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Free-space laser system for secure air-to-ground quantum communications

Florian Moll

Published Dec 9, 2013• 6 citations
Publisher

Abstract

A novel optical communication system enables quantum key distribution between a ground station and an airplane in flight.
DOI
10.1117/2.1201311.005189

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We present QuBUS, a mobile optical ground station designed for free-space optical communication (FSOC) and quantum key distribution (QKD). Built into a transportable lab container, QuBUS includes key subsystems such as an optical antenna with uplink lasers, a freeform telescope, acquisition and tracking hardware, relay optics, and an adaptive optics system for single-mode fiber coupling. The station is being prepared for an air-to-ground quantum communication experiment involving an aircraft-based transmitter. Initial 7-km horizontal link tests validate core functionalities, demonstrating QuBUS’s potential as a compact and deployable platform for future secure quantum communication networks.
The quantum space age has officially begun and many important milestones and achievements have recently been demonstrated, such as the exciting launch and results of the first quantum demonstration satellite, Micius. Previously with terrestrial applications, quantum key distribution was limited in distance to a few hundred kilometers through either free space or optical fiber. This had dampened progress towards a global quantum cryptographic network, but with the recent progress towards space implemented quantum systems, the door has been opened once again. \n \nIn this thesis, we begin by studying the effect of using an adaptive optics system to improve the efficiency of a free space link to a satellite for quantum key distribution. Adaptive optics has been used extensively in astronomy and has the potential to increase the average optical intensity received by the satellite. We study the effect of the atmosphere on the beam as it propagates from the ground station to the satellite. In the up-link configuration, the atmosphere is of special concern as it affects the beam at the beginning of the propagation, making the end effect worse. \n \nOne of the important components of a free space quantum key distribution satellite system is a fine pointing unit. We have, along with industry partners, designed and implemented such a unit for free space optical links. The device was designed to have little to no effect on the polarization of the photons used to transmit the key bits. The device was tested, both in the laboratory and outside and quantum key distribution was successfully performed while the fine pointing was active. \n \nThe main experiment of the thesis demonstrates quantum key distribution to a moving airplane from a ground station. The components of a quantum key distribution receiver prototype were tested locally around the University of Waterloo campus as well as some tests using private airplanes. The collaboration with the National Research Council of Canada really allowed the project to take flight by granting us access to a research aircraft to deploy our receiver prototype. This project spanned over three years and culminated in a two week flight campaign out of Ottawa and Smiths Falls Ontario. \n \nUsing only five flight hours we were able to successfully transmit finite size quantum secure keys from our optical ground station, located at Smiths Falls--Montague Airport to a Twin Otter Research Aircraft housing our quantum key distribution receiver prototype. Many of the components implemented in the receiver were designed and built with spaceflight in mind and have a clear path to flight for space application. \n \nFinally, we study the feasibility of implementing a quantum key distribution receiver onto a nano satellite. In partnership with the University of Toronto Institute for Aerospace Studies Space Flight Laboratory, we studied the various aspects such as optics, detection, cooling, power, mass, etc., to determine if it would be possible to perform quantum key distribution to a nano satellite. The main difference of this project from the previous Quantum Encryption and Science Satellite is the simpler pointing system, which doesn't utilize fine pointing. \n \nThrough various studies, experiments, and component design, we have shown the feasibility of implementing quantum key distribution to a moving aircraft in an up-link configuration. This work contributes to the long line of achievements leading towards satellite implementations of quantum key distribution for eventual global quantum cryptography.
Quantum Key Distribution (QKD), either fiber based or free-space, allows for provably secure key distribution solely based on the laws of quantum mechanics. Feasibility of QKD systems in aircraft-ground links was demonstrated with a successful key exchange. Experiment flights were undertaken during night time at the site of the German Aerospace Center (DLR) Oberpfaffenhofen, Germany. The aircraft was a Dornier 228 equipped with a laser communication terminal, originally designed for optical data downlinks with intensity modulation and direct detection. The counter terminal on ground was an optical ground station with a 40 cm Cassegrain type receiver telescope. Alice and Bob, as the transmitter and receiver systems usually are called in QKD, were integrated in the flight and ground terminals, respectively. A second laser source with 1550 nm wavelength was used to transmit a 100 MHz signal for synchronization of the two partners. The so called BB84 protocol, here implemented with faint polarization encoded pulses at 850nm wavelength, was applied as key generation scheme. Within two flights, measurements of the QKD and communication channel could be obtained with link distance of 20 km. After link acquisition, the tracking systems in the aircraft and on ground were able to keep lock of the narrow QKD beam. Emphasis of this paper is put on presentation of the link technology, i.e. link design and modifications of the communication terminals. First analysis of link attenuation, performance of the QKD system and scintillation of the sync signal is also addressed.