Quantum secure direct communication and deterministic secure quantum communication Abstract In this review article, we review the recent development of quantum secure direct communication (QSDC) and deterministic secure quantum communication (DSQC) which both are used to transmit secret message, including the criteria for QSDC, some interesting QSDC protocols, the DSQC protocols and QSDC network, etc. The difference between these two branches of quantum communication is that DSQC requires the two parties exchange at least one bit of classical information for reading out the message in each qubit, and QSDC does not. They are attractive because they are deterministic, in particular, the QSDC protocol is fully quantum mechanical. With sophisticated quantum technology in the future, the QSDC may become more and more popular. For ensuring the safety of QSDC with single photons and quantum information sharing of single qubit in a noisy channel, a quantum privacy amplification protocol has been proposed. It involves very simple CHC operations and reduces the information leakage to a negligible small level. Moreover, with the one-party quantum error correction, a reation has been established between classical linear codes and quantum one-party codes, hence it is convenient to transfer many good classical error correction codes to the quantum world. The one-party quantum error correction codes are especially designed for quantum dense coding and related QSDC protocols based on dense coding.Keywords quantum secure direct communication, deterministic secure quantum communication, network, quantum privacy amplification, one-party quantum error correcting codes PACS numbers 03.67.Hk, 03.65.Ud, 42.50.Dv

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...

Preparation of a quantum register is an important step in quantum computation and quantum information processing. It is straightforward to build a simple quantum state such as |i1i2 · · · in with ij being either 0 or 1, but is a non-trivial task to construct an arbitrary superposed quantum state. In this Letter, we present a scheme that can most generally initialize a quantum register with an arbitrary superposition of basis states. Implementation of this scheme requires O(N n 2 ) standard 1-and 2-bit gate operations, without introducing additional quantum bits. Application of the scheme in some special cases is discussed.PACS numbers: 03.67. Lx, 89.70.+c, 89.80.+h Research on quantum computers and quantum information processing has been a fast developing interdisciplinary field over the past years. As a new branch of science overlapping quantum physics and classical information theory, it resembles in some ways both of the subfields, but differs from each of them in many other respects. In quantum computation and quantum information processing, the concept of quantum superposition of basis states |i 1 i 2 · · · i n is used and massive parallelism is achieved [1]. For instance, a significant speed-up over classical computers, at least theoretically, has been gained in prime-factorization [2] and quantum searching [3]. Nevertheless, some simple operations for a classical computer can not be easily implemented in a quantum computer. A vivid example is the need of introducing the quantum error correction scheme to overcome the decoherence problem in quantum computers. This has been obtained with admirable genius [4] whereas the corresponding classical coding scheme is straightforward.Quantum computing is realized by quantum gate operations. It has been shown that a finite set of basic gate operations can be used to construct any quantum computation gate operation [5]. This universality of quantum computation has been studied by many authors [6][7][8]. A quantum circuit, which is a network of gate operations for certain purpose, has been constructed, for example, for basic arithmetic [9] and efficient factorization [10].Initializing a quantum register to an arbitrary superposition of basis states is a seemingly simple, yet difficult problem. Addition of two numbers in a classical computer could not be easier, but addition of two quantum states a 1 |ψ 1 + a 2 |ψ 2 is not easy at all. However, superposition is the basic ingredient in quantum computing and quantum information processing. An efficient scheme for initializing an arbitrary superposition for a quantum register is very much desired. An efficient scheme for initializing a quantum register for a known function of amplitude distribution was given by Ventura and Martinez (VM) with n + 1 additional quantum bits (qubits) [11].In this Letter, we present a general scheme that initializes a quantum register without introducing additional qubits. For some quantum computing tasks, introduction of additional qubits is not permitted. Thus our scheme may be appreciate...