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[2022-2 콜로키움] 2022년 11월 09일 16:30, 박경득 박사 (국방과학연구소)
2022-11-02 17:26:23 조회수247

  Quantum-correlation-based free-space optical link

  • 일 시 : 2022년 11월 09일 수요일 16:30

  • 연 사 : 박 경 득 박사 (국방과학연구소)

  • 장 소 : 자연과학관 B117호

  • HOST : 오 차 환 교수님

  • 초 록

 Quantum key distribution (QKD) offers a method of distributing and sharing encryption keys among known users. Thus far, enormous advances have been made in QKD from ground-to-ground, plane-to-ground, and ground-to-satellite transmission. As a result, free space optical (FSO) systems have naturally attracted considerable attention as a critical technology for large-scale quantum networks, QKD in urban areas, retro-reflective quantum communication links, and unmanned mobile FSO links. Moreover, FSO links are wirelessly installed and portable. Thus, FSO links are useful in a metropolitan networks affected by mass data traffic, and also free-space QKD would be of interest to various military scenarios to distribute secure cryptographic keys.

Nevertheless, because it must operate outdoors, several technical issues need to be considered. Because of the strong background noise from scattered sunlight, a FSO link should be designed with a highly stable coupling of the signal photons into single-mode fibers (SMF) under atmospheric turbulence. Moreover, atmospheric turbulence can lead to various detrimental effects - including optical loss, beam wander, arrival-angular fluctuation, and received-signal fluctuations, which result in a reduction in the signal-to-noise ratio (SNR). Consequently, proper optical feedback is required to compensate for such turbulence effects and to maximize the signal photon-fiber coupling in the quantum channel.

We have experimentally demonstrated a 250 m FSO link based on a non-classical photon-pair source. In the transmitter stage, the quantum signal is combined with a weak tracking laser, both beams being sent to an active reflector 125 m away from the transceiver station. The reflected signal photons are then correctly aligned to a SMF in the quantum channel receiver using a closed-loop tracking system and a wirelessly-controlled hexapod stage.

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