Quantum Direct Communication Research Articles

A Quantum Network System of QSS-QDC Using χ-Type Entangled States

A multiuser quantum direct communication network system for N users utilizing χ-type entangled states is proposed. The network system is composed of a communication center, N users, and N quantum lines linking the center and the N users. There is no quantum line among users, and therefore only N quantum lines are necessary for communication between users. Using one χ-type entangled state, in this protocol we are able to send two bits of information through direct communication and, at the same time, share two bits of quantum keys. The security of the protocol is then analyzed.

Improved Eavesdropping Detection Strategy in Quantum Direct Communication Protocol Based on Five-Particle Cluster State

In order to improve the eavesdropping detection efficiency in two-step quantum direct communication protocol, an improved eavesdropping detection strategy using four-particle GHZ state is proposed, in which four-particle GHZ state is used to detect eavesdroppers. During the security analysis, the method of the entropy theory is introduced, and two detection strategies are compared quantitatively by using the constraint between the information which eavesdropper can obtain and the interference introduced. If the eavesdroppers intend to obtain all information, the eavesdropping detection rate of the original two-step quantum direct communication protocol by using EPR pair block as detection particles is 50%; while the proposed strategy's detection rate is 88%. In the end, the security of the proposed protocol is discussed. The analysis results show that the eavesdropping detection strategy presented is more secure.

Improved quantum “Ping-pong” protocol based on GHZ state and classical XOR operation

In order to transmit the secure message, a deterministic secure quantum direct communication protocol which was called “Ping-pong” protocol was proposed by Bostrom and Felbinger [Bostrom K, et al. Phys Rev Lett, 2002, 89: 187902]. But the protocol was proved very vulnerable, and can be attacked by an eavesdropper. An improved “Ping-pong” protocol is presented to overcome the problem. The GHZ state particles are used to detect eavesdroppers, and the classical XOR operation which serves as a one-time-pad is used to ensure the security of the protocol. During the security analysis, the method of the entropy theory is introduced, and three detection strategies are compared quantitatively by using the constraint between the information which an eavesdropper can obtain and the interference introduced. If the eavesdropper gets the full information, the detection rate of the original “Ping-pong” protocol is 50%; the detection rate of the second protocol which used two particles of EPR pair as detection particles is also 50%; and the detection rate of the presented protocol is 75%. In the end, the security of the proposed protocol is discussed. The analysis results show that the improved “Ping-pong” protocol in this paper is more secure than the other two.

Security of the quantum direct communication based on pairs of completely entangled qudits

Quantum secure direct communication protocols offer confidential transmission of classic information over the quantum channel without a prior key agreement. The ping-pong based protocols provide asymptotic security and detailed analysis of the security level provided by each variant of the protocol is required. The paper presents a general method of calculation of the eavesdropped information as a function of the attack detection probability. The method is applied to the ping-pong protocol based on completely entangled pairs of qudits. The lower and upper bound on the amount of the leaked information is provided.

Quantum direct communication with mutual authenticationQuantum direct communication with mutual authenticationQuantum direct communication with mutual authentication

In this paper, we first point out that some recently proposed quantum direct communication (QDC) protocols with authentication are vulnerable under some specific attacks, and the secrete message will leak out to the authenticator who is introduced to authenticate users participating in the communication. We then propose a new protocol that is capable of achieving secure QDC with authentication as long as the authenticator would do the authentication job faithfully. Our quantum protocol introduces a mutual authentication procedure, uses the quantum Bell states, and applies unitary transformations in the authentication process. Then it exploits and utilizes the entanglement swapping and local unitary operations in the communication processes. Thus, after the authentication process, the client users are left alone to communicate with each other, and the authenticator has no access to the secrete message. In addition, our protocol does not require a direct quantum link between any two users, who want to communicate with each other. This may also be an appealing advantage in the implementation of a practical quantum communication network.

Efficient Quantum Secure Direct Communication with Authentication

Two protocols of quantum direct communication with authentication [Phys. Rev. A 73 (2006) 042305] were recently indicated to be insecure against the authenticator Trent attacks [Phys. Rev. A 75 (2007) 026301]. We present two efficient protocols by using four Pauli operations, which are secure against inner Trent attacks as well as outer Eve attacks. Finally we generalize them to multiparty quantum direction communication.

Multiparty Quantum Chatting Scheme

We propose a new multiparty simultaneous quantum direct communication scheme based on Green–Horne–Zeilinger (GHZ) states and dense coding. For achieving high efficiency without leaking any information, four encoding schemes are prepared in advance. The present scheme has the capacity of transmitting (M + 1)M classical bits per group of M-particle GHZ states when there exist M parties. The technique of rearranging particles makes the legal users coequally exchange their messages in the same length. Both high efficiency and excellent security against the common attacks are virtues of this new scheme.

