Decoherence in crystals of quantum molecular magnets

Quantum decoherence is a central concept in physics. Applications such as quantum information processing depend on understanding it; there are even fundamental theories proposed that go beyond quantum mechanics, in which the breakdown of quantum theory would appear as an 'intrinsic' decoherence, mimicking the more familiar environmental decoherence processes. Such applications cannot be optimized, and such theories cannot be tested, until we have a firm handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate and testable predictions for environmental decoherence in molecular-based quantum magnets. Experiments on molecular magnets have successfully demonstrated quantum-coherent phenomena but the decoherence processes that ultimately limit such behaviour were not well constrained. For molecular magnets, theory predicts three principal contributions to environmental decoherence: from phonons, from nuclear spins and from intermolecular dipolar interactions. We use high magnetic fields on single crystals of Fe(8) molecular magnets (in which the Fe ions are surrounded by organic ligands) to suppress dipolar and nuclear-spin decoherence. In these high-field experiments, we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally, and there are no other visible decoherence sources. In these high fields, we obtain a maximum decoherence quality-factor of 1.49?×?10(6); our investigation suggests that the environmental decoherence time can be extended up to about 500 microseconds, with a decoherence quality factor of ～6?×?10(7), by optimizing the temperature, magnetic field and nuclear isotopic concentrations.     

Counterfactual quantum computation through quantum interrogation

The logic underlying the coherent nature of quantum information processing often deviates from intuitive reasoning, leading to surprising effects. Counterfactual computation constitutes a striking example: the potential outcome of a quantum computation can be inferred, even if the computer is not run. Relying on similar arguments to interaction-free measurements (or quantum interrogation), counterfactual computation is accomplished by putting the computer in a superposition of 'running' and 'not running' states, and then interfering the two histories. Conditional on the as-yet-unknown outcome of the computation, it is sometimes possible to counterfactually infer information about the solution. Here we demonstrate counterfactual computation, implementing Grover's search algorithm with an all-optical approach. It was believed that the overall probability of such counterfactual inference is intrinsically limited, so that it could not perform better on average than random guesses. However, using a novel 'chained' version of the quantum Zeno effect, we show how to boost the counterfactual inference probability to unity, thereby beating the random guessing limit. Our methods are general and apply to any physical system, as illustrated by a discussion of trapped-ion systems. Finally, we briefly show that, in certain circumstances, counterfactual computation can eliminate errors induced by decoherence.     

超低渗透油层温度-应力-渗流的流固耦合效应

目的在于研究由于注入流体的温度过低而引起超低渗透油层渗透率和孔隙结构变化的温度-应力-渗流的流固耦合效应.实验选用了长庆超低渗油藏的岩心,在恒压和各种不同温度下测定了岩心的渗透率.实验结果表明:①随着温度的降低,岩心的渗透率不断降低,大约2/3的超低渗岩心在25℃左右时会出现裂缝直至断裂.在25~15℃温度段内,流体通过超低渗透多孔介质的渗透率对温度最敏感,其变化程度最大(要么降低的幅度最大,要么出现裂缝),且这种变化是部分不可逆的.②随着温度的升高,岩心的渗透率稍有增加,当温度再开始下降时,渗透率下降,温度下降到原始地层温度时渗透率的值比初始值要大,但增加的幅度不大.因此,注水开发低渗透和超低渗油田时应尽量避免冬季投产和施工作业.在正常的开发状态下应在合理的生产压差下,合理选择或保持适当的注水温度,尽量降低冷伤害对开发效果的影响,保持油藏的稳产和高产.     

Strong coupling of a single photon to a superconducting qubit using circuit quantum electrodynamics

The interaction of matter and light is one of the fundamental processes occurring in nature, and its most elementary form is realized when a single atom interacts with a single photon. Reaching this regime has been a major focus of research in atomic physics and quantum optics for several decades and has generated the field of cavity quantum electrodynamics. Here we perform an experiment in which a superconducting two-level system, playing the role of an artificial atom, is coupled to an on-chip cavity consisting of a superconducting transmission line resonator. We show that the strong coupling regime can be attained in a solid-state system, and we experimentally observe the coherent interaction of a superconducting two-level system with a single microwave photon. The concept of circuit quantum electrodynamics opens many new possibilities for studying the strong interaction of light and matter. This system can also be exploited for quantum information processing and quantum communication and may lead to new approaches for single photon generation and detection.     

