Kouichi Yamazaki

On Reconciliation of Discrepant Sequences Shared through Quantum Mechanical Channels

In the practical quantum key distribution system, discrepancies may occur on the secret random sequences shared through a quantum mechanical channel even in the absence of an eavesdropper because of an imperfect system configuration and noises. The discrepancies may be spread and expanded through a following privacy amplification. Therefore all the discrepancies must be removed at this point. The reconciliation protocol was proposed by Bennett et al. As long as the authors know, however, detailed investigation on the protocol has not been reported. In this report, we investigate the reconciliation protocol proposed by Bennett et al. and consider to optimize it.

Properties of Quantum Cryptography Based on Orthogonal States: Goldenberg and Vaidman Scheme

Properties of quantum key distribution scheme based on orthogonal quantum states proposed by Goldenberg and Vaidman (GV scheme) are theoretically clarified. First, it is shown that multiple-particle sources can be applied to the GV scheme instead of single-particle ones. Then, probabilities that an eavesdropper is detected is given for an eavesdropping method which is a simple modification of that used to show the security of the scheme. Finally, the security of the GV scheme against the eavesdropping method is discussed based on information theory.

Elimination of nonlinear PM and FM noises in optical amplifier repeater systems by squeezer

Recently, noise phenomenon, so called Gordon-Haus effect, caused by nonlinearity of fiber and spontaneous emission from optical amplifier has received much attention. In general, it is very difficult to eliminate such an effect, because the Kerr effect of fibers and the spontaneous emission noise of a conventional optical amplifier are inherent in those physics. In this paper, we show that the Gordon-Haus effect can be perfectly eliminated by applying the squeezer which is studied in the research field of squeezed state.

A quantum theoretical analysis of nonlinear phase noise

The authors clarify noise generation mechanisms due to optical fiber nonlinearity in laser amplifiers and squeezers in coherent communication systems, and discuss the mechanism of reduction of phase noise due to optical fiber nonlinearity by using a squeezer in optical coherent repeater systems. They show that considering the classical and the quantum behaviors of the Kerr effect separately makes it easier to understand the mechanism instinctively. They expect further technical development of squeezer in order to realize long-haul coherent communications systems.<<ETX>>

Eavesdropping on Key Distribution Scheme based on Shot Noise of Intense Laser Pulses

We investigate the security of the Yuen‐Kim key distribution scheme with conceptual channels implemented by intense laser pulses. Several kinds of individual attacks are considered.

Influence of the Correlation by Beam Splitting in YK Protocol QKD System

The quantum key distribution (QKD) system can provide the absolutely secure communication for distant users [C. H. Bennett and G. Brassard, Proc. of the IEEE Int. Conf. on Computers, Systems, and Signal Processing, pp. 175–179, 1984]. But it requires single photon sources, highly sensitive detectors, and newly constructed quantum channel between the legitimate users. On the other hand, classical (or quantum) noise based key distribution system is one of the candidates to realize the key distribution system with today’s technology [U. M. Maurer, IEEE Trans. Inform. Theory, 39, No.3, pp.733–742, 1993, H. P. Yuen and A. M. Kim, Phys. Lett. A, 241, pp.135–138, 1998]. In such a system, the independence of the noise between a legitimate receiver and an eavesdropper is presupposed. For long haul distant communication, however, amplification of the carrier is indispensable. This amplification process generates correlated fluctuations between them.We analyze how much correlation occurs when a coherent state is amp...

Signal-to-noise Ratio of Optical System Consisting of Squeezer and Optical Kerr Medium

Abstract In this paper, we analyse the properties of a quantum state transformation system consisting of a squeezer and an optical Kerr medium. An average, a variance, and a signal-to-noise ratio (SNR) of a quadrature-phase amplitude are calculated analytically. We show that the SNR is increased by this system; in particular the use of a squeezer with a large squeezing gain gives a drastic improvement in the SNR. Furthermore, the reason for the improvement in SNR and the usefulness of the squeezed state are discussed.

On the error correcting code for noisy interleaved optical PPM channel

An optical PPM communication system with interleaving technique is analyzed for non-ideal situation in which background noise photons disturb the system. The authors propose a decoding algorithm of error control codes for this system. The correlation between code words forming PPM blocks is used in order to improve the performance of the code. In order to show the improvement, a single error correcting binary code is considered as an example, and the algorithm is investigated. As a result, it is shown that a single-error-correcting binary code can correct not only all single error and one or two-tupple erasures but detect more than two-tupple errors with high probability.<<ETX>>