Johnson(-like)–Noise–Kirchhoff-loop based secure classical communicator characteristics, for ranges of two to two thousand kilometers, via model-line

A pair of Kirchhoff-loop–Johnson(-like)–Noise communicators, which is able to work over variable ranges, was designed and built. Tests have been carried out on a model-line performance characteristics were obtained for ranges beyond the ranges of any known direct quantum communication channel and they indicate unrivalled signal fidelity and security performance of the exchanged raw key bits. This simple device has single-wire secure key generation and sharing rates of 0.1, 1, 10, and 100 bit/second for corresponding copper wire diameters/ranges of 21 mm/2000 km, 7 mm/200 km, 2.3 mm/20 km, and 0.7 mm/2 km, respectively and it performs with 0.02% raw-bit error rate (99.98% fidelity). The raw-bit security of this practical system significantly outperforms raw-bit quantum security. Current injection breaking tests show zero bit eavesdropping ability without triggering the alarm signal, therefore no multiple measurements are needed to build an error statistics to detect the eavesdropping as in quantum communication. Wire resistance based breaking tests of Bergou–Scheuer–Yariv type give an upper limit of eavesdropped raw-bit ratio of 0.19% and this limit is inversely proportional to the sixth power of cable diameter. Hao's breaking method yields zero (below measurement resolution) eavesdropping information.

The effectiveness of quantum operations for eavesdropping on sealed messages

A quantum protocol is described which enables a user to send sealed messages and that allows for the detection of active eavesdroppers. We examine a class of eavesdropping strategies, those that make use of quantum operations, and we determine the information gain versus disturbance caused by these strategies. We demonstrate this tradeoff with an example and we compare this protocol to quantum key distribution, quantum direct communication, and quantum seal protocols.

Comment on “Quantum direct communication with authentication”

Two protocols of quantum direct communication with authentication [Phys. Rev. A {\bf 73}, 042305 (2006)] are recently proposed by Lee, Lim and Yang. In this paper we will show that in the two protocols the authenticator Trent should be prevented from knowing the secret message of communication. The first protocol can be eavesdropped by Trent using the the intercept-measure-resend attack, while the second protocol can be eavesdropped by Trent using single-qubit measurement. To fix these leaks, I revise the original versions of the protocols by using the Pauli-Z operation $\sigma_z$ instead of the original bit-flip operation $X$. As a consequence, the protocol securities are improved.

Quantum telecommunication with atomic ensembles

Quantum mechanics provides a mechanism for absolutely secure communication between remote parties. For distances greater than 100 km, direct quantum communication via optical fiber is not viable, owing to fiber losses, and intermediate storage of the quantum information along the transmission channel is necessary. This leads to the concept of the quantum repeater, proposed by Briegel et al. [Phys. Rev. Lett. 81, 169 (1998)]. Duan [Nature 414, 413 (2001)] have proposed to use atomic ensembles as the basic memory elements for the quantum repeater. We provide an overview of our program on the use of atomic ensembles as an interface for quantum information transfer and the prospects for long-distance quantum networks.

Controlled Bidirectional Quantum Direct Communication by Usinga GHZ State

A controlled bidirectional quantum secret direct communication schemeis proposed by using a Greenberger–Horne–Zeilinger (GHZ) state. In thescheme, two users can exchange their secret messages simultaneously witha set of devices under the control of a third party. The security of thescheme is analysed and confirmed.

Quantum direct communication with authentication

We propose two Quantum Direct Communication (QDC) protocols with user authentication. Users can identify each other by checking the correlation of Greenberger-Horne-Zeilinger (GHZ) states. Alice can directly send a secret message to Bob using the remaining GHZ states after authentication. Our second QDC protocol can be used even though there is no quantum link between Alice and Bob. The security of the transmitted message is guaranteed by properties of entanglement of GHZ states.

Eavesdropping on the two-way quantum communication protocols with invisible photons

The crucial issue of quantum communication protocol is its security. In this Letter, we show that all the deterministic and direct two-way quantum communication protocols, sometimes called ping-pong (PP) protocols, are insecure when an eavesdropper uses the invisible photon to eavesdrop on the communication. With our invisible photon eavesdropping (IPE) scheme, the eavesdropper can obtain full information of the communication with zero risk of being detected. We show that this IPE scheme can be implemented experimentally with current technology. Finally, a possible improvement of PP communication protocols security is proposed.

Comment on “Secure direct communication with a quantum one-time pad”

In the paper [Phys. Rev. A \textbf{69}, 052319 (2004)], a quantum direct communication protocol is proposed which is claimed to be unconditionally secure even for the case of a noisy channel. We show that this is not the case by giving an undetectable attack scheme.

Deterministic Secure Direct Communication by Using SwappingQuantum Entanglement and Local Unitary Operations

A deterministic direct quantum communication protocol is proposed byusing swapping quantum entanglement and local unitary operations. Thepresent protocol is secure for the proof of the security of the presentscheme, the same as that in the two-step protocol [Phys. Rev. A68 (2003) 042317]. Additionally, the advantages and disadvantages ofthe present protocol is also discussed.