Prof. Masato Koashi Profile

Prof. Masato Koashi

at Univ of Tokyo

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Proceedings Article | 13 October 2015 Paper

Establishing security of quantum key distribution without monitoring disturbance

Masato Koashi

Proceedings Volume 9648, 96480Y (2015) https://doi.org/10.1117/12.2197813

KEYWORDS: Phase shift keying, Quantum key distribution, Quantum communications, Information security, Quantum mechanics, Interferometers, Statistical analysis, Photodetectors, Geometrical optics, Binary data

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In conventional quantum key distribution (QKD) protocols, the information leak to an eavesdropper is estimated through the basic principle of quantum mechanics dictated in the original version of Heisenberg's uncertainty principle. The amount of leaked information on a shared sifted key is bounded from above essentially by using information-disturbance trade-off relations, based on the amount of signal disturbance measured via randomly sampled or inserted probe signals. Here we discuss an entirely different avenue toward the private communication, which does not rely on the information disturbance trade-off relations and hence does not require a monitoring of signal disturbance. The independence of the amount of privacy amplification from that of disturbance tends to give it a high tolerance on the channel noises. The lifting of the burden of precise statistical estimation of disturbance leads to a favorable finite-key-size effect. A protocol based on the novel principle can be implemented by only using photon detectors and classical optics tools: a laser, a phase modulator, and an interferometer. The protocol resembles the differential-phase-shift QKD protocol in that both share a simple binary phase shift keying on a coherent train of weak pulses from a laser. The difference lies in the use of a variable-delay interferometer in the new protocol, which randomly changes the combination of pulse pairs to be superposed. This extra randomness has turned out to be enough to upper-bound the information extracted by the eavesdropper, regardless of how they have disturbed the quantum signal.

Proceedings Article | 7 May 2015 Paper

A protocol of quantum key distribution without relying on information-disturbance trade off

Masato Koashi

Proceedings Volume 9505, 950502 (2015) https://doi.org/10.1117/12.2184965

KEYWORDS: Quantum key distribution, Photodetectors, Interferometers, Information security, Double positive medium, Phase shifts, Sensors, Statistical analysis, Receivers, Mach-Zehnder interferometers

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We propose a new scheme of quantum key distribution (QKD) called the round-robin differential-phase-shift (RRDPS) protocol, which is based on a principle entirely different from the conventional QKD protocols based on the information-disturbance trade off. In the RRDPS protocol, the amount of privacy amplification is essentially constant and there is no need to change it according to the observed amount of disturbance. This means that it is hard for an eavesdropper to guess the bit value regardless of the amount of disturbance she has caused. The new scheme has a better tolerance on bit errors and is free from the cost of monitoring eavesdropping attempts. In contrast to the conventional QKD schemes, the amount of privacy amplification is the same even if the quality of the transmission channel becomes poorer and the bit error rate increases. This leads to a higher bit error threshold, typically over 30% and with no theoretical bound less than 50%. The fact that the protocol does not require precise estimation of the amount of signal disturbance is advantageous when the finite-key effect is taken into account; the RRDPS protocol can produce a key even when the total number of transmitted bits is small.

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