BEGIN:VCALENDAR VERSION:2.0 PRODID:-//linuxsoftware.nz//NONSGML Joyous v1.4//EN BEGIN:VEVENT SUMMARY:Towards High Dimentional Practical Quantum Communication DTSTART:20260505T090000Z DTEND:20260505T110000Z DTSTAMP:20260505T042052Z UID:2a67456a-e4b7-4a35-ad43-aa64f72b33d2 SEQUENCE:2 CREATED:20260504T134631Z DESCRIPTION:Quantum communication promises information-theoretic security grounded in the laws of physics\, addressing the computational vulnerabili ty of classical public-key cryptosystems such as RSA to quantum algorithms . However\, deployable quantum communication systems are still not optimis ed for practical conditions due to high optical complexity and cost\, very low key-rate exchange over long distances\, and limited information capac ity in two-dimensional encodings.This thesis develops and experimentally v alidates a unified prepare-and-measure platform that addresses the challen ges above. At its core is a polarization-encoded optical setup based on a self-compensating SAGNAC loop modulator and a time-multiplexed measurement stage requiring only two single-photon detectors\, achieving 99.78\\% fid elity across two mutually unbiased bases. Using this optical system\, thre e demonstrations of increasing practical relevance are presented. (i) A si mplified BB84 quantum key distribution (QKD) protocol benchmarked over a d eployed 7 km dark fibre link on the Instituto Superior Técnico campus wit h both SPAD and SNSPD detectors\, marking it the first demonstration using top-of-the-line SNSPD detectors in Portugal\; (ii) A numerical feasibilit y study of a CubeSat-based downlink for satellite-to-ground QKD and quantu m keyless private communication (QKPC)\, optimising protocol parameters ac ross a realistic LEO overpass and yielding a peak transmission rate of 80. 8 kHz and 700 MHz at zenith\, respectively\; (iii) A quantum-secure time t ransfer protocol that uses the quantum signals for both key generation and time transfer\, and further uses the QKD-generated key to authenticate ti ming data over a 4.2 m bidirectional fibre link\, achieving a clock offset precision of 0.105 ± 0.018 ps over 8 s averaging windows. The remaining work extends the prepare-and-measure scenario to high-dimensional quantum states by combining time-bin and polarization degrees of freedom\, exploit ing the increased information capacity\, enhanced noise robustness\, and h igher security thresholds of qudit encodings to push practical quantum com munication beyond the qubit regime. LAST-MODIFIED:20260504T134654Z LOCATION:Sala de reuniões TN| Sala 5.09 (Piso 5 - Torre Sul\, Alameda) URL:http://df.vps.tecnico.ulisboa.pt/pt/eventos/towards-high-dimentional-p ractical-quantum-communication/ X-ALT-DESC;FMTTYPE=text/html:

Quantum communicati on promises information-theoretic security grounded in the laws of physics \, addressing the computational vulnerability of classical public-key cryp tosystems such as RSA to quantum algorithms. However\, deployable quantum communication systems are still not optimised for practical conditions due to high optical complexity and cost\, very low key-rate exchange over lon g distances\, and limited information capacity in two-dimensional encoding s.

This thesis develops and experimentally validates a unified pr epare-and-measure platform that addresses the challenges above. At its cor e is a polarization-encoded optical setup based on a self-compensating SAG NAC loop modulator and a time-multiplexed measurement stage requiring only two single-photon detectors\, achieving 99.78\\% fidelity across two mutu ally unbiased bases. Using this optical system\, three demonstrations of i ncreasing practical relevance are presented. (i)

A simplified BB 84 quantum key distribution (QKD) protocol benchmarked over a deployed 7 k m dark fibre link on the Instituto Superior Técnico campus with both SPAD and SNSPD detectors\, marking it the first demonstration using top-of-the -line SNSPD detectors in Portugal\; (ii) A numerical feasibility study of a CubeSat-based downlink for satellite-to-ground QKD and quantum keyless p rivate communication (QKPC)\, optimising protocol parameters across a real istic LEO overpass and yielding a peak transmission rate of 80.8 kHz and 7 00 MHz at zenith\, respectively\; (iii) A quantum-secure time transfer pro tocol that uses the quantum signals for both key generation and time trans fer\, and further uses the QKD-generated key to authenticate timing data o ver a 4.2 m bidirectional fibre link\, achieving a clock offset precision of 0.105 ± 0.018 ps over 8 s averaging windows.

The remaining w ork extends the prepare-and-measure scenario to high-dimensional quantum s tates by combining time-bin and polarization degrees of freedom\, exploiti ng the increased information capacity\, enhanced noise robustness\, and hi gher security thresholds of qudit encodings to push practical quantum comm unication beyond the qubit regime.

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