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DTSTART:20240310T070000
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DTSTART:20231105T060000
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UID:69e5b545cf657
DTSTART;TZID=America/Toronto:20240522T083000
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URL:https://uwaterloo.ca/institute-for-quantum-computing/events/paul-oh-phd
 -thesis-defense
SUMMARY:Paul Oh PhD Thesis Defense
CLASS:PUBLIC
DESCRIPTION:ENTANGLED PHOTON SOURCE FOR A LONG-DISTANCE QUANTUM KEY DISTRIB
 UTION\n\nRemote\n\nSatellite-based Quantum Key Distribution (QKD) leverage
 s quantum\nprinciples to offer unparalleled security and scalability for g
 lobal\nquantum networks\, making it a promising solution for next-generati
 on\nsecure communication systems. However\, many technical challenges need
 \nto be overcome. This thesis focuses on theoretical modeling and\nexperim
 ental validation for long-distance QKD\, as well as the\ndevelopment and t
 esting of the quantum source necessary for its\nimplementation\, to take s
 trides towards realization. While various\napproaches exist for demonstrat
 ing long-distance QKD\, here we focus on\ndiscussing the approach of sendi
 ng entangled photon pairs from an\noptical quantum ground station (OQGS)\,
  one through free-space on one\nend (uplink)\, and the other one through g
 round on the other end. This\nis also because our research team at the Qua
 ntum Photonics Laboratory\n(QPL)\, collaborating with the Canadian Space A
 gency (CSA)\, is planning\nto demonstrate Canada's first ground-to-space Q
 KD in the near future.\nThe mission is called Quantum Encryption and Scien
 ce Satellite\n(QEYSSat) mission\, which is planned to deploy a Low-Earth O
 rbit (LEO)\nsatellite for the purpose for demonstrating QKD. \n\nIn the th
 esis\, we first discuss the considerations relevant to\nestablishing a lon
 g-distance quantum link. Since a substantial amount\nof research has alrea
 dy been conducted on optical fiber communication\nthrough ground-based met
 hods\, our focus is specifically directed\ntowards ground-to-space (i.e.\,
  free space) quantum links. One of the\nmost concerning aspects in free- s
 pace quantum communication is signal\nattenuation caused by environmental 
 factors. We particularly examine\npointing errors that arise from satellit
 e tracking systems. To\ninvestigate this further\, we designed a tracking 
 system employing a\nspecific tracking algorithm and conducted tracking tes
 ts to validate\nits accuracy\, using the International Space Station (ISS)
  as a test\nsubject. Our findings illustrate the potentially significant i
 mpact of\ninaccurate ground station-to- satellite alignment on link attenu
 ation\,\naccording to our theoretical model. Given that photons serve as t
 he\nsignals for the QKD\, we also investigate the background light noise\n
 resulting from light pollution\, which is another concerning aspect\, as\n
 it could worsen the link attenuation. Conducting light pollution\nmeasurem
 ents around our Optical Quantum Ground Station (OQGS)\, we\nestimate the m
 inimum photon pair rate required for successful QKD\,\ntaking into account
  both the obtained values from these measurements\nand the expected level 
 of link loss. \n\nHaving determined the minimum photon pair rate and other
  requirements\nfor the long-distance QKD\, we proceed to fully elaborate o
 n the\ndevelopment process of the Entangled Photon Source (EPS)\, which is
  one\nof the crucial devices for demonstrating entanglement-based QKD. We\
 nuse a nonlinear crystal for generating photon pairs\, and\nexperimentally
  obtain the photon pair rate produced from it. Here\, the\nthesis also inc
 ludes a detailed explanation of the customization\nprocess for the crystal
  oven. Next\, we implement a beam displacer\nscheme along with the Sagnac 
 loop scheme to create a robust\ninterferometer\, responsible for creating 
 quantum entanglement. In\naddition\, we demonstrate a novel approach to ef
 fectively compensate\nfor the major weaknesses of the interferometer\, nam
 ely spatial and\ntemporal walk-offs. Finally\, we conduct the entanglement
  test and\ndemonstrate its suitability for long-distance QKD. As a side pr
 oject\,\nwe \n\ninvestigate the performance degradation of nonlinear cryst
 als in\nresponse to proton radiation\, exploring the potential of deployin
 g the\nEPS in space for downlink QKD in the future. This thesis provides a
 \ncomprehensive analysis and testing of elements required for\nlong-distan
 ce QKD\, contributing to the advancement of future global\nquantum network
 s. \n\nSupervisor: Thomas Jennewein
DTSTAMP:20260420T051029Z
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