中性原子量子计算研究进展

相互作用可控、相干时间较长的中性单原子体系具备在1 mm2的面积上提供成千上万个量子比特的规模化集成的优势,是进行量子模拟、实现量子计算的有力候选者.近几年中性单原子体系在实验上取得了快速的发展,完成了包括50个单原子的确定性装载、二维和三维阵列中单个原子的寻址和操控、量子比特相干时间的延长、基于里德伯态的两比特量子门的实现和原子态的高效读出等,这些工作极大地推动了该体系在量子模拟和量子计算方面的应用.本文综述了该体系在量子计算方面的研究进展,并介绍了我们在其中所做的两个贡献:一是实现了"魔幻强度光阱",克服了光阱中原子退相干的首要因素,将原子相干时间提高了百倍,使得相干时间与比特操作时间的比值高达105;二是利用异核原子共振频率的差异建立了低串扰的异核单原子体系,并利用里德伯阻塞效应首次实现了异核两原子的量子受控非门和量子纠缠,将量子计算的实验研究拓展至异核领域.最后,分析了中性单原子体系在量子模拟和量子计算方面进一步发展面临的挑战与瓶颈.     

20超导量子比特薛定谔猫态制备

<正>最近几年,量子计算、量子模拟与量子信息研究发展迅速,人们特别关注量子计算平台的可扩展性,即如何实现更多量子比特的相干操控。超导量子计算平台可集成多个量子比特,相干时间长,操控和读出精度高,是实用化、可扩展量子计算主要技术路线之一。衡量量子计算平台可     

超导量子比特中的量子跳跃方法

量子力学自建立以来很长一段时间被当作是描述系综运动的理论。但是上世纪80年代对囚禁离子和原子的研究观测到了量子跳跃现象,迫使人们发展了一些能够用于单量子系统的方法,其中之一就是量子跳跃方法。本文总结了我们近来把量子跳跃方法应用到以约瑟夫森隧道结为基础的超导量子比特的工作。我们不但实验观测到宏观量子系统中的量子跳跃,还可以利用量子跳跃现象来研究固体中普遍存在的两能级系统及其引起的超导量子比特中的退相干。     

离子阱中量子逻辑门的实现

在实际建造量子计算机时,需要寻找合适的量子系统,实现量子逻辑门,并同时要克服消相干问题.人们提出了许多物理系统,比如:腔QED模型、囚禁离子系统、核磁共振系统、量子点等等.     

离子阱量子计算规模化的研究进展

离子阱系统是当前实现量子计算最为领先的物理系统之一,已经在数十量子比特的规模下实现了保真度达到容错量子计算阈值的量子态制备、测量、通用量子逻辑门等基本量子操作.未来离子阱量子计算的一个重要研究方向,是在保持量子比特高性能的同时,进一步扩展量子比特的数量,最终达到解决实际问题所需的规模.本文介绍当前离子阱量子计算研究中主流的规模化方案,如离子输运方案和离子-光子量子网络方案等,以及各方案中存在的限制因素,进而探讨如二维离子阵列、双重量子比特等新的规模化方案及其前景.     

基于超导量子比特的电磁感应透明研究进展

超导量子计算是目前被认为最有希望实现量子计算机的方案之一. 超导量子比特是超导量子计算的核心部件. 如何尽可能的增加超导量子比特的退相干时间, 大规模的集成超导量子比特已成为超导量子计算研究的主要方向. 超导量子比特作为宏观的人工原子, 有许多量子光学现象都能够在其中观测到. 利用超导量子比特实现电磁感应透明为研究超导量子比特的退相干机理提供了新手段, 为研究非线性光学、光存储、光的超慢速传输等量子光学效应开辟了新思路. 本文介绍了电磁感应透明的理论基础, 总结了目前针对超导量子比特的电磁感应透明研究进展, 对比了一般气体原子与超导量子比特的电磁感应透明区别, 并对超导量子比特实现电磁感应透明的潜在应用进行了总结和展望.     

