Rapid development of supercomputers and the prospect of quantum computers are posing increasingly serious threats to the security of communication. Using the principles of quantum mechanics, quantum communication offers provable security of communication and is a promising solution to counter such threats. Quantum secure direct communication (QSDC) is one important branch of quantum communication. In contrast to other branches of quantum communication, it transmits secret information directly. Recently, remarkable progress has been made in proof-of-principle experimental demonstrations of QSDC. However, it remains a technical feat to bring QSDC into a practical application. Here, we report the implementation of a practical quantum secure communication system. The security is analyzed in the Wyner wiretap channel theory. The system uses a coding scheme of concatenation of low-density parity-check (LDPC) codes and works in a regime with a realistic environment of high noise and high loss. The present system operates with a repetition rate of 1 MHz at a distance of 1.5 kilometers. The secure communication rate is 50 bps, sufficient to effectively send text messages and reasonably sized files of images and sounds.

Device-independent" not only represents a relaxation of the security assumptions about the internal working of the quantum devices, but also can enhance the security of the quantum communication. In the paper, we put forward the first device-independent quantum secure direct communication (DI-QSDC) protocol, where no assumptions are made about the way the devices work or on what quantum system they operate. We show that in the absence of noise, the DI-QSDC protocol is absolutely secure and there is no limitation for the communication distance. However, under practical noisy quantum channel condition, the photon transmission loss and photon state decoherence would reduce the communication quality and threaten its absolute security. For solving the photon transmission loss and decoherence problems, we adopt noiseless linear amplification (NLA) protocol and entanglement purification protocol (EPP) to modify the DI-QSDC protocol. With the help of the NLA and EPP, we can guarantee the absolute security of the DI-QSDC and effectively improve its communication quality.PACS numbers: 03.67. Dd, 42.50.Dv, 42.50.Ex I. INTRODUCTIONQuantum cryptography can provide an absolute approach to guarantee the security of communication based on the basic principles of quantum mechanics. Quantum cryptography began with quantum key distribution (QKD), i. e., BB84 protocol [1]. Besides QKD, quantum cryptography includes some other branches, such as quantum secret sharing [2] and quantum secure direct communication (QSDC) [3]. QKD can share a series of secure keys between the sender (Alice) and the receiver (Bob) [1,[4][5][6][7]. In QKD, in order to realize the secure communication, the sender and the receiver should ensure that the encryption and decrypt processes are absolutely secure. Moreover, they also require one-time pad and perfect key management. Different from QKD, QSDC provides us another secure communication approach. QSDC allows the sender to transmit secret messages to the receiver without sharing a key first. QSDC has no key, no ciphertext, and either no key management.The first QSDC protocol was proposed by Long et al., which exploited the properties of entanglement and a block transmission technique [3]. QSDC can also be used to achieve QKD with high capacity [8]. In 2003, the standard of QSDC was proposed [9]. Later, QSDC based on single photon and high dimension system were proposed [10,11]. In the aspect of experiment, in 2016, Hu et al. experimentally realized the QSDC with single photons in a noisy environment using frequency coding [12]. In 2017, Zhang et al. successfully completed the QSDC ex- * shengyb@njupt.edu.cn † gllong@tsinghua.edu.cn periment with quantum memory [13]. Recently, Zhu et al. realized the first long-distance QSDC experiment in fibre [14].As a quantum cryptography mode, QSDC is also required to have absolute security. In QSDC protocols, photons should be transmitted in quantum channel for two rounds. For ensuring its absolute security and correctness, a security checking should be performed a...

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