Eavesdropping on Quantum Secure Direct Communication with W State in Noisy Channel

Security of the quantum secure direct communication protocol （i.e., the C-S QSDC protocol） recently proposed by Cao and Song [Chin. Phys. Lett. 23 （2006） 290] is analyzed in the case of considerable quantum channel noise. The eavesdropping scheme is presented, which reveals that the C-S QSDC protocol is not secure if the quantum bit error rate （QBER） caused by quantum channel noise is higher than 4.17%. Our eavesdropping scheme induces about 4.17% QBER for those check qubits. However, such QBER can be hidden in the counterpart induced by the noisy quantum channel if the eavesdropper Eve replaces the original noisy channel by an ideal one. Furthermore, if the QBER induced by quantum channel noise is lower than 4.17%, then in the eavesdropping scheme Eve still can eavesdrop part of the secret messages by safely attacking a fraction of the transmitted qubits. Finally, an improvement on the C-S QSDC protocol is put forward.     

A New Quantum Secure Direct Communication Protocol Using Decoherence-Free Subspace

A new quantum secure direct communication （QSDC） protocol is proposed by using decoherence free subspace （DFS） to avoid insecurity of the present QSDC protocols in a quantum noise channel. This protocol makes it easily for Bob and Alice to find eavesdropping in channel because the collective dephasing noise disappears in DFS. The probability of successful attack by Eve in this protocol is smaller than in BB84 protocol. Thus this protocol realizes secure QSDC and is feasible with present-day technology.     

An efficient quantum secure direct communication scheme with authentication

In this paper an efficient quantum secure direct communication （QSDC） scheme with authentication is presented, which is based on quantum entanglement and polarized single photons. The present protocol uses Einstein-Podolsky-Rosen （EPR） pairs and polarized single photons in batches. A particle of the EPR pairs is retained in the sender＇s station, and the other is transmitted forth and back between the sender and the receiver, similar to the‘ping-pong＇ QSDC protocol. According to the shared information beforehand, these two kinds of quantum states are mixed and then transmitted via a quantum channel. The EPR pairs are used to transmit secret messages and the polarized single photons used for authentication and eavesdropping check. Consequently, because of the dual contributions of the polarized single photons, no classical information is needed. The intrinsic efficiency and total efficiency are both 1 in this scheme as almost all of the instances are useful and each EPR pair can be used to carry two bits of information.     

A Special Eavesdropping on One-Sender Versus N-Receiver QSDC Protocol

We analyse the security of a quantum secure direct communication （QSDC） protocol and find that an eavesdropper can utilize a special property of GHZ states to elicit all or part of the transmitted secrets without being detected. The particular attack strategy is presented in detail. We give an improved version of this protocol so that it can resist this attack.     

Forcible-Measurement Attack on Quantum Secure Direct Communication Protocol with Cluster State

The security of the quantum secure direct communication （QSDC） protocol with cluster state is analysed. It is shown that the secret would be partially leaked out when an eavesdropper performs forcible measurements on the transmitted particles. With the help of the result in minimum error discrimination, an upper bound （i.e. 40%） of this leakage is obtained. Moreover, the particular measurements which makes the leakage reach this bound are given.     

Cryptanalysis of Multiparty Quantum Secret Sharing of Quantum State Using Entangled States

Security of a quantum secret sharing of quantum state protocol proposed by Guo et al. [Chin. Phys. Lett. 25 （2008） 16] is reexamined. It is shown that an eavesdropper can obtain some of the transmitted secret information by monitoring the classical channel or the entire secret by intercepting the quantum states, and moreover, the eavesdropper can even maliciously replace the secret message with an arbitrary message without being detected. Finally, the deep reasons why an eavesdropper can attack this protocol are discussed and the modified protocol is presented to amend the security loopholes.     

Quantum secure direct communication via partially entangled states

We present in this paper a quantum secure direct communication （QSDC） protocol by using partially entangled states. In the scheme a third party （Trent） is introduced to authenticate the participants. After authentication, Alice can directly, deterministically and successfully send a secret message to Bob. The security of the scheme is also discussed and confirmed.     

Cryptanalysis of an Improved Flexible Ping-Pong Protocol in Perfect and Imperfect Quantum Channels

We recently proposed a flexible quantum secure direct communication protocol [Chin. Phys. Lett. 23 （2006） 3152]. By analyzing its security in the perfect channel from the aspect of quantum information theory, we find that an eavesdropper is capable of stealing all the information without being detected. Two typical attacks are presented to illustrate this point. A solution to this loophole is also suggested and we show its powerfulness against the most general individual attack in the ideal case. We also discuss the security in the imperfect case when there is noise and loss.     

Experimental Decoy State Quantum Key Distribution Over 120km Fibre

Decoy state quantum key distribution （QKD）, being capable of beating PNS attack and being unconditionally secure has become attractive recently. However, in many QKD systems, disturbances of transmission channel make the quantum bit error rate （QBER） increase, which limits both security distance and key bit rate of real-world decoy state QKD systems. We demonstrate the two-intensity decoy QKD with a one-way Faraday- Michelson phase modulation system, which is free of channel disturbance and keeps an interference fringe visibility （99%） long period, over a 120 km single mode optical fibre in telecom （1550nm） wavelength. This is the longest distance fibre decoy state QKD system based on the two-intensity protocol.     

Quantum Secure Direct Communication Using W State

A theoretical scheme of quantum secure direct communication using teleportation is proposed. In the scheme, the sender needs to prepare a class of three-particle W states to use as quantum channel. The two communicators may communicate after they test the security of the quantum channel. The security of the protocol is ensured by quantum entanglement and quantum no-cloning theorem. The receiver can obtain the secret message determinately if the quantum channel is secure.