三维传输子量子比特的退相干参数表征

超导量子比特的退相干时间是决定超导量子计算能否实现的重要指标之一. 文章以三维传输子量子比特(3D transmon)为研究对象, 在氧化硅衬底上制备了三维传输子量子比特, 并在超低温下(10 mK), 采用拉比振荡(Rabi oscillation)、能量弛豫(energy relaxation)、 拉姆齐条纹(Ramsey fringe)、自旋回波(spin echo)的方法, 对其进行了详细的退相干时间常数表征. 结果显示该量子比特的退相干时间在几百纳秒. 根据几种退相干时间的关系进行计算, 可以看出, 低频噪声目前不是影响量子比特退相干的最主要因素, 而氧化硅中的缺陷可能是样品退相干时间的主要瓶颈. 关键词： 三维传输子量子比特 拉比振荡 拉姆齐条纹 自旋回波     

级联动力学退耦合与无消相干子空间实现量子计算

量子计算与量子信息过程中一个最主要的障碍量子消相干问题.克服量子消相干的策略,一般可以分为两大类方式:量子编码和动力学控制.近年来,无消相干子空间的量子编码以其能够有效的避免消相干过程得到了人们的广泛关注,在理论和实验上都取得了较大的进展.另外,利用强快脉冲实现量子系统与环境的动力学退耦合也可以有效的压缩消相干过程,动力学退耦合的噪声控制形式也得到了广泛研究.本文利用强快脉冲实现量子比特与环境的部分退耦合,然后用两个量子比特编码到无消相干子空间,从而可以克服系统与环境的一般耦合带来的消相干.另外,我们考虑了量子比特之间的相互作用,两个量子比特的耦合会导致量子信息在量子比特之间的非控制流动,使量子计算失败.我们利用对两个量子比特实行非同步的脉冲控制,使编码的量子态在比特间固有相互作用影响下保持稳定.我们给出了一维比特串结构的量子计算机模型,每个计算单元内包括三个逻辑比特,每个逻辑比特采用不同的脉冲控制保护量子比特态的稳定.通过调节控制脉冲,有顺序的"打开"不同的相互作用,可以得到普适的逻辑门组,从而实现了可扩展、容错的普适量子计算机.     

囚禁离子非线性Jaynes-Cummings模型量子场熵演化特性

利用VonNeuuman量子约化熵理论研究了驻波激光场与囚禁在谐振势中的离子单量子共振相互作用系统中量子场熵的时间演化特性,通过数值计算详细讨论了Lamb-Dick参数、离子质心在驻波激光场中的位置以及囚禁离子初始状态对量子场熵演化特性的影响.结果表明:Lamb-Dick参数影响囚禁离子与驻波激光场之间量子纠缠的频率和幅度,其值越大离子与光场之间的平均纠缠程度越低;随着离子质心从驻波激光场的波节向波腹移动,二者之间量子纠缠的振荡频率逐渐变慢,纠缠强度逐渐减弱;随着囚禁离子处于激发态概率的减小,离子与光场之间的量子纠缠呈现先增强后减弱的变化趋势.这些特性对于纠缠态的制备以及利用囚禁离子进行量子通讯等信息处理过程有一定的参考价值.     

LA声子对激子qubit纯退相干的影响

在一个抛物量子点中,以激子的真空态和基态作为量子比特(qubit),采用求密度矩阵元的方法,计算了由形变势下声学声子引发的激子量子比特纯退相干.找到了激子量子比特纯退相干因子对时间、温度和量子点受限长度的依赖关系.研究发现,激子量子比特的退相干因子在2.5ps的时间范围内随时间的增加而迅速增加,其纯退相干时间在ps量级;在温度即使为绝对温度0K时由LA声子引发的退相干依然存在,在温度大于3K后退相干因子随温度的增大而开始迅速增大;并同时发现量子点受限长度对退相干因子有重要影响,激子越受限退相干越快.研究结果表明,对激子量子比特使用适当大小量子点,且保持环境低温,并采用低能超快光学操作可以有效地抑制声子对激子量子比特纯退相干的影响. 关键词： 量子点 量子信息 量子比特     

