在腔耦合的超导量子比特中实现几何量子信息处理

用量子空腔耦合的超导电荷比特器件被认为是实现量子信息处理的相当有希望的体系之一.如何在这种可集成的量子体系中实现高保真度的操作是量子信息处理领域的重要课题.文章介绍作者最近提出的在量子腔耦合的超导量子比特中用具有内禀容错功能的几何操作来实现普适量子逻辑门,产生多比特量子纠缠及实现量子纠错编码的一个可行方案.     

量子计算与量子模拟

量子计算和量子模拟在过去的几年里发展迅速,今后涉及多量子比特的量子计算和量子模拟将是一个发展的重点.本文回顾了该领域的主要进展,包括量子多体模拟、量子计算、量子计算模拟器、量子计算云平台、量子软件等内容,其中量子多体模拟又涵盖量子多体动力学、时间晶体及多体局域化、量子统计和量子化学等的模拟.这些研究方向的回顾是基于对现阶段量子计算和量子模拟研究特点的考虑,即量子比特数处于中等规模而量子操控精度还不具有大规模逻辑门实现的能力,研究处于基础科研和实用化的过渡阶段,因此综述的内容主要还是希望管窥今后的发展.     

量子相变与量子纠缠

作为量子体系一种内在的非局域性关联,量子纠缠已经成为一个可利用的重要资源并广泛应用于许多领域.强关联体系中的量子相变,作为凝聚态理论中的一个重要现象,也一直是人们研究的热点.本文介绍了量子纠缠的定义,纠缠的判据,以及纠缠的度量.接着,通过一个具体模型对如何利用量子纠缠描述量子相变进行讨论和分析.研究发现,在强关联体系量子相变中量子纠缠扮演着非常重要的角色.     

基于金刚石氮-空位色心自旋系综与超导量子电路混合系统的量子节点纠缠

量子纠缠是实现量子计算和量子通信的核心基础,本文提出了在金刚石氮-空位色心(NV centers)自旋系综与超导量子电路耦合的混合系统中实现两个分离量子节点之间纠缠的理论方案.在该混合系统中,把金刚石NV centers自旋系综和与之耦合的超导共面谐振器视为一个量子节点,两个量子节点之间通过一个空的超导共面谐振器连接.具有较长相干时间的NV centers自旋系综作为一个量子存储器,用于制备、存储和发送量子信息;易于外部操控的超导量子电路可执行量子逻辑门操作,快速调控量子信息.为了实现两个分离量子节点之间的纠缠,首先对系统的哈密顿量进行正则变换,将其等价为两个NV centers自旋系综与同一个超导共面谐振器之间的JC耦合;然后采用NV centers自旋-光子混合比特编码的方式,通过调节超导共面谐振器的谐振频率,精确控制体系演化时间,高保真度地实现了两个分离量子节点之间的量子纠缠.本方案还可以进一步扩展和集成,用于构建多节点纠缠的分布式量子网络.     

基于量子Fisher信息的量子计量进展

量子计量是超冷原子气体研究中的一个热点领域.超冷原子体系独特的量子性质(量子纠缠)和量子效应有助于大幅度提高待测物理量的测量精度,这已经成为量子精密测量中的共识.量子Fisher信息对该领域的发展起了非常重要的作用.本文首先介绍量子Fisher信息的基本概念和量子计量的主要内容;然后简要回顾这些理论在提高测量精度方面的应用,特别是多粒子量子纠缠态的产生及其判定;再介绍线性和非线性原子干涉仪的相关进展;最后论述量子测量过程中的统计方法的研究进展.     

几何量子计算

实现可集成的量子计算的关键步骤是实现保真度足够高的一组普适量子逻辑门，最近几年发展的几何量子计算使用几何位相来实现量子逻辑门，其特点是利用几何位相的整体几何性质来避免某些局域的无规噪声的影响，从而实现较高保真度的量子门，文章先简要介绍常规几何量子逻辑门的概念，然后重点介绍最近提出的非常规几何量子计算：量子计算中使用的逻辑门的总位相既包含有几何位相，又包含有动力学位相，但它仅依赖于一些几何特征，而且，对于任意的量子位输入态，在量子门操作过程中积累的位相要么是零，要么是仅依赖几何特征的位相。     

量子计算纠错取得突破性进展

谷歌团队利用超导量子芯片,实现了两种规模的量子纠错表面码,利用辅助比特进行多次测量和初始化操作进行纠错,表明使用更多量子比特表面码的逻辑比特,其性能表现更好,达到了量子纠错规模化的盈亏平衡点,本文简要回顾量子纠错的机制,讨论谷歌量子纠错结果的意义,以及系列量子纠错进展,最后将展望量子计算的发展方向.     

