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:20260616T184738Z UID:2a67456a-e4b7-4a35-ad43-aa64f72b33d2 SEQUENCE:1 CREATED:20260504T134623Z DESCRIPTION: Quantum communication promises information-theoretic security grounded in the laws of physics\, addressing the computational vulnerabil ity of classical public-key cryptosystems such as RSA to quantum algorithm s. However\, deployable quantum communication systems are still not optimi sed for practical conditions due to high optical complexity and cost\, ver y low key-rate exchange over long distances\, and limited information capa city in two-dimensional encodings.This thesis develops and experimentally validates a unified prepare-and-measure platform that addresses the challe nges above. At its core is a polarization-encoded optical setup based on a self-compensating SAGNAC loop modulator and a time-multiplexed measuremen t stage requiring only two single-photon detectors\, achieving 99.78\\% fi delity across two mutually unbiased bases. Using this optical system\, thr ee demonstrations of increasing practical relevance are presented. (i) A s implified BB84 quantum key distribution (QKD) protocol benchmarked over a deployed 7 km dark fibre link on the Instituto Superior Técnico campus wi th both SPAD and SNSPD detectors\, marking it the first demonstration usin g top-of-the-line SNSPD detectors in Portugal\; (ii) A numerical feasibili ty study of a CubeSat-based downlink for satellite-to-ground QKD and quant um keyless private communication (QKPC)\, optimising protocol parameters a cross 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 transfer protocol that uses the quantum signals for both key generation an d time transfer\, and further uses the QKD-generated key to authenticate t iming data over a 4.2 m bidirectional fibre link\, achieving a clock offse t 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\, exploi ting the increased information capacity\, enhanced noise robustness\, and higher security thresholds of qudit encodings to push practical quantum co mmunication beyond the qubit regime. LAST-MODIFIED:20260504T134623Z LOCATION:Sala de reuniões TN| Sala 5.09 (Piso 5 - Torre Sul\, Alameda) URL:http://df.vps.tecnico.ulisboa.pt/en/events/towards-high-dimentional-pr actical-quantum-communication/ X-ALT-DESC;FMTTYPE=text/html:

Quantum communicat ion promises information-theoretic security grounded in the laws of physic s\, addressing the computational vulnerability of classical public-key cry ptosystems such as RSA to quantum algorithms. However\, deployable quantum communication systems are still not optimised for practical conditions du e to high optical complexity and cost\, very low key-rate exchange over lo ng distances\, and limited information capacity in two-dimensional encodin gs.

This thesis develops and experimentally validates a unified p repare-and-measure platform that addresses the challenges above. At its co re is a polarization-encoded optical setup based on a self-compensating SA GNAC loop modulator and a time-multiplexed measurement stage requiring onl y two single-photon detectors\, achieving 99.78\\% fidelity across two mut ually unbiased bases. Using this optical system\, three demonstrations of increasing practical relevance are presented. (i)

A simplified B B84 quantum key distribution (QKD) protocol benchmarked over a deployed 7 km dark fibre link on the Instituto Superior Técnico campus with both SPA D and SNSPD detectors\, marking it the first demonstration using top-of-th e-line SNSPD detectors in Portugal\; (ii) A numerical feasibility study of a CubeSat-based downlink for satellite-to-ground QKD and quantum keyless private communication (QKPC)\, optimising protocol parameters across a rea listic LEO overpass and yielding a peak transmission rate of 80.8 kHz and 700 MHz at zenith\, respectively\; (iii) A quantum-secure time transfer pr otocol that uses the quantum signals for both key generation and time tran sfer\, and further uses the QKD-generated key to authenticate timing 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.

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