Progress of quantum entanglement in a trapped-ion based quantum computer

In recent years, there have been significant progress toward building a practical quantum computer, demonstrating key ingredients such as single-qubit gates and a two-qubit entangling gate. Among various physical platforms for a potential quantum computing processor, a trapped-ion system has been one of the most promising platforms due to long coherence times, high-fidelity quantum gates, and qubit connectivity. However, scaling up the number of qubits for a practical quantum computing faces a core challenge in operating high-fidelity quantum gates under influence from neighboring qubits. In particular, for the trapped-ion system, unwanted quantum crosstalk between qubits and ions’ quantum motional states hinder performing high-fidelity entanglement as the number of ions increases. In this review, we introduce a trapped-ion system and explain how to perform single-qubit gates and a two-qubit entanglement. Moreover, we mainly address theoretical and experimental approaches to achieve high-fidelity and scalable entanglement toward a trapped-ion based quantum computer.     

Evaluation of the frequency instability limited by Dick effect in the microwave ~(199)Hg~+ trapped-ion clock

In the microwave ~(199)Hg~+ trapped-ion clock, the frequency instability degradation caused by the Dick effect is unavoidable because of the periodical interrogating field. In this paper, the general expression of the sensitivity function g(t)to the frequency fluctuation of the interrogating field with Nπ-pulse(N is odd) is derived. According to the measured phase noise of the 40.5-GHz microwave synthesizer, the Dick-effect limited Allan deviation of our ~(199)Hg~+ trapped-ion clock is worked out. The results indicate that the limited Allan deviations are about 1.75 × 10~(-13)τ~(1/2) and 3.03 × 10~(-13)τ~(1/2) respectively in the linear ion trap and in the two-segment extended linear ion trap under our present experimental parameters.     

Visible quantum cutting via downconversion in LiGdF4 : Er, Tb upon Er 4f→4f5d excitation

Visible quantum cutting (the emission of two photons of visible light per absorbed photon) is reported for LiGdF₄ codoped with Er³⁺ and Tb³⁺. In this compound visible quantum cutting is achieved through downconversion, i.e., by making use of energy transfer between different lanthanide ions. Upon VUV excitation in the 4f¹⁰5d levels of Er³⁺, part of the excitation energy can be transferred to Gd³⁺ by cross-relaxation, yielding both Er³⁺ and Gd³⁺ in an excited state. This is followed by the emission of two visible photons: one from Er³⁺ due to the ⁴S_3/2→⁴I_15/2 transition, and, after energy transfer from Gd³⁺ to Tb³⁺, one from Tb³⁺ due to the ⁵D_J→⁷F_J transition. It is shown that this process occurs, next to other energy transfer processes from the Er³⁺ 4f¹⁰5d states to Gd³⁺ yielding only one visible photon.     

用于GaAs太阳能电池的NaYF4中Tb3+-Er3+耦合对的光谱转换

观测到一种以Tb³⁺-Er³⁺进行光谱转换的量子剪裁现象。一个高能紫外光子(Tb³⁺的⁷F₆→⁵L₁)被量子剪裁成两个低能光子:一个是近红外光子(Er³⁺的⁴I_9/2→⁴I_15/2),另一个是蓝色光子(Tb³⁺的⁵D₄→⁷F₆),它们两个 都可以被GaAs太阳能电池有效地吸收。量子剪裁效率高达188%,接近理论极限的200%。从Tb³⁺(⁵L₁→ ⁵D₄) 到Er³⁺(⁴I_15/2→⁴I_9/2)的能量传递的能量失配是237 cm^-1,比NaYF₄中的声子能400 cm^-1小,能量传递是近共振的。Tb³⁺施主间的能量迁移可以近似地用扩散模型处理, 从Tb³⁺-Er³⁺对之间能量传递的初始过程发现,偶极-偶极相互作用占主导地位。     

