Secure Long-Distance Quantum Communication over Optical Fiber Quantum Channels [chapter]

Laszlo Gyongyosi, Sandor Imre
2012 Optical Fiber Communications and Devices  
The safety of quantum cryptography relies on the no-cloning theorem. According to nocloning theorem, any eavesdropping activity on the quantum channel necessarily perturbs the state of the qubits, thus Alice and Bob can detect the presence of Eve in the communication. In quantum cryptography Eve cannot clone the sent qubits perfectly, thus she has to use an ancilla quantum state, interact with the sent quantum state. This chapter will analyze the DPS QKD protocol, using efficient computational
more » ... nformation geometric algorithms. The DPS QKD protocol was introduced for practical reasons, since the earlier QKD schemes were too complicated to implement in practice. The DPS QKD protocol can be an integrated part of current network security applications, hence it's practical implementation is much easier with the current optical devices and optical networks. Moreover, the DPS QKD protocol can be implemented in long-distance quantum communications, between the quantum repeater nodes. As follows, we will focus on this QKD scheme, however there are many other QKD schemes available, see (Branciard et al. The DPS QKD scheme was designed to offer a well-implementable and more efficient practical solution with better key generation rates to realize quantum cryptography, than classical QKD approaches. As follows, it provides the best way to achieve long-distance QKD over optical-fiber quantum channels. In the DPS quantum cryptography protocol, the sender and the receiver use weak coherent state pulses, and logical bits are encoded in the relative phase of the pulses. The sender encodes every logical bit in two signals, and at the receiver's side, Bob use the two signals to decode the sent logical bit. The relevance of the DPS QKD protocol could have been increased dramatically in practical applications, since the differential phase shift QKD protocol is much more simpler in hardware design than the well known QKD protocols, such as BB84 or the Six-state QKD protocols. On the other side, contrary to it's easy implementation and it's much simpler working mechanism, the DPS QKD's protocol unconditional security is still not proven. The proposed geometrical analysis shows a method to quantify the secure key generation rate of the DPS QKD protocol, which is still missing from the literature. The possible attacks against the DPS protocol have been studied deeply. In this section we analyze the information-theoretical impacts of quantum cloner based attacks against the DPS QKD protocol. As the most general attack against the protocol, we analyze coherent attacks, based on two different types of quantum cloner machines. The first section is organized as follows. First is a short brief on the DPS QKD protocol, and then we show the results on the informationtheoretic security analysis of DPS QKD protocol. Finally, we summarize the results. In the second part of the chapter we discuss long-distance optical-fiber based quantum communications. The DPS QKD protocol In practical implementations of QKD protocols, Alice, the sender, uses weak coherent pulses (WCP) instead of a single photon source. As has been shown, WCP based protocols have a security threat, since an eavesdropper can perform a photon number splitting attack against the protocol (Inoue et al., 2003) , (Honjo et al., 2004) . These kinds of attacks are based on the fact that some weak coherent pulses contain more than one photon in the same polarization www.intechopen.com Secure Long-Distance Quantum Communication over Optical Fiber Quantum Channels
doi:10.5772/26976 fatcat:p6blq6mzyzhztcaxz7iih5hkhi