表面等离激元量子信息应用研究进展

近年来表面等离激元得到了越来越多的关注和研究,得益于其能把电磁场束缚在金属-介质界面附近的亚波长尺度范围内.本文回顾了近年来表面等离激元在量子信息领域中的理论和实验研究,包括表面等离激元的基本量子性质、表面等离激元量子回路、在量子尺度下与物质的相互作用及其潜在应用.量子表面等离激元开辟了对表面等离激元基本物理性质研究的新方向,可以应用于高度集成化的量子集成光学回路,同时也可以用来增强光与量子发光体的相互作用.     

单原子脉塞和单原子激光——腔量子电动力学的实验平台

单原子脉塞和单原子激光是腔量子电动力学研究的重要实验平台。本文综述关于几类单原子脉塞和单原子激光的研究工作,包括德国Walther小组的单原子脉塞、超冷原子注入的单原子脉塞,法国Haroche小组的腔量子电动力学系统、注入原子的单原子激光、以及囚禁原子和囚禁离子的单原子激光。我们介绍相关的理论工作、实验系统、以及主要结果。     

Simple schemes for generation of W-type multipartite entangled states and realization of quantum- information concentration

We propose simple schemes for generating W-type multipartite entangled states in cavity quantum electrodynamics (CQED). Our schemes involve a largely detuned interaction of Λ-type three-level atoms with a single-mode cavity field and a classical laser, and both the symmetric and asymmetric W states can be created in a single step. Our schemes are insensitive to both the cavity decay and atomic spontaneous emission. With the above system, we also propose a scheme for realizing quantum-information concentration which is the reverse process of quantum cloning. In this scheme, quantum-information originally coming from a single qubit, but now distributed into many qubits, is concentrated back to a single qubit in only one step.     

Lattice Wess-Zumino-Witten model and quantum groups

Quantum groups play the role of symmetries of integrable theories in two dimensions. They may be detected on the classical level as Poisson-Lie symmetries of the corresponding phase spaces. We discuss specifically the Wess-Zumino-Witten conformally invariant quantum field model combining two chiral parts which describe the left- and right-moving degrees of freedom. On one hand, the quantum group plays the role of the symmetry of the chiral components of the theory. On the other hand, the model admits a lattice regularization (in Minkowski space) in which the current algebra symmetry of the theory also becomes quantum, providing the simplest example of a quantum group symmetry coupling space-time and internal degrees of freedom. We develop a free field approach to the representation theory of the lattice sl (2)-based current algebra and show how to use it to rigorously construct an exact solution of the quantum SL (2) WZW model on lattice.     

Perfect photon absorption based on the optical parametric process

The perfect photon absorption is studied in a cavity quantum electrodynamics(CQED) system, in which an optical parameter amplifier(OPA) is coupled to the cavity mode. This makes it possible to control the optical phase to realize the perfect photon absorption. It is found that in the presence of one and two injected fields, the perfect photon absorption is present in these two cases and can be controlled by adjusting the parametric phase. Moreover, different from the previous predictions of perfect photon absorption in atomic CQED systems, the perfect photon absorption can be changed significantly by the relative phase. Our work provides a new platform to use the parametric processes to make an available way to control the behaviors of photons and to take advantage of the optical phase to achieve the perfect photon absorption.     

Entanglement Properties in the System of Three Atoms Trapped in Three Distant Cavities Connected by Two Optical Fibers

This paper discusses the entanglement dynamics of a coupled cavity quantum electrodynamics (CQED) setup, which comprises three two-level atoms resonantly interacting with three cavities that are coupled by two optical fibers. The influences of atom-cavity coupling constant on the entanglement between atoms and that between cavities are discussed. The results obtained from the numerical method show that the entanglement between non-adjacent atoms or that between adjacent cavities has a nonlinear relation with increasing of the atom-cavity coupling coefficient. On the other hand, the entanglement between non-adjacent cavities is strengthened and the entanglement between adjacent atoms is weakened with increasing of atom-cavity coupling constant.     

Charge Qubit Storage and Its Engineered Decoherence via Microwave Cavity

We study the entanglement of the superconducting charge qubit with the quantized electromagnetic field in a microwave cavity. It can be controlled dynamically by a classical external field threading the SQUID within the charge qubit. Utilizing the controllable quantum entanglement, we can demonstrate the dynamic process of the quantum storage of information carried by charge qubit. On the other hand, based on this engineered quantum entanglement, we can also demonstrate a progressive decoherence of charge qubit with quantum jump due to the coupling with the cavity field in quasi-classical state.     

