【作者】 潘国柱；

【导师】 杨名； 宋伟；

【作者基本信息】 安徽大学 ， 光学， 2010， 硕士

【摘要】 量子信息学是一门信息理论和量子理论交叉的新兴学科,它在信道容量、安全和运算速度等许多方面显示出了比经典信息所无法比拟的优势,这些优势归根结底来源于其相干性。然而,现有的量子系统由于其或多或少与环境相互作用导致消相干,从而影响量子信息的处理过程,最终有可能导致信息的丧失。如何克服系统的消相干已经成为许多科学家共同关注的目标。迄今为止,人们提出的解决方案主要有三种：一是量子信息编码；二是通过动力学解耦(dynamical decoupling)的方法；三是拓扑量子计算的方案。其中量子编码的方案由于其适用范围的普遍性而受到广泛关注。量子信息编码主要通过量子纠错和量子避错两种方法来保护量子纠缠态不受环境噪声的影响。前者适用于任意的量子系统的纠错但是需要引入大量的辅助位,后者量子避错的方法由于利用了系统的对称性,因而在处理集体消相干噪声的情形时成为一种十分有效的方案。该方案最先由中国科学技术大学段路明和郭光灿提出,他们用两个原子形成一个原子对作为一个逻辑比特,该逻辑比特构成一个对于集体相位噪声不变的无消相干子空间(Decoherence-free subspace—DFS),在此子空间中进行量子信息处理能够有效的避免消相干。本文主要针对该量子避错方案,提出在此无消相干子空间中基于腔量子电动力学(QED)的物理体系来实现量子信息处理的物理方案,取得的主要成果如下：(1)基于腔QED系统提出了如何在无消相干空间中实现量子SWAP门。本方案建立在单边泄漏腔的输入输出方程基础上,利用原子作为存储比特,光子作为飞行比特,当一个单光子脉冲和一个囚禁原子的高品质因子腔相互作用时,可以实现两原子的量子相位门,单逻辑比特的Hadamard门和两逻辑比特的量子相位门,在此基础上,我们提出了如何实现一个逻辑比特和一个飞行比特间的量子信息SWAP门方案。此方案不受原子退相位噪声的影响,且光子的损失只影响实验的成功率,不影响保真度。(2)基于腔QED系统提出了如何在无消相干空间中实现量子密集编码。用腔QED系统实现量子信息的一个主要障碍是腔场的消相干,然而,在这个方案中,通过采用逻辑比特来对量子信息进行编码,消相干被有效的克服。此方案建立在单边泄漏腔的输入输出方程基础上,当通信一方对逻辑比特进行操作时,另一方可以利用逻辑比特的各种门操作来识别,从而达到了通过量子通道来传递经典信息,并且信道容量增大的目的。更多还原

【Abstract】 Quantum information is a new subject, which is the combination of quantum theory and information theory. Compared with classical information science, it manifests distinct advantages in many respects. For example, information capacity, information security, operates speed, and so on. These advantages are due to the coherence of quantum system. However, decoherence induced from the more or less interaction between quantum system and environment, which will interfere the quantum information processing, even spoil the quantum information. How to resist decoherence has become the aim of many physicist. There are three schemes have been proposed so far, The first is quantum information coded, the second is dynamically decoupled from the environment, and the third is topological quantum computation. In these schemes, quantum information coded has attracted widespread attention for its universal application, which prevents the quantum entanglement from noise mainly through two methods, one is error correction coded, and the other is error protection coded. The former is applicable to any quantum system, but need to introduce a large number of auxiliary bits. The latter, for its symmetry, become a very effective scheme in dealing with collective dephasing. The scheme is proposed by Duan Lu-ming and Guo Guang-can from University of Science and Technology, they use an atom pair consist of two atoms as a logic qubit, which forms a Decoherence-free subspace immune to collective dephasing. The quantum information processing can effectively resist decoherence in the Decoherence-free subspace. The paper proposes some schemes for Physical Implementation of Quantum Information Processing in Decoherence-free Subspace based on cavity quantum electrodynamics.Our main results include: (1)We propose a scheme of how to realize quantum SWAP gate in decoherence-free subspace based on cavity QED system. The scheme is built on the input-output formulation of a single-sided cavity, where atom is used as storage qubit and photon is used as flying qubit. We can implement the controlled phase gate between two atoms, the Hadamard gate of one logic qubit and the controlled phase gate between two logic qubits by means of cavity assisted interactions with single-photon pulse. Here we propose a scheme of how to realize the SWAP gate between one logic qubit and one flying qubit. The scheme is immune to dephasing and the loss of photon only affects the successful probability but has no influence on the fidelity.(2) We propose a scheme of how to implement quantum dense coding in decoherence-free subspace based on cavity QED system. One of the main obstacles for the implementation of quantum information in cavity QED is the decoherence of the cavity field. However, In our scheme, by using logic qubit to encode quantum information, the decoherence can be effectively resisted. The scheme is built on the input-output formulation of a single-sided cavity, when one party operates on the logic qubit, the other party can identify it with the help of the quantum logic qubit gate. So we can transfer the classical information by quantum channel, and information capacity increases.更多还原