Quantum secure direct communication transmits secret messages directly without another classical transmission. We describe its principle and working procedure through example protocols. Distinctions from QKD are pointed out, and its wide applications are given.
Quantum key distribution (QKD) is considered as an ideal method to make secret message unreadable to eaves-dropper but intelligible to the two authorized parties of the communication [1][2][3][4][5][6].In the research of experimental quantum key distribution, single photons and entangled photon pairs are used as the carriers.In quantum key distribution, secret keys are generated first between the communication parties.The security of quantum key distribution is guaranteed by the laws of quantum mechanics.After the quantum key distribution is completed, the communication parties should share secret keys, then the sender encrypts the secret message using the secret keys to form the ciphertext and transmits the ciphertext through a classical channel.The receiver receives the ciphertext and then decrypt the ciphertext to get the secret message.Altogether there are four steps in a secret communication process with QKD: key generation, encryption, transmission and decryption.Here in this review, we review some new development in quantum communication, quantum direct communication (QDC).Quantum direct communication is a form of quantum communication where secret messages can be transmitted through a quantum channel with or without additional classical communications.There are two forms of quantum direct communication, quantum secure direct communication (QSDC) [7{9] and deterministic secure quantum communication (DSQC) [10,11].In QSDC, secret messages are transmitted directly between the communication parties, from sender Alice to receiver Bob, without additional classical communication except those for the necessary eavesdropping check.In other words, the quantum key distribution process and the classical communication of ciphertext are condensed into one single quantum communication procedure in QSDC.Deterministic secure quantum communication is another type of quantum direct communication, such as those proposed in Ref. [10,11], where classical communication is required in order to read out the secret message.As mentioned earlier, to complete a secure communication with the help of QKD, one usually encodes the secret message with an encryption scheme, and the ciphered text is transmitted through a classical channel.With a quantum channel, this procedure can be varied.
Quantum secure direct communication (QSDC) is one of the most important branches of quantum communication. In contrast to the quantum key distribution (QKD) which distributes a secure key between distant parties, QSDC directly transmits secret message instead of sharing key in advance. To establish a secure QSDC protocol, on the one hand, the security of the quantum channel should be confirmed before the exchange of the secret message. On the other hand, the quantum state should be transmitted in a quantum data block since the security of QSDC is based on the error rate analysis in the theories on statistics. Compared with the deterministic quantum key distribution (DQKD) which can also be used to transmit deterministic information, QSDC schemes do not need extra classical bits to read the secret message except for public discussion. In this article, we introduce the basic principles of QSDC and review the development in this field by introducing typical QSDC protocols chronologically. The first QSDC protocol was proposed by Long and Liu, which can be used to establish a common key between distant parties. In their scheme, the method for transmitting quantum states in a block by block way and in multiple steps was proposed and the information leakage before eavesdropping detection was solved. Subsequently, Deng et al. presented two pioneering QSDC schemes, an entangled-state-based two-step QSDC scheme and a single-photon-state-based quantum one-time pad scheme, in which the basic principle and criteria for QSDC were pointed out. From then on, many interesting QSDC schemes have been proposed, including the high-dimension QSDC scheme based on quantum superdense coding, multi-step QSDC scheme based on Greenberger-Horne-Zeilinger states, QSDC scheme based on quantum encryption with practical non-maximally entangled quantum channel, and so on. We also introduce the anti-noise QSDC schemes which were designed for coping with the collective-dephasing noise and the collective-rotation noise, respectively. In 2011, Wang et al. presented the first QSDC which exploited the hyperentangled state as the information carrier and several QSDC schemes based on the spatial degree of freedom (DOF) of photon, single-photon multi-DOF state and hyperentanglement were proposed subsequently. In addition to the point-to-point QSDC schemes, we also review the QSDC networks. Finally, a perspective of QSDC research is given in the last section.
Quantum secure direct communication is one of the major branches of quantum cryptography. In quantum secure direct communication, secret information can be transmitted directly through a quantum channel without the use of a private key. Quantum secure direct communication(QSDC) offers higher security and instantaneousness in communication. Because it does not use private key, key management is dispensed with. Unlike quantum key distribution where key is transferred from quantum terminal to the classical terminal at the user's site, the coding operations are solely in the quantum terminal, it further provides security in the user's site. QSDC is also a powerful basic cryptographic primitive to build other quantum communication protocols, such as quantum bidding, quantum signature and quantum dialogue and so on. Here, we briefly introduce the basic ideas of QSDC, describe the major protocols, report current status, and give a perspective.
Communication security has to evolve to a higher plane in the face of the threat from the massive computing power of the emerging quantum computers. Quantum secure direct communication (QSDC) constitutes a promising branch of quantum communication, which is provably secure and overcomes the threat of quantum computing, whilst conveying secret messages directly via the quantum channel. In this survey, we highlight the motivation and the status of QSDC research with special emphasis on its theoretical basis and experimental verification. We will detail the associated point-to-point communication protocols and show how information is protected and transmitted. Finally, we discuss the open challenges as well as the future trends of QSDC networks, emphasizing again that QSDC is not a pure quantum key distribution (QKD) protocol, but a fully-fledged secure communication scheme.
Quantum secure direct communication (QSDC) transmits information securely through a quantum channel, distinct among various quantum communications. Proposed by me and Liu 20 years ago, QSDC has now been developed to a stage that is ready for practical application. QSDC advances classical communication of reliable communication over a noisy channel, which is guaranteed by Shannon's theory, to a higher plane of both secure and reliable communication over a noisy channel even in the presence of eavesdropping. Here we briefly introduce the basic principles, the techniques for practical applications.