Air-to-ground quantum communication

Journal name:
Nature Photonics
Volume:
7,
Pages:
382–386
Year published:
DOI:
doi:10.1038/nphoton.2013.46
Received
Accepted
Published online

Abstract

Quantum key distribution1, 2 (QKD) is the first commercial application in the new field of quantum information, with first routine applications in government and financial sectors3 and with successful demonstrations of trusted node networks4, 5. Today, the main goal is efficient long-range key distribution via either quantum repeaters6 or satellites7, 8, 9, with a view to enabling global secure communication. En route to achieving QKD via satellites, a free-space demonstration of secure key distribution was performed between two ground stations10, over a distance of 144 km. This scenario is comparable to links between satellites in low Earth orbit and ground stations with respect to both attenuation and fluctuations. However, key exchange with rapidly moving platforms remained to be demonstrated. Here, we prove, for the first time, the feasibility of BB84 QKD between an aeroplane and a ground station. By establishing a stable and low-noise quantum communication channel with the aeroplane moving at 290 km h−1 at a distance of 20 km—that is, 4 mrad s−1—our results are representative of typical communication links to satellites11 or to high-altitude platforms.

At a glance

Figures

  1. Overview of the classical communication system of the German Aerospace Center's Institute of Communications and Navigation.
    Figure 1: Overview of the classical communication system of the German Aerospace Center's Institute of Communications and Navigation.

    Initially developed to provide efficient communication for large-area monitoring22 the system is able to provide a stable link using optical tracking with beacon lasers on both ground station and aircraft side. a, The Dornier 228 used in this experiment. Inset: optical dome housing the CPA. b, Aeroplane track, with the red section indicating the positions during QKD transmission. c, OGS telescope. d, Schematic of aircraft and ground terminals with the QKD system integrated (coloured boxes). LD, laser diode; EDFA, erbium-doped fibre amplifier; IMU, inertial measurement unit; DSP, digital signal processor; WFoV, wide field of view camera; NFoV, narrow field of view camera; 4QD, four-quadrant diode; CPA, coarse pointing assembly; FPA, fine pointing assembly; FL, fibre laser; RFE, receiver front-end.

  2. QKD and classical communication hardware.
    Figure 2: QKD and classical communication hardware.

    a, Illustration of the Alice module. Four laser diodes emit short pulses (1 ns, 10 MHz, 850 nm). Their light is overlapped using (polarizing) beamsplitters (BS, PBS) in a spatial filter. A half waveplate (λ/2) is used to set the angle between the two BB84 bases. Two precision-mounted mirrors and a dichroic mirror (DCM) are used to overlap the 850 nm output with the classical link (1,550 nm). b, Photograph of the module mounted in the FELT2 terminal. For safety reasons, the FELT2 has to be covered during flight by a fibreglass hood and was then controlled via Ethernet and USB using laptops only. c, Design of the Bob module and the free-space polarization controller (waveplates λ/4, λ/2). Four avalanche photodiodes (APDs) analyse the state of the QKD signals randomly in the H/V and +/−45° basis. d, Optical bench attached to the back of the OGS main mirror with the Bob module and polarization controller mounted in the upper part. The incoming beam from the telescope first hits the mirror of the FPA and is then divided spectrally at the DCM. While the 1,550 nm light from the aircraft beacon is analysed to maintain pointing and recover classical data, the 850 nm signal is directed to QKD analysis behind an interference filter (full-width at half-maximum, 10 nm).

  3. Count rates registered during one aircraft passage (duration 10 min and 4 s).
    Figure 3: Count rates registered during one aircraft passage (duration 10 min and 4 s).

    The raw detector event rate (red) is dominated by background from the anticollision flashes of ~1.5 ms length. One can clearly distinguish between the flash light close to the terminal dome and the other one at the back of the aircraft. For the analysis, the detection events were filtered to remove this background using a simple rate threshold. The sifted key rate (green) and QBER (blue) are given as averages over intervals of 1 s. Solid lines show mean values for the total data.

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Affiliations

  1. Fakulät für Physik, Ludwig-Maximilians-Universität, 80799 München, Germany

    • Sebastian Nauerth,
    • Markus Rau,
    • Stefan Frick &
    • Harald Weinfurter
  2. Institut für Kommunikation und Navigation, Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), 82234 Weßling, Germany

    • Florian Moll,
    • Christian Fuchs &
    • Joachim Horwath
  3. Max-Planck-Institut für Quantenoptik, 80539 Garching, Germany

    • Harald Weinfurter

Contributions

All authors contributed equally to the realization of the experiment, discussed the results and commented on the manuscript at all stages. S.N., M.R. and S.F. designed, built and operated the QKD hardware new to this project. F.M., C.F. and J.H. initially developed the optical communications (classical) system, took care of modifications and operations and organized the flight campaign, including airworthiness certification. S.N. evaluated the data and H.W. supervised the work.

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The authors declare no competing financial interests.

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