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.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:kth-349368 |
| Date | January 2024 |
| Creators | Zetterberg, Oliver |
| Publisher | KTH, Skolan för teknikvetenskap (SCI) |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | TRITA-SCI-GRU ; 2024:178 |
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