核磁共振中的量子控制

近年来, 随着量子信息科学的发展, 对由量子力学原理描述的微观世界的主动调控已成为重要的前沿研究领域. 为构造实际的量子信息处理器, 一个关键的挑战是: 如何对处于噪声环境下的量子体系实现一系列高精度的任意操作, 以完成目标量子信息处理任务. 为此, 人们将经典系统控制论的思想方法延伸到量子体系的领域, 提出了大量的量子控制方法以及相关的数值技术(如量子优化控制、量子反馈控制等), 并取得了丰富的研究成果. 核磁共振自旋体系具备成熟的系统理论和操控技术, 为量子控制方法的实用性研究提供了优秀的实验测试平台. 因此, 基于核磁共振的量子控制成为量子控制领域的重要方向. 本文简要介绍了量子控制的基本概念和方法; 从系统控制论的角度对核磁共振自旋体系的基本原理和重要控制任务做了阐述; 介绍了近些年来在该领域发展的相关控制方法及其应用; 对基于核磁共振体系的量子控制的进一步的研究做了几点展望.     

量子信息掩蔽

量子信息掩蔽是量子信息处理中的一个新兴概念,它将量子信息完全转移至各个量子实体的关联之中,从而使单个量子系统不再包含掩蔽前的任何信息。量子信息掩蔽在量子比特承诺、秘密共享等方面都具有重要的应用;然而与量子态的克隆、广播、隐藏等操作类似,人们无法使用一个普适的两体幺正演化来实现量子信息的掩蔽。本文系统地介绍近期量子信息掩蔽方向的一系列研究进展：首先描述了可实现量子信息掩蔽态集合的几何特征,给出掩蔽操作的实现方法,讨论了其在信息论中的含义以及与量子纠错码等概念的联系;然后介绍量子信息掩蔽的实验实现,展现量子信息掩蔽在高维体系等复杂系统中的可行性,并展示其在量子秘密共享和噪声免疫的量子通信等方面的应用。     

量子相干

量子相干不仅是量子力学中的一个基本概念,同时也是重要的量子信息处理的物理资源.随着基于资源理论框架的量子相干度量方案的提出,量子相干度的量化研究成为近年来人们关注的一个热点问题.量子相干作为一种物理资源也十分脆弱,极容易受到环境噪声的影响而产生退相干,因此开放系统中的量子相干演化和保持也是人们广泛关注的课题.另外,量子相干在量子多体系统、量子热动力学、量子生物学等领域也有着潜在的应用价值.本文介绍量子相干度量的资源理论框架和基于该框架定义的相对熵相干性、l1范数相干性、基于量子纠缠的相干性、基于凸顶结构的相干性和相干鲁棒性等量子相干度量函数,概述开放系统中量子相干演化的动力学行为、典型信道的量子相干产生和破坏能力以及量子相干的冻结等现象,同时例举量子相干在Deutsch-Jozsa算法、Grover算法以及量子多体系统相变问题研究等方面的重要应用.量子相干研究仍处于快速发展之中,期望本综述能为该领域的发展带来启示.     

One-way gates based on EPR, GHZ and decoherence-free states of W class

The logical gates using quantum measurement as a primitive of quantum computation are considered. It is found that these gates achieved with EPR, GHZ and W entangled states have the same structure, allow encoding the classical information into states of quantum system and can perform any calculations. A particular case of decoherence-free W states is discussed as in this very case the logical gate is decoherence-free.     

A quantum computer in the scheme of an atomic quantum transistor with logical encoding of qubits

A scheme of a multiqubit quantum computer on atomic ensembles using a quantum transistor implementing two qubit gates is proposed. We demonstrate how multiatomic ensembles permit one to work with a large number of qubits that are represented in a logical encoding in which each qubit is recorded on a superposition of single-particle states of two atomic ensembles. The access to qubits is implemented by appropriate phasing of quantum states of each of atomic ensembles. An atomic quantum transistor is proposed for use when executing two qubit operations. The quantum transistor effect appears when an excitation quantum is exchanged between two multiatomic ensembles located in two closely positioned QED cavities connected with each other by a gate atom. The dynamics of quantum transfer between atomic ensembles can be different depending on one of two states of the gate atom. Using the possibilities of control for of state of the gate atom, we show the possibility of quantum control for the state of atomic ensembles and, based on this, implementation of basic single and two qubit gates. Possible implementation schemes for a quantum computer on an atomic quantum transistor and their advantages in practical implementation are discussed.     

On Harmonic Approximation for Large Josephson Junction Coupling Charge Qubits

We revisit the harmonic approximation (HA) for a large Josephson junction interacting with some charge qubits through the variational approach for the quantum dynamics of the junction-qubit coupling system. By making use of numerical calculation and analytical treatment, the conditions under which HA works well can be precisely presented to control the parameters implementing the two-qubit quantum logical gate through the couplings to the large junction with harmonic oscillator Hamiltonian.     

