融合了量子物理学与现代信息学，量子通信将量子所具有的不可克隆性、叠加态、纠缠等神奇的特性应用于现代通信，使信息能够实现保密快速传输。由于科技的发展，庞大的数据量和远距离、大带宽的通信需求使得量子信息学具有巨大的应用前景。相比于经典通信，量子通信拥有绝对安全的通信能力。近些年，一种新兴的量子保密通信方式——量子安全直接通信发展起来，它能够直接在量子信道中传递量子态并荷载信息，而不需要传统量子密码通信中的密钥传输的过程，降低了信息传输过程中加密、解密的要求。信道中的噪声是指由于通信过程中存在干扰，导致接收方接收到的信息与发送方所发送的信息不一致的现象。信道容量为每个符号能在信道中可靠传输的最大的信息量，用于衡量信息传输的效率，也是经典信息论中一个十分重要的研究方向。但是在量子通信中，尤其是量子安全直接通信协议中的噪声信道容量目前还未有系统化的研究。

基于已知的几种量子噪声信道模型与几种经典的量子安全直接通信协议，本论文的主要工作包括以下三个方面：

一、以量子去极化信道和量子振幅阻尼信道为噪声模型，分别研究了在两种经典的量子安全直接通信协议（量子一次便笺方案与传统两步协议）中，从发送方Alice到接收方Bob之间单光子或纠缠光子对的量子态的演化过程以及其保真度和生存函数随噪声参数的变化。结果表明，信道的噪声参数越大，其信道的保真度和生存函数越小，直至收敛为零。

二、得到了在量子一次便笺方案和传统两步协议中，分别使用去极化噪声信道和振幅阻尼噪声信道时，信道的经典容量以及量子容量下界的计算公式；进一步探究了在自由空间中的量子通信过程中，实际通信过程的传输距离与光子的信息传输速率的关系，通过Matlab仿真实验得到通信能够传输的最大距离。结果表明，信道中的噪声干扰对量子容量的影响比经典容量更大。尤其是当信道噪声参数较大时，信道可以传送经典信息，但却不能传送量子信息。两种噪声信道相比较，对于经典信息的传递，振幅阻尼信道容量更大，而对于量子信息的传递，去极化信道的容量更大。

三、研究了在基于高维编码空间的两步量子安全直接通信协议中，量子去极化信道和量子消除信道的经典容量与通信维度以及噪声之间的关系。通过提高编码空间的维度，或减小传输过程中的损耗及噪声都可以有效增加量子信道的容量。

Quantum communication combines quantum physics with modern informatics. Quantum communication applies the magical characteristics of quantum to modern communication, such as non-cloning, superposition states and entanglement, so that information can be transmitted in a secure and fast way. Due to the development of science and technology, the huge data volume and the long-distance and wide bandwidth communication demand make quantum informatics have a great application prospect. Compared with classical communication, quantum communication has absolute security communication capability. In recent years, a new quantum secure communication method, quantum secure direct communication, has developed. It can transmit quantum state and load information directly in quantum channel, without the need of key transmission in traditional quantum cryptography communication, which reduces the requirements of encryption and decryption in the process of information transmission. The noise in the channel refers to the interference in the communication process, which makes the information received by the receiver is not identical with the information sent by the sender. The channel capacity is the largest number of information bits that can be reliably transmitted by each symbol in the channel. It is also a very important research direction in classical information theory to measure the efficiency of information transmission. However, the noise channel capacity in quantum communication, especially in the direct quantum secure communication protocol, has not been systematically studied.

Based on several known quantum noise channel models and several classical quantum security direct communication protocols, the main work of this paper includes the following three aspects:

1. In this paper, the evolution of the quantum states of a single photon or entangled photon pair between Alice and the receiver Bob, and the variation of fidelity and survival function with noise parameters are studied in two classical quantum secure direct communication protocols (quantum first note scheme and traditional two-step protocol), respectively. The results show that the larger the noise parameters of the channel, the smaller the fidelity and survival function of the channel until the convergence is zero.

2. The classical capacity and the lower bound of quantum capacity are obtained when depolarized noise channel and amplitude damping noise channel are used in quantum one-time note scheme and traditional two-step protocol; The relationship between the transmission distance of the real communication process and the information transmission rate of photons in the quantum communication in free space is further explored. The maximum distance that can be transmitted is obtained by MATLAB simulation experiment. The results show that the influence of noise interference on quantum capacity is greater than that of classical capacity. Especially when the noise parameters of the channel are large, the channel can transmit classical information, but it can not transmit quantum information. Compared with the two noise channels, the capacity of amplitude damping channel is larger for classical information transmission, while for quantum information transmission, the capacity of depolarization channel is larger.

3. In the two-step quantum secure direct communication protocol based on high dimensional coding space, the relationship between the classical capacity of quantum depolarization channel and quantum elimination channel, communication dimension and noise is studied. The capacity of quantum channel can be increased by increasing the dimension of coding space, or reducing the loss and noise in transmission. When the dimension range is 0 to 40, the effect of improving the classical capacity of the channel is more obvious, and the convergence is more than 40.