Quantum secure direct communication (QSDC) transmits secret messages directly over a quantum channel without the prior distribution of a key. Here, we apply Wyner's wiretap channel theory to analyze the security of QSDC protocols. The ideal protocol is treated as the main channel, and the effect of eavesdropping is treated as the wiretap channel. Entanglement-based QSDC protocols are analyzed in detail at first. We calculated the channel capacity of the wiretap channel, and hence, the secrecy channel capacity of the protocol. The security of single-photon–based QSDC protocols is studied through the equivalence between the entanglement-based protocols and single-photon–based protocols. We present a modified version of the single-photon-based DL04 protocol, which gives a higher secrecy capacity.
Quantum secure direct communication (QSDC) transmits secret messages directly over a quantum channel without the prior distribution of a key. Here, we apply Wyner's wiretap channel theory to analyze the security of QSDC protocols. The ideal protocol is treated as the main channel, and the effect of eavesdropping is treated as the wiretap channel. Entanglement-based QSDC protocols are analyzed in detail at first. We calculated the channel capacity of the wiretap channel, and hence, the secrecy channel capacity of the protocol. The security of single-photon–based QSDC protocols is studied through the equivalence between the entanglement-based protocols and single-photon–based protocols. We present a modified version of the single-photon-based DL04 protocol, which gives a higher secrecy capacity.
Quantum secure direct communication (QSDC) is an important branch of quantum communication that is capable of directly transmitting secret messages over a quantum channel. It may be viewed as a concrete realization of Wyner's wiretap channel theory, which ensures the reliable and secure communication of information in the presence of noise and eavesdropping. Hence it is a fully-fledged quantum-communications protocol, which does not require a separate secret key negotiation phase. By contrast, its quantum key distribution (QKD) counterpart represents a secret key-negotiation protocol, which has to be followed up by a separate classical communication session. The essential difference between these two modes of quantum communication lies in the employment of a block-based data transmission technique, proposed by Long and Liu in 2000. However, the original block-based data transmission requires quantum memory, which is not widely available at the time of writing. Recently, this difficulty has been overcome by using classical coding theory, which has been successfully applied to the single-qubit DL04 QSDC. Here we will present a single-photon-memory QSDC protocol based on entangled pairs of photons. We commence by comparing QSDC to QKD, followed by an example of the single-photon QSDC and single-photon QKD protocol. Then we continue by modifying the so-called two-step QSDC protocol designed for deterministic QKD by reducing the number of qubits in a block into a single one, in which Alice prepares Einstein-Podolsky-Rosen (EPR) photon pairs and partitions them into two parts: the so-called pioneer qubit and the follow-up qubit. The pioneer photon is transferred first to Bob, while the follow-up photon is used either for performing encoding or for eavesdropping detection. Bob extracts the candidate key by combining the two particles of the EPR pair to perform Bell-basis measurement. Then the protocol is transformed into a single-photon-memory QSDC using coding theory. Our theoretical analysis shows that the resultant protocol is robust to individual attacks. Additionally, a high communication efficiency is achieved.
Quantum secure direct communication (QSDC) is a recently developed practical solution, which transmits secret messages between legitimate parties, without setting up a cryptographic key. The QSDC protocols are mainly classified into entanglement-based and single photon-based approaches. Their security has been analyzed using Wyner's wiretap channel theory, and their secrecy capacity has been calculated [1]. Hitherto, QSDC protocols have been implemented on optical fiber and free-space channels, all in discrete-variable (DV) encoding that employs single-photon detectors [1]–[2]. However, the implementation of continuous-variable (CV) QSDC, in which the keys are encoded into the quadratures of quantized electromagnetic fields (e.g., coherent states) and detected via homodyne techniques, is preferred due to: its lower costs, excellent integrability with existing optical communication systems, easy implementation from state preparation to measurement, and fast experimental realization. Furthermore, it has been proved that CV-QSDC protocols using squeezed states show higher tolerance in purely lossy channels and enhanced robustness against highly noisy ones, as well [3]. Nevertheless, none of the CV-QSDC protocols employing either coherent or squeezed quantum states have been implemented yet.
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 (QSDC) can directly transmit secrete messages through a quantum channel. Device-independent (DI) QSDC can guarantee the communication security relying only on the observation of the Bell-inequality violation, but not on any detailed description or trust of the inner workings of users' devices. In the paper, we propose a DI-QSDC protocol with practical highly efficient single-photon sources. The communication parties construct the entanglement channel from single photons by adopting the heralded architecture, which makes the message-leakage rate independent of the photon-transmission loss. The secure communication distance and the practical communication efficiency of the current DI-QSDC protocol are about 6 times and 600 times of those in the original DI-QSDC protocol. Combining with the entanglement purification, the parties can construct the nearly perfect entanglement channel and completely eliminate the message leakage. This DI-QSDC protocol may have useful applications in the future quantum communication field.
Quantum secure direct communication (QSDC) enables the message sender to directly transmit messages to the message receiver through quantum channel without keys. Environmental noise is the main obstacle for QSDC's practicality. For enhancing QSDC's noise robustness, we introduce the quantum error correction (QEC) code into QSDC and propose the QSDC protocol based on the redundancy code. This QSDC protocol correlates atomic state with the electron–photon entangled pairs and transmits photons in quantum channels for two rounds. The parties can construct the remote atomic logical entanglement channel and decode messages with the heralded photonic Bell state measurement (BSM) and single electron measurement. This QSDC protocol is unconditionally secure in theory and has some advantages. First, benefiting from the heralded photonic BSM, it can eliminate the influence from photon transmission loss and has the potential to realize long-distance secure message transmission. Second, taking use of the error correction function of the repetition code, the error rate caused by the decoherence during the second round of photon transmission can be reduced, which can reduce the message error and increase the secret message capacity. Third, the whole protocol is feasible under current experimental condition. Our QSDC protocol can be extended to use other stronger QEC code. It provides a promising method to promote QSDC's practicality in the future.
The extensive studies on the quantum communication, which provides a novel means of secure communication, have been carried out in last decades. Among the various types of quantum communication, quantum secure direct communication (QSDC) can securely transmit message directly through a quantum channel. This paper reports a practical scheme of QSDC without quantum memory, taking advantage of the wiretap channel theory and classical coding theory. The proposed approach solves one of the biggest obstacle of practical applications of QSDC, the quantum memory, which is indispensable in original QSDC protocols. The design and detailed scheme for implementation of the proposed protocol are given.