量子点操控的光子探测和圆偏振光子发射

半导体量子点是研究光子与电子态相互作用的优选固态体系,并在光子探测和发射两个方向上展现出独特的技术机遇.其中基于量子点的共振隧穿结构被认为在单光子探测方面综合性能最佳,但受到光子数识别、工作温度两个关键性能的制约.利用腔模激子态外场耦合效应,有望获得圆偏振态可控的高频单光子发射.本文介绍作者提出的量子点耦合共振隧穿（QD-cRTD）的光子探测机理,利用量子点量子阱复合电子态的隧穿放大,将QD-cRTD光子探测的工作温度由液氦提高至液氮条件,光电响应的增益达到10⁷以上,并具备双光子识别能力;同时,由量子点能级的直接吸收,原型器件获得了近红外的光子响应.在量子点光子发射机理的研究方面,作者实现了量子点激子跃迁和微腔腔模共振耦合的磁场调控,在Purcell效应的作用下增强激子自旋态的自发辐射速率,从而增强量子点中左旋或右旋圆偏振光的发射强度,圆偏度达到90%以上,形成一种光子自旋可控发射的新途径.     

Entangling single photons on a beamsplitter

We report on a scheme for the creation of time-bin entangled states out of two subsequent single photons. Both photons arrive on the same input port of a beamsplitter and the situation in which the photons leave the beamsplitter on different output ports is post-selected. We derive a full quantum mechanical analysis of such time-bin entanglement for emitters subject to uncorrelated dephasing processes and apply this model to sequential single photons emerging from a single semiconductor quantum dot. Our results indicate that the visibility of entanglement is degraded by decoherence effects in the quantum dot, but can be restored by use of CQED effects, namely the Purcell effect.     

An alternative quantum theory for single particles and a proposed experimental test

An alternative quantum theory for single particles bounded in the external field proposed in 1986 (Huang X. Y., Phys. Lett. A., 1986, 115: 310) is further developed from which the energy of the state for the single particle takes one of the eigenvalues of the quantum Hamiltonian, and the usual quantum mechanics for the particle in a stationary state holds only in the statistical sense. In light of the theory, the particle of definite energy, ground-state-energy for instance, can exhibit a novel periodic behavior. This result for the ground-state-energy state neutron in the Earth’s gravitational field is experimentally testable using ultracold neutron beam passing through the same apparatus that was devised in 2002 to identify the energy quantization of neutron in the field (Nesvizhevsky V. V., et al., Nature, 2002, 415: 297).      

自组织量子点的量子态光学探测

半导体量子点主要包括在真空中外延生长的自组织量子点和在溶液中采用化学方法合成的胶体量子点,由于量子限制效应所导致的分立能级结构使得它们通常被称为“人工原子”。和自然原子不同,半导体量子点的能级结构强烈依赖于其尺寸和形状,这样就提供了更为灵活的方法来控制固体材料中的光与材料的相互作用。近年来,许多类原子的量子光学现象(包括量子干涉、Rabi振荡和Mollow荧光)都已经在单个的自组织量子点中揭示出来。与此形成对比的是,上述所有的类原子量子光学特性目前还没有在单个的胶体量子点中观察得到。在本文中,我们将侧重于介绍我们科研组以及我们和别的科研组合作对单个自组织量子点的单量子态在光学探测和相干控制方面完成的一系列工作。对单个的胶体量子点,我们认为量子相干特性的测量和控制将在新近合成的非荧光闪烁或荧光闪烁得到抑制的材料体系中得以实现.     

半导体中的自旋弛豫--从体材料到量子阱、量子线、量子点

本文对半导体中的自旋弛豫过程给出一个简要的回顾,介绍了半导体材料从体材料到量子阱、量子线、量子点不同维数的结构中各种自旋弛豫过程,主要关注了自旋去相位和相干控制。对于不同材料中的各种弛豫机制,关注的重点在于如何能够在实验上以一种可以控制的方式来改变可调参数从而达到控制自旋弛豫过程。这些参数主要有电场、磁场、温度、应变、有效g因子等等。本文的组织上,首先介绍研究前景,第1部分简要介绍了自旋弛豫的四种机制。第2部分按照维数的不同将半导体中自旋弛豫分为3个部分:体材料、量子阱、量子线、量子点,在每一部分中又基本上按照电子、空穴、激子的顺序进行了简要的总结:对于不同的载流子,考虑了自旋弛豫对可调参数的依赖关系。这些结果要么试图解释了已有的实验结果,要么从理论上给出预言从而给实验指明了方向,为室温下可以使用的自旋电子学器件设计提供了依据,为固态量子计算和量子信息处理铺平了道路。最后简单地给出展望。     

Determining the parity of a permutation using an experimental NMR qutrit

We present the NMR implementation of a recently proposed quantum algorithm to find the parity of a permutation. In the usual qubit model of quantum computation, it is widely believed that computational speedup requires the presence of entanglement and thus cannot be achieved by a single qubit. On the other hand, a qutrit is qualitatively more quantum than a qubit because of the existence of quantum contextuality and a single qutrit can be used for computing. We use the deuterium nucleus oriented in a liquid crystal as the experimental qutrit. This is the first experimental exploitation of a single qutrit to carry out a computational task.     

