Murtaza, Ghulam (2023) Molecular Quantum Emitters for Quantum Key Distribution. [Tesi di dottorato]

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Abstract

Many quantum emitters have been extensively investigated till date, but important challenges remains open. Such challenges include ambient temperature operation as well as the portability of the quantum emitter within communication devices. In this thesis we have discussed the investigation of DBT:AC-based single photon sources for quantum photonics technologies, obtaining promising results. The presented SPS exhibits high levels of purity and interference ability, both of which are essential for the success of quantum technologies. In particular, the triggered generation of highly indistinguishable single photons from a single organic dye molecule under non-resonant pulsed excitation is achieved without the aid of any photonic resonance, resulting in a HOM interference visibility of over 78%, limited only by the residual dephasing present at the operating temperature of 3K. The remarkable spectral stability demonstrated in this experiment, where the Hong-Ou-Mandel(HOM)visibility remains largely unaffected even for photons separated by up to 125 ns, holds tremendous potential for the practical implementation of quantum technologies. Multiple photons are integral to the functioning of linear optical quantum computing, where the stable and predictable behavior of the HOM visibility observed in this experiment represents a significant step forward in the quest for reliable and efficient quantum computing. However, the brightness of the source at detector is currently limited to around 2%, which corresponds to a brightness at the first lens of around 5%. Therefore, the integration of the emitter with photonic devices becomes essential for implementation in quantum applications. Recent studies have demonstrated the potential of this type of system to be seamlessly integrated into hybrid photonic structures, where photonic resonance can modify both the radiation pattern and spectral distribution of the emission, thereby enhancing the source brightness to the state-of-the-art level. Moreover, the successful implementation of a proof-of-concept Quantum Key Distribution (QKD) setup employing a deterministic single-photon source operating at room temperature is demonstrated in this study. The use of a molecular emitter as a single-photon source has several advantages, including its room-temperature operation and long-term photostability, making it an attractive candidate for practical quantum communication applications. The experimental results indicate that the expected SKR(mol) (0.5, Mbps at zero losses) of the molecular emitter is competitive with state-of-the-art experiments at cryogenic and room temperature. This performance can be further improved by optimizing the nano-photonics of the sample configuration and the optical setup, offering a promising avenue for future research. Furthermore, the analysis of the overall source efficiency suggests that the use of the generated single-photon states can offer potential advantages compared to the decoy state performances. The room-temperature operation and long-term photostability of the emitter make the proposed hybrid technology particularly interesting for satellite quantum communication. Finally, we demonstrated real-time state preparation by means of a polarization modulator. Two alternative approaches, such as FRM and a Sagnac circuit, were applied to compensate for polarization mode dispersion arising in the lithium niobate crystal. In the case of WCP, the QBER is ≈ 1.6% employing any of the compensation methods. We also used the SPS operated at 3k for real-time states encoding, demonstrating a QBER of ≈ 2.1%. The future upgrade to real-time state-preparation and -measurement has the potential to significantly enhance the performance and reliability of the proposed QKD setup. This would enable a continuous monitoring and optimization of the single-photon source, which is crucial for achieving high SKR(mol) values and for detecting potential eavesdropping attacks. The integration of real-time state-preparation and -measurement with the emitter at low temperatures might enable the implementation of advanced quantum communication protocols, such as quantum teleportation and superdense coding, which have the potential to revolutionize the field of quantum communication.

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