fast tillstånd, WS2 för att förbättra icke-linjär ljusgenerering inom Si3N4 och hBN som utmärkt singelfoton-sändare. Vi demonstrerar optiskt aktiva kvantprickar som enstaka foton sändare i telekom C-bandet och deras kompatibilitet med kommersiell telekomutrustning. Vi kontrollerar finstruktursdelningen av dessa enheter m

FOGNINI, ANDREAS WALTER, open, Advances in the field of quantum computation could disclose powerful means applicable in many areas of research such as life sciences and metrology among others. However, concerning the field of telecommunication, the so-called “quantum supremacy” in computational power provided by quantum computers poses a huge threat to classical encryption schemes making the transmission of sensitive data unsafe. Progress in Quantum Key Distribution has demonstrated to solve the issue of eavesdropping by using photon qubits but only in short-range transfer. In order to enhance the reliability and range of the quantum communication protocols, the source of photons have to comply with stringent requirements like high single-photon purity, near-to-perfect entanglement, high brightness and high operation rate. With this thesis we have presented and confirmed a model that predicts consistently the entanglement evolution for an AlGaAs quantum dot. This system seems not to suffer from any dephasing mechanism if pumped with two-photon resonant excitation. The experimental results point out that the behavior of these solid-state emitters only depend on the exciton precession, especially considering that AlGaAs Qdots do possess significant nuclear spin. Thus, we could infer that the interaction between the charge carriers involved in the emission and the nuclei is not a relevant dephasing mechanism within the time of the exciton radiative decay. We show that, although no dephasing mechanism occurs for the quantum dot under investigation, reaching a steady concurrence level of one during the whole radiative decay, that indicates perfect entanglement, does still remain a challenge. In light of these findings, we present here two possible workarounds. First, by improving the time resolution of the detection system we aimed at increasing the measurable degree of entanglement, because, according to the theoretical model, the concurrence is lowered by the timing jitter of the detection system. T, I progressi nel campo del calcolo quantistico potrebbero fornire rilevanti applicazioni in molte aree di ricerca come la biologia e la metrologia su tutte. Tuttavia, per quanto riguarda il campo delle telecomunicazioni, la cosiddetta "supremazia quantistica" nella potenza di calcolo del computer quantistico rappresenta un’enorme minaccia per gli schemi di crittografia classici, mettendo a rischio la riservatezza di dati sensibili. I progressi nella distribuzione delle chiavi quantistiche hanno dimostrato di poter risolvere il problema dell’eavesdropping utilizzando qubit di fotoni ma nel solo caso di trasferimento a corto raggio. Per migliorare l’affidabilità e il raggio dei protocolli di comunicazione quantistica, la sorgente dei fotoni deve soddisfare requisiti rigorosi come l’elevata purezza di singolo fotone, l’entanglement quasi perfetto, l’elevata luminosità e l’elevata velocità di funzionamento. Con questa tesi abbiamo presentato e confermato un modello che predice in maniera consistente l’evoluzione dell’entanglement per un quantum dot di AlGaAs che sembra non soffrire di alcun meccanismo di sfasamento se pompato con un’eccitazione risonante a due fotoni. I risultati sperimentali sottolineano che il comportamento di questi emettitori a stato solido dipende solo dalla precessione degli eccitoni, soprattutto considerando che i quantum dots di AlGaAs possiedono uno spin nucleare significativo. Pertanto, si potrebbe dedurre che l’interazione tra i portatori di carica coinvolti nell’emissione e i nuclei non sia un meccanismo di sfasamento rilevante entro il tempo di decadimento radiativo dell’eccitone. Inoltre, in questo manoscritto si dimostra che, sebbene non si verifichi alcun meccanismo di sfasamento nel quantum dot in esame, il raggiungimento di un livello di concorrenza pari a uno, sinonimo di entanglement perfetto, rimane un compito difficile. Alla luce di questi risultati, presentiamo due possibili soluzioni. Innanzitutto, il miglioramento della risoluzio, 1, open, Guardiani, A

Optically active semiconductor quantum dots have proven to be excellent single- and entangled-photon sources, with applications in quantum optics and quantum photonics. These sources are considered crucial in the development of future photonic quantum technology, such as quantum communication, quantum computation and quantum metrology. In future quantum networks, they allow to share quantum information through optical fiber links and implement secure communication protocols based on quantum key distribution. However, there are several challenges when developing quantum dot devices in order to unlock the full potential of these quantum emitters. The ideal quantum dot source efficiently generates triggered single- and entangled-photons on-demand. It provides further high collection-efficiency, low multi-photon probability, near-unity indistinguishability and high entanglement fidelity. Finally, it also offers compatibility with other systems by providing photons with the desired spectral properties and enabling efficient photon coupling. In this thesis the development and fabrication of bright and strain-tunable quantum dot devices for single- and entangled-photon generation has been studied. It covers highly-symmetric GaAs quantum dots emitting in the near-infrared, InAs quantum dots generating photons in the telecom C-band and InAsP quantum dots embedded in InP nanowires enabling deterministic integration into photonic circuits. The main aspects of operating these quantum dots in cryogenic micro-photoluminescence experiments are described, with focus on enhancing the collection efficiency using solid immersion lenses. For strain-tunability, the focus lies on the fabrication of piezoelectric actuators as substrates for the integration of quantum dot samples by polymer-based bonding. Finally, this thesis describes the simulation, fabrication and measurement of a novel device featuring quantum dots embedded in broad-band parabolic mirror microcavities for enhanced ligh, Optiskt aktiva halvledarkvantprickar har visats vara utmärkta källor för enstaka och intraslade fotoner med tillämpningar inom kvantoptik och kvantfotonik. Denna typ av källor anses vara kritiska för utvecklingen av framtida fotonikbaserade kvantteknologier så som kvantkommunikation, kvantdatorer och kvantmetrologi. I framtida kvantnätverk kommer dessa källor tillåta utbytet av kvantinformation genom optiska fiberlänkar och de kommer även att möjligöra kryptografiskt säker kommunikation genom kvantnyckeldistribution. Det finns emellertid flera utmaningar inom utvecklingen av dessa kvantprickkällor för att nå dess fulla potential. En ideal kvantprickkälla genererar effektivt enstaka och intrasslade fotoner på beställning. Den har också hög uppsamlingskoefficient, låg sannolikhet för multi-foton generation, oskiljbar intrassling och hög kvantfidelitet. Slutligen är en ideal källa kompatibel med andra system genom att generera fotoner med önskvärda spektral egenskaper och genom att möjligöra effektiv koppling av dessa fotoner. I denna avhandling har utvecklingen och tillverkningen av ljusa och töjnings-justerbara kvantprickskällor för enstaka och intrasslade fotoner studerats. Avhandlingen täcker högsymmetriska GaAs kvantprickar som genererar fotoner i det nära infraröda spektrat, InAs kvantprickar som genererar fotoner i telekommunikations C-bandet och InAsP kvantprickar inbädade i InP nanotråd som möjliggör deterministisk integration med fotoniska kretsar. De kritiska aspekterna av att använda dessa kvantprickar i kryogena mikro-fotoluminescens-experiment beskrivs, med fokus på att öka insamlingseffektiviteten med hjälp av solida immersionslinser. Fokuset för möjligheten att justera kvantpricken med hjälp av töjning ligger på fabrikationen av piezoelektriska ställdon som substrat för integration med kvantprickar med hjälp av polymerbaserad bindning. Slutligen beskriver denna avhandling simulering, tillverkning och karakterisering av en ny enhet med kvantprickar inbäd