半导体中的自旋弛豫——从体材料到量子阱、量子线、量子点

本文对半导体中的自旋弛豫过程给出一个简要的回顾,介绍了半导体材料从体材料到量子阱、量子线、量子点不同维数的结构中各种自旋弛豫过程,主要关注了自旋去相位和相干控制。对于不同材料中的各种弛豫机制,关注的重点在于如何能够在实验上以一种可以控制的方式来改变可调参数从而达到控制自旋弛豫过程。这些参数主要有电场、磁场、温度、应变、有效g因子等等。本文的组织上,首先介绍研究前景,第1部分简要介绍了自旋弛豫的四种机制。第2部分按照维数的不同将半导体中自旋弛豫分为3个部分:体材料、量子阱、量子线、量子点,在每一部分中又基本上按照电子、空穴、激子的顺序进行了简要的总结:对于不同的载流子,考虑了自旋弛豫对可调参数的依赖关系。这些结果要么试图解释了已有的实验结果,要么从理论上给出预言从而给实验指明了方向,为室温下可以使用的自旋电子学器件设计提供了依据,为固态量子计算和量子信息处理铺平了道路。最后简单地给出展望。     

Coherent Interband and Intersubband Dynamics in Terahertz-Driven GaAs Quantum Wells

We theoretically investigate the optical absorption spectra and charge density by subjecting a GaAs quantum well to both an intense terahertz (THz)-frequency driving field and an optical pulse within the theory of density matrix. In presence of a strong THz field, the optical transitions in quantum well subbands are altered by the THz field. The alteration has a direct impact on the optical absorption and the charge density. The excitonic peak splitting and THz optical sideband in the absorption spectra show up when changing the THz field intensity and/or frequency. The Autler-Towns splitting is a result from the THz nonlinear dynamics of confined excitons. On the other hand, the carrier charge density is created as wave packets formed by coherent superposition of several eigenstates. The charge density exhibitsquantum beats for short pulses and/or wider wells and is modulated by the THz field.     

腔QED中利用超导量子干涉仪实现Toffoli门

基于腔量子电动力学技术,提出了利用三能级超导量子干涉仪实现Toffoli门的理论方案.利用超导量子干涉仪与腔场发生耦合,以及与外加经典脉冲发生共振跃迁来实现量子态的演化控制.该方案可以拓展到N比特Toffoli门的实现.最后,讨论了逻辑门的实验可行性,四比特Toffoli门的作用时间约为30 nm,它远小于腔衰减时间和较高能级的能量驰豫时间,从而足以实现量子态的操控.并且随着比特数的增多,Toffoli门作用时间的增幅较慢.     

Preparation of multi-atom cluster state and teleportation of arbitrary two-atom state via thermal cavity

We propose one cavity QED (CQED) scheme for generating an arbitrary 2-level-atom cluster state. Besides, by using a 4-atom cluster state as quantum channel, we propose another CQED scheme for teleporting any unknown two-atom state. In both schemes, the dynamics processes are essentially quite similar. The Rabi frequency of the classical driving field is much bigger than the detuning between the atoms and the cavity. Hence both schemes are insensitive to the cavity decay. The necessary time for implementation is much shorter than the Rydberg-atom lifespan, therefore atom decays do not need to be considered. Moreover, in the teleportation scheme the discrimination of the 16 mutually orthogonal 4-atom cluster states is transformed into the discrimination of single-atom product states, consequently the discrimination difficulty is degraded and the scheme is more easily implemented.     

Probabilistic teleportation of multi-particle partially entangled state

Utilizing the generalized measurement described by positive operator-wlued measure, this paper comes up with a protocol for teleportation of an unknown multi-particle entangled （GHZ） state with a certain probability. The feature of the present protocol is to weaken requirement for the quantum channel initially shared by sender and receiver. All unitary transformations performed by receiver are summarized into a formula. On the other hand, this paper explicitly constructs the efficient quantum circuits for implementing the proposed teleportation by means of universal quantum logic operations in quantum computation.