高准确度的钙离子光频标

近年来,冷原子技术和激光技术促进了高精度光频标的发展,有望在建立时间基准、推动基础研究和满足国家需求等方面发挥重要的作用.本文介绍了中国科学院武汉物理与数学研究所近年来在高准确度钙离子(~(40)Ca~+)光频标研究方面的进展:采用新的ULE腔系统,实现了729 nm钟跃迁激光器1—100 s的频率稳定度均优于2×10~(-15),通过对外场和环境效应的控制及克服,特别是囚禁离子运动效应的抑制,获得单个钙离子光频标的不确定度优于5.5×10~(-17);通过两台光频标的比对,测得20000 s的稳定度也进入10~(-17)量级;基于高精度钙离子光频标平台,进行了相关精密测量的工作,包括:基于全球定位系统的超高精度远程光频绝对值测量方案,第二次测量了钙离子的光频跃迁绝对值,该测量结果再次被国际时间频率咨询委员会采纳,更新了钙离子的频率推荐值;精确测量了钙离子的钟跃迁魔幻波长,由此提出新型的全光囚禁离子光频标的方法;精密测量了钙离子的亚稳态寿命等参数.以上工作推动了基于冷原子的精密测量工作.     

Λc重子谱疑难(英文)

Λ_c+重子家族存在一个疑难,也就是重夸克有效理论预言的轨道角动量为L=2的多重态中JP=3/2+的态在实验上是缺失的,而且已发现的Λ_c（2765）+和Λ_c（2940）+的JP量子数仍然未知。在本工作中,我们使用双夸克图像计算了重子的质量,研究了Λ_c（2765）+和Λ_c（2940）+的可能归类以及仍缺失的具有L=2和JP=3/2+量子数的Λ_c+态,进而指出Λ_c（2880）+的峰结构中实际上已经包含了L=2、JP=3/2+这一缺失态,并建议Λ_c（2765）+和Λ_c（2940）+的量子数应分别为2S（1/2+）和2P（1/2-）。There is a puzzle in the Λ_c+ family, i.e., one member with JP=3/2+ is missing in a L=2 multiplet which the heavy quark effective theory predicts, and JP's of Λ_c(2765)+ and Λ_c(2940)+ are unknown. Using a light diquark picture to calculate baryon masses, we study possible assignments of two Λ_c's with unknown JP and the missing Λ_c+ with 3/2+ for L=2, and we find the most probable possibility that the peak corresponding to Λ_c(2880)+ actually includes a missing member with spin 3/2+ for L=2 and that quantum numbers of Λ_c(2765)+ and Λ_c(2940)+ are 2S(1/2+) and 2P (1/2-), respectively.     

在NaGdF4：Tm3+, Dy3+中Gd3+间的能量迁移及Gd3+-Dy3+能量传递量子效率

基于量子剪裁基本原理,通过光谱技术研究NaGdF₄:Tm³⁺,Dy³⁺在一个真空紫外光子激发下获得两个蓝色光子的可能性。在这种化合物中,量子剪裁通过下转换,即通过应用不同镧系离子间的能量传递进行。通过对Tm 4f¹²-4f¹¹5d激发,部分能量从Tm³⁺离子5d态直接传递给Gd³⁺,然后在Gd³⁺-Tm³⁺之间发生交叉弛豫,剩余能量从Gd³⁺传递给Dy³⁺,产生两个可见光子发射,一个来自Tm³⁺的¹G₄-³H₆跃迁,另一个来自Dy³⁺的⁴F_9/2-⁶H_15/2跃迁。主要研究获得以NaGdF₄:Tm³⁺,Dy³⁺为基础的新型具有更高效率,更高稳定性和更强真空紫外(VUV)吸收量子剪裁发光粉的可能性。各种光谱技术,如光致发光、激发和衰减等被用来表征不同Dy³⁺浓度掺杂NaGdF₄中Gd³⁺晶格间能量迁移引起的施主Gd³⁺和受主Dy³⁺之间的能量传递。结果表明Gd³⁺离子之间存在能量迁移,随之交换相互作用引起施主与受主(Gd³⁺-Dy³⁺)之间的能量传递。通过Bursh-tein等人关于激发态的弛豫理论,施主-受主能量传递参数k_DS可以从Gd³⁺的⁶P_7/2发射的衰减计算出。Gd³⁺-Dy³⁺能量传递量子效率也可以得到。NaGdF₄:Tm³⁺和NaGdF₄:Tm³⁺,Dy³⁺是由水热法制备的,NaGdF₄:Dy³⁺是由文献[4]方法制备的。发射光谱和激发光谱通过自制的VUV光谱仪和F-4500测量。衰减曲线由OPO激光器激发获得Gd³⁺-Dy³⁺之间能量传递量子效率在受主浓度大约在N_A=0.6%时达到最佳值,并且明显地观测到浓度猝灭效应。     

