Research and Development


Research loop in the project and contribution of Subject B

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(3-1) Efficient error correcting codes / Titech

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(3-2) Efficient Privacy amplification / Nagoya Univ., Mitsubishi

Proposal of dual unversal2 hash functionsa

— From Quantum Cryptography to Modern Cryptography —

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Research loop to close security loopholes

— Toward guaranteeing the implementation security —

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(3-3) Theoretical Countermeasures against security loopholes / NTT

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According to the existing analysis, called GLLP, the key rate R decreases significantly with slight imperfections in preparing the states. We have proposed "loss-tolerant protocol" to solve this problem, in which we exploit basis mismatched instances and only three flawed states.

K. Tamaki, M. Curty, G. Kato, H.-K. Lo, and K. Azuma, Phys. Rev. A 90, 052314 (2014).

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We have generalized our loss-tolerant protocol for easier characterization of the source and for taking into account the finite-size effect of the sending pulses.

Easier characterization based on modulation regimes Finite-size effect
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  • item pointOur protocol is now implemented by a number of experiments
  • item pointOur results are a step forward toward guaranteeing the implementation security of QKD

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(3-4) Device characterization experiments / Hokkaido Univ.

Requirements on device characteristics

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Criteria for phase randamization

We related randomization to a mearsurable quantity: visibility of interface

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Inter-pulse phase correlation for 10GHz

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Intensity fluctuation in Decoy-state QKD

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Intensity fluctuation in Gain-Switched LD

  • item pointTarget range: 3σ / μ < 5%, determined from security theories
  • item pointIntensity fluctuation was measured for a gain-switched LD
  • item pointGain switching LD can satisfy the above criterion by strong excitation
    • recursed item pointDC bias is limited by the phase correlation between pulses
    • recursed item pointPulse current is limited by a LD-driver
  • item pointLarge fluctuation was observed for weak excitation
  • item pointWe found unstable gain switching when the pulse period is comparable with carrier lifetime
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State preparation flaw

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Improved implementation

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Supression of phase uncertainty

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(3-5) Contributions to basic research


Before 2014, finite key length analysis of the B92 protocol had not been done. We applied the convex optimization technique previously introduced by us into the QKD area, and we obtained secure key rate of the B92 as above. (ISIT 2014).

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