Suppressing decoherence of quantum algorithms by jump codes

The stabilizing properties of one-error correcting jump codes are explored under the realistic non-ideal condition of finite recovery times. For this purpose the quantum algorithm of the tent-map is decomposed into a universal set of Hamiltonian quantum gates which ensure perfect correction of spontaneous decay processes under ideal circumstances even if they occur during a gate operation. An entanglement gate is presented which is capable of entangling any two logical qubits of different one-error correcting code spaces. With the help of this gate simultaneous spontaneous decay processes affecting physical qubits of different code spaces can be corrected and decoherence can be suppressed significantly.     

Observation of coherent oscillation of a single nuclear spin and realization of a two-qubit conditional quantum gate

Rabi nutations of a single nuclear spin in a solid have been observed. The experiments were carried out on a single electron and a single 13C nuclear spin of a single nitrogen-vacancy defect center in diamond. The system was used for implementation of quantum logical NOT and a conditional two-qubit gate (CROT). Density matrix tomography of the CROT gate shows that the gate fidelity achieved in our experiments is up to 0.9, good enough to be used in quantum algorithms.     

飞秒激光直写光量子逻辑门

量子比特在同一时刻可处于所有可能状态上的叠加特性使得量子计算机具有天然的并行计算能力,在处理某些特定问题时具有超越经典计算机的明显优势.飞秒激光直写技术因其具有单步骤高效加工真三维光波导回路的能力,在制备通用型集成光量子计算机的基本单元—量子逻辑门中发挥着越来越重要的作用.本文综述了飞秒激光直写由定向耦合器构成的光量子比特逻辑门的进展.主要包括定向耦合器的功能、构成、直写和性能表征,集成波片、哈达玛门和泡利交换门等单量子比特逻辑门、受控非门和受控相位门等两量子比特逻辑门的直写加工,并对飞秒激光加工三量子比特逻辑门进行了展望.     

Quantum computation based on a quantum switch in cavity QED

A feasible scheme for constructing quantum logic gates is proposed on the basis of quantum switches in cavity QED. It is shown that the light field which is fed into the cavity due to the passage of an atom in a certain state can be used to manipulate the conditioned quantum logical gate. In our scheme, the quantum information is encoded in the states of Rydberg atoms and the cavity mode is not used as logical qubits or as a communicating “bus”; thus, the effect of atomic spontaneous emission can be neglected and the strict requirements for the cavity can be relaxed.     

Remote Implementation of a Fredkin Gate via Virtual Excitation of an Atom-Cavity-Fiber System

Simple operations and robust results are always of interest for any quantum tasks. Herein, a novel scheme is proposed for implementing a Fredkin gate via the virtual excitation of an atom-cavity-fiber system. The scheme is to control the nonlocal state-swap of two spatially separated target atoms according to the state of the control atom at hand. In the scheme, only the control atom at hand needs the laser to drive and the virtual excitation of the atom-cavity-fiber system effectively suppresses the decoherence. By numerical simulations, appreciated parameters are chosen and it is shown that the Fredkin gate can be implemented with high fidelity. Although the operation time error has slightly stronger influence on the fidelity than atom-cavity coupling strength error, the robustness of the scheme can be effectively improved against the operation time error by adopting Gaussian pulse to replace the constant pulse. In addition, the scheme can be generalized to implement alternative Fredkin gates by controlling the non-local state-swap of two remote atoms or of two remote and spatially separated atoms, which will be undoubtedly of benefit to the distributed quantum computation and remote quantum information processing.     

Speed Geometric Quantum Logical Gate Based on Double-Hamiltonian Evolution under Large-Detuning Cavity QED Model

We introduce the double-Hamiltonian evolution technique approach toinvestigate the unconventional geometric quantum logical gate with dissipation under the model of many identical three-level atoms in a cavity, driven by a classical field. Our concrete calculation is made for the case of two atoms for the large-detuning interaction of the atoms with the cavity mode. The main advantage of our scheme is of eliminating the photon flutuation in the cavity mode during the gating. The corresponding analytical results will be helpful for experimental realization of speed geometric quantum logical gate in real cavities.     

利用非稳定子态容错实现密集旋转操作

Non-Clifford操作不能在量子纠错码上自然横向实现, 但可通过辅助量子态和在量子纠错码上能横向实现的Clifford操作来容错实现, 从而取得容错量子计算的通用性. 非平庸的单量子比特操作是Non-Clifford操作, 可以分解为绕z轴和绕x轴非平庸旋转操作的组合. 本文首先介绍了利用非稳定子态容错实现绕z轴和绕x轴旋转的操作, 进而设计线路利用魔幻态容错制备非稳定子态集, 最后讨论了运用制备的非稳定子态集模拟任意非平庸单量子比特操作的问题. 与之前工作相比, 制备非稳定子态的线路得到简化, 成功概率提高, 且在高精度模拟任意单量子比特操作时所消耗的非稳定子态数目减少了50%. 关键词： 容错量子计算 非稳定子态 魔幻态 Clifford操作