Study On Quantum Key Distribution Using Coherent State

Quantum informatics is a rising interdiscipline of informatics and quantum mechanics. Quantum communication is an important branch of quantum informatics, which has advantages in improving the arithmetic speed, ensuring the security of information, increasing data capacity and improving information accuracy.In recent decades, Quantum cryptography, from the original single photon discrete system to widely-discussed quantum key distribution (QKD) using coherent state system, has developed with the more simple experiment devices, the higher secret key rate, and more efficient behavior. Also, much more classes of attack against the QKD system have emerged.In this paper, two parts are mainly focused on, one for the analysis to the performances of the three main schemes of conciliation; the other for the analysis of the typical attacks to the QKD system under the ordinary channel.First, the basic concept and the progress of quantum information is introduced. Based on the quantum theory, expression of light field under quantum theory is deduced. The basic state - coherent state is studied, as well as the characteristics of the quantum noise. Furthermore, based on the Heisenberg uncertainty relation, the characteristic of squeezing state is studied, in which One of the two orthogonal quadrature of the light field is lower than the uncertainty limits, while the other is higher than the limit. After briefly introducing the basic concept of entangle state, at last the QKD system is presented and analyzed.Second, the basic principle of QKD system using coherent states is analyzed. The three schemes of reconciliation are analyzed in detail. The security condition of QKD system is summarized according to the different schemes. According to the different characteristics of the channel, the advantages and disadvantages of the three schemes of reconciliation are analyzed. First of all, only the attenuation is taken into regarded, in this case, the best attack is beam-splitting attack which does not bring in the noises. Then considering the channel noise, all the noises are assumed to be brought in by Eve, in order to be more secure. Through the analysis, it is reported that direct reconciliation has more advantages in a channel with large transmission, and postselection has much more tolerance to noise.Finally, the attack capacity of Gaussian cloning attack and amplifying channel attack is analyzed and compared. And the two kinds of attack are all analyzed under individual attack and collective attack, also the direction of the reconciliation is taken into account. The later attack has better attack capacity except for when it is used in individual attack using direct reconciliation; however, the former attack has a smooth distribution in the security area, rather than the later one has a sharp increasing when amplification is close to 1.