Numerical analysis of motional mode coupling of sympathetically cooled two-ion crystals

A two-ion pair in a linear Paul trap is extensively used in the research of the simplest quantum-logic system; however,there are few quantitative and comprehensive studies on the motional mode coupling of two-ion systems yet. This study proposes a method to investigate the motional mode coupling of sympathetically cooled two-ion crystals by quantifying three-dimensional(3 D) secular spectra of trapped ions using molecular dynamics simulations. The 3 D resonance peaks of the ~(40)Ca~+ – ~(27)Al~+ pair obtained by using this method were in good agreement with the 3 D in-and out-of-phase modes predicted by the mode coupling theory for two ions in equilibrium and the frequency matching errors were lower than 2%.The obtained and predicted amplitudes of these modes were also qualitatively similar. It was observed that the strength of the sympathetic interaction of the ~(40)Ca~+ – ~(27)Al~+ pair was primarily determined by its axial in-phase coupling. In addition,the frequencies and amplitudes of the ion pair's resonance modes(in all dimensions) were sensitive to the relative masses of the ion pair, and a decrease in the mass mismatch enhanced the sympathetic cooling rates. The sympathetic interactions of the ~(40)Ca~+ – ~(27)Al~+ pair were slightly weaker than those of the ~(24)Mg~+– ~(27)Al~+ pair, but significantly stronger than those of ~9Be~+ – ~(27)Al~+ . However, the Doppler cooling limit temperature of ~(40)Ca~+ is comparable to that of ~9Be~+ but lower than approximately half of that of ~(24)Mg~+. Furthermore, laser cooling systems for ~(40)Ca~+ are more reliable than those for ~(24)Mg~+and ~9Be~+ . Therefore, ~(40)Ca~+ is probably the best laser-cooled ion for sympathetic cooling and quantum-logic operations of ~(27)Al~+ and has particularly more notable comprehensive advantages in the development of high reliability, compact, and transportable ~(27)Al~+ optical clocks. This methodology may be extended to multi-ion systems, and it will greatly aid efforts to control the dynamic behaviors of sympathetic cooling as well as the development of low-heating-rate quantum logic clocks.     

Fast holonomic quantum computation based on solid-state spins with all-optical control

Holonomic quantum computation is a quantum computation strategy that promises some built-in noise-resilience features. Here, we propose a scheme for nonadiabatic holonomic quantum computation with nitrogen-vacancy center electron spins, which are characterized by fast quantum gates and long qubit coherence times. By varying the detuning, amplitudes, and phase difference of lasers applied to a nitrogen-vacancy center, one can directly realize an arbitrary single-qubit holonomic gate on the spin. Meanwhile, with the help of cavity-assisted interactions, a nontrivial two-qubit holonomic quantum gate can also be induced. The distinct merit of this scheme is that all the quantum gates are obtained via an all-optical geometric manipulation of the solid-state spins. Therefore, our scheme opens the possibility for robust quantum computation using solid-state spins in an all-optical way.