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NTT technical review

Differential Phase Shift Quantum Key Distribution (DPS-QKD) Experiments

康弘 都倉, Toshimori Honjo

Published Sep 1, 2011• 3 citations
Publisher

Abstract

NTT Basic Research Laboratories has been researching and developing differential phase shift quantum key distribution (DPS-QKD), a new QKD protocol.This article introduces the basics of this protocol, proof-of-principle experiments, the development of a prototype system, and a field experiment done at the Tokyo QKD Network demonstration in October 200.* Sifted key: The sifted key is the initial raw key generated through photon transmission using a QKD protocol such as DPS-QKD or BB84.It has some errors due to system imperfections, so the final key is distilled through the error-correction and privacy-amplification processes.

DOI
10.53829/ntr201109fa8

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NTT Basic Research Laboratories has been researching and developing differential phase shift quantum key distribution (DPS-QKD), a new QKD protocol.This article introduces the basics of this protocol, proof-of-principle experiments, the development of a prototype system, and a field experiment done at the Tokyo QKD Network demonstration in October 200.* Sifted key: The sifted key is the initial raw key generated through photon transmission using a QKD protocol such as DPS-QKD or BB84.It has some errors due to system imperfections, so the final key is distilled through the error-correction and privacy-amplification processes.
A unique quantum key distribution (QKD) protocol, called DPS (differential-phase-shift) QKD, has been proposed and developed at NTT and Osaka University, which utilizes a coherent pulse train instead of individual photons as in traditional QKD protocols such as BB84. Its security is based on the fact that every phase difference of a highlyattenuated coherent pulse train cannot be fully measured. This protocol has features of simple setup, potential for a high key creation rate, and robustness against photon-number-splitting attack. This paper presents recent research activities on DPS-QKD.
Differential-phase-shift (DPS) quantum key distribution (QKD) is one of the major QKD protocols that can be implemented with a simple setup using a laser source and a passive detection unit. Recently, an information-theoretic security proof of this protocol was established by Mizutani et al. [npj Quantum Inf. 5, 87 (2019)], assuming an infinitely large number of emitted pulses. To implement the DPS protocol in a real-life world, it is indispensable to analyze the security with the finite number of emitted pulses. The extension of the security proof to the finite-size regime requires the accommodation of the statistical fluctuations to determine the amount of privacy amplification. In doing so, Azuma's inequality is often employed, but unfortunately we show that in the case of the DPS protocol, this results in a substantially low key rate. This low key rate is due to a loose estimation of the sum of probabilities regarding three-photon emission whose probability of occurrence is very small. The main contribution of our work is to show that this obstacle can be overcome by exploiting the recently found novel concentration inequality, Kato's inequality. As a result, the key rate of the DPS protocol is drastically improved. For instance, assuming typical experimental parameters, a 3-Mbit secret key can be generated over 77 km for 8.3 hours, which shows the feasibility of DPS QKD under a realistic setup.
Differential phase-shift (DPS) quantum key distribution (QKD) is a unique QKD protocol that is different from traditional ones, featuring simplicity and practicality. In this work, we simulated the DPS-QKD experiment conducted by (Liu et al., 2013), using OptiSystem 7. To the best of our knowledge, this is the first simulation work on DPS-QKD using a single photon source.We used a random number generator to get the phase modulation pattern of N=5, 7,9,11 and 13, while for the 3 and 15 pulse cases, the pattern adopted in the experiment was used. When the number of pulse (N) was 3, a quantum bit error rate (QBER) of 3.0%, which is lower than the minimum QBER of 4.12% required for unconditional security, was obtained. The key creation efficiency increases with the increase in the number of pulse up to 15, as it reaches 93.4% but at the expense of the increment in QBER. The result of our simulation is, on some aspect, in agreement with the experimental result. However, we were able to extend the transmission distance from 3 meter, as in the experiment, to 10 meter. The coincidence count obtained was also in total agreement with the one obtained from the experiment. The result of the average QBER indicated that increase in the pulse number N causes the QBER to raise up due to longer rise and fall time of phase modulation step which affect the MZ inference. Therefore, we suggest using a faster waveform generator with shorter rise and fall times will remarkably lower the QBER. Extending the transmission coverage to a longer distance while, at the same time reducing the QBER with full unconditional security will part of the future research.
A high speed physical random bit generator is applied for the first time to a gigahertz clocked quantum key distribution system. Random phase-modulation in a differential-phase-shift quantum key distribution (DPS-QKD) system is performed using a 1-Gbps random bit signal which is generated by a physical random bit generator with chaotic semiconductor lasers. Stable operation is demonstrated for over one hour, and sifted keys are successfully generated at a rate of 9.0 kbps with a quantum bit error rate of 3.2% after 25-km fiber transmission.