Secure quantum key distribution

Journal name:
Nature Photonics
Volume:
8,
Pages:
595–604
Year published:
DOI:
doi:10.1038/nphoton.2014.149
Received
Accepted
Published online

Abstract

Secure communication is crucial in the Internet Age, and quantum mechanics stands poised to revolutionize cryptography as we know it today. In this Review, we introduce the motivation and the current state of the art of research in quantum cryptography. In particular, we discuss the present security model together with its assumptions, strengths and weaknesses. After briefly introducing recent experimental progress and challenges, we survey the latest developments in quantum hacking and countermeasures against it.

At a glance

Figures

  1. Progress in free-space QKD implementations.
    Figure 1: Progress in free-space QKD implementations.

    a, First free-space demonstration of QKD19 realized two decades ago over a distance of 32 cm. The system uses a light-emitting diode (LED) in combination with Pockels cells to prepare and measure the different signal states. b, Entanglement-based QKD set-up connecting the two Canary Islands La Palma and Tenerife6. The optical link is 144 km long. OGS, optical ground station; GPS, Global Positioning System; PBS, polarizing beamsplitter; BS, beamsplitter; HWP, half-wave plate. c, Schematic of a decoy-state BB84 QKD experiment between ground and a hot-air balloon20. This demonstration may be considered a first step towards realizing QKD between ground and low-Earth-orbit satellites. MON, monitor window; ATT, attenuator; DM, dichroic mirror; 532LD, 532 nm laser; FSM, fast steering mirror; 671LD, 671 nm laser; 532D, 532 nm detector; IF, interference filter; CMOS, complementary metal–oxide–semiconductor. Figure adapted with permission from: a, ref. 19, © 1992 IACR; b, ref. 6, © 2007 NPG; c, ref. 20, © 2013 NPG.

  2. Experimental QKD.
    Figure 2: Experimental QKD.

    a, Schematic of the decoy-state BB84 protocol26, 27, 28, 29, 30, 31, 32, 33, 34, 35 based on polarization coding. Four lasers are used to prepare the polarizations needed in BB84. Decoy states are generated with an amplitude modulator (AM). On Bob's side, a 50:50 beamsplitter (BS) is used to passively ensure a random measurement basis choice. Active receivers are also common. PM, phase modulator; F, optical filter; I, optical isolator; HWP, half-wave plate; PBS, polarizing beamsplitter; QRNG, quantum random number generator. b, Lower bound on the secret key rate (per pulse) in logarithmic scale for a BB84 set-up with two decoys (blue line)29. In the short-distance regime, the key rate scales linearly with the transmittance, η. Standard BB84 protocol without decoy states (dark brown line)23, 25; its key rate scales as η2. c, Photograph of a fibre-coupled modularly integrated decoy-state BB84 transmitter based on polarization coding37; it produces decoy-state BB84 signals at a repetition rate of 10 MHz. d, Performance of the SwissQuantum network9. This network was operated for more than 18 months in Geneva, Switzerland. The data shown in the figure correspond to a QKD link of 14.4 km; they highlight the stability of current QKD set-ups. QBER, quantum bit error rate. Figure adapted with permission from: c, ref. 37, © 2013 LANL; d, ref. 9, © 2011 IOP.

  3. QKD networks.
    Figure 3: QKD networks.

    a, Schematic of the layer structure of the Tokyo QKD network13, which is based on a trusted node architecture. From the users' perspective, the QKD layer and the key management layer can be treated as a black box that supplies them with a secure key. They can be used in applications such as secure video meetings and secure communication via smart phones. Different QKD networks have been implemented in other countries (see, for instance, refs 7,8,9,10,11,12). b, (Upper subfigure) Downstream versus upstream passive quantum access network. In the upstream approach14, single-photon detectors are located only at the network node. This may reduce the costs of the network and allow its detectors' bandwidth to be used more efficiently. Estimated secret key rate per user for an upstream solution as a function of the distance and the number of active users in the network for various network capacities (lower subfigure). Figure b adapted with permission from ref. 14 © 2013 NPG.

  4. Examples of quantum hacking.
    Figure 4: Examples of quantum hacking.

    a, Experimentally measured detection efficiency mismatch between two detectors from a commercial QKD system versus time shifts76. Eve could exploit this to perform a time-shift attack75; that is, she could shift the arrival time of each signal such that one detector has a much higher detection efficiency than the other. b, Working principle of the detector blinding attack80. By shining intense light onto the detectors, Eve can make them leave Geiger-mode operation (used in QKD) and enter linear-mode operation. In so doing, she can control which detector produces a 'click' each given time and learn the entire secret key without being detected. c, Full-field implementation of a detector blinding attack on a running entanglement-based QKD set-up83. HWP, half-wave plate; PBS, polarizing beamsplitter; BS, beamsplitter; LD, laser diode; SPDC, spontaneous parametric downconversion, BBO; β-barium-borate crystal; FPC, fibre polarization controller; TS, timestamp unit; PA, polarization analyser; FSG, faked-state generator. Figure adapted with permission from: a, ref. 76 © 2008 APS; b, ref. 80 © 2010 NPG; c, ref. 83 © 2011 NPG.

  5. Examples of countermeasures against quantum hacking.
    Figure 5: Examples of countermeasures against quantum hacking.

    a, Schematic of DI-QKD96, 97, 98, 9. Alice and Bob can prove the security of the protocol based on the violation of an appropriate Bell inequality. To overcome the channel loss, the system can include a fair-sampling device100, 101. In principle, DI-QKD can remove all side channels in a QKD implementation. b, Schematic representation of MDI-QKD74. Alice and Bob prepare WCPs in different BB84 polarization states and send them to an untrusted relay Charles, who is supposed to perform a Bell-state measurement that projects the incoming signals into a Bell state. MDI-QKD removes all detector side channels, which can be regarded as the Achilles heel of QKD. MDI-QKD has the advantage over DI-QKD of being feasible with current technology. Indeed, proof-of-principle demonstrations have been already done104, 105, and real QKD implementations have been realized106, 107. BS, beamsplitter; PBS, polarizing beamsplitter; D, single-photon detector. c, Field-test proof-of-principle demonstration of MDI-QKD realized in Calgary, Canada104. MC, master clock; AM, amplitude modulator; PM, phase modulator; ATT, variable attenuator; POC, polarization controller; FS, frequency shifter. Figure c reproduced with permission from ref. 104 © 2013 APS.

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Author information

  1. All authors contributed equally to this work.

    • Hoi-Kwong Lo,
    • Marcos Curty &
    • Kiyoshi Tamaki

Affiliations

  1. Center for Quantum Information and Quantum Control, Department of Physics and Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario M5S 3G4, Canada

    • Hoi-Kwong Lo
  2. EI Telecomunicación, Department of Signal Theory and Communications, University of Vigo, E-36310 Vigo, Pontevedra, Spain

    • Marcos Curty
  3. NTT Basic Research Laboratories, NTT Corporation, 3-1, Morinosato-Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan

    • Kiyoshi Tamaki

Competing financial interests

H.-K.L. is a named inventor on US Patent #8,554,814, “Random signal generator using quantum noise” (2013), which is related to the methods described in ref. 59. M.C. is a named inventor on patents and pending patents related to the methods described in refs 57 and 58. K.T. declares no competing financial interests other than his employment with NTT, Basic Research Lab.

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