Strong coupling in a single quantum dot-semiconductor microcavity system

Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140 microeV.     

天然气水合物储藏降压开采产量流固耦合模拟

针对当前天然气水合物储藏降压开采产能模型局限于水合物分解引起的储集层孔隙度及渗透率变化、忽视流固耦合作用等问题,提出了考虑"水合物分解效应"、流固耦合作用以及储集层应力敏感性,建立了水合物储藏气、水两相非等温流固耦合数学模型并进行程序开发.以墨西哥湾某水合物储藏为例,进行了水合物储藏降压开采产能模拟,剖析了流固耦合作用对水合物储藏开采动态的影响机制.结果表明,流固耦合作用引起的岩石孔隙收缩虽有助于提高储集层弹性驱动能,但岩石孔隙收缩导致储集层孔渗能力降低的影响居主导地位,故流固耦合的总体效果导致水合物储藏产气速率以及累积产气量等生产指标较不考虑流固耦合时偏低.     

自旋相干叠加态的反聚束效应

两个不同的自旋相干态的叠加态具有非经典的特性,反聚束效应是这类叠加态的非经典效应之一.通过研究实、虚两个自旋相干态的叠加态的反聚束效应,发现粒子数目和自旋相干态的参数|η|共同决定了这一类叠加态具有反聚束效应的量子态的数量.     

自旋相干叠加态的反聚束效应

两个不同的自旋相干态的叠加态具有非经典的特性,反聚束效应是这类叠加态的非经典效应之一。通过研究实、虚两个自旋相干态的叠加态的反聚束效应,发现粒子数目和自旋相干态的参数|η|共同决定了这一类叠加态具有反聚束效应的量子态的数量。     

内嵌富勒烯量子位的消相干研究

利用马尔可夫近似方法计算了X@C60（X=^14N,^15N,^31P）量子位中电子自旋在核子的精细作用和外磁场作用下的密度算符演化行为,得出量子位驰豫时间在10～100ns范围,与其它文献相符．     

双模交叉克尔型耦合系统中宏观量子叠加态的制备

利用双模交叉克尔型耦合光力系统中的光机耦合来制备宏观量子叠加态.考虑二能级原子与单模光腔的强耦合相互作用,并引入单模光腔与单模机械振子间双模交叉克尔型耦合.通过分析发现交叉克尔效应可以放大机械振子的位移,并且当耦合系数趋近于机械振子的振动频率时,可以制备出机械振子的可区分宏观量子叠加态.     

双量子阱中有自旋轨道耦合的场助电子共振隧穿

研究在自旋轨道耦合和周期振动场的作用下,电子隧穿双量子阱结构的透射系数和自旋极化率．通过数值计算发现：隧穿后电子的自旋简并消除,得到与自旋相关的共振峰．电子隧穿宽势阱时出现对称的Breit-Wigner共振峰,而隧穿窄势阱时出现不对称的Fano共振峰．研究也发现通过调节入射能量和中间势垒的宽度,可以改变共振峰的振幅和位置．利用这个原理可以设计可调的自旋过滤器,实现对自旋的调控．     

复杂结构井开发低渗油藏的效果及关键问题研究

复杂结构井具有增大泄油面积、有效连通储层裂缝、提高储层导流能力的优势.通过对国内外低渗油藏开发现状的调研,对比分析了两种复杂结构井(压裂水平井和多分支水平井)开发低渗油藏的效果;研究了地应力与裂缝参数的关系.结果表明开发低渗油藏多分支水平井比压裂水平井有更多的优势,且两种复杂结构井在设计水平井眼走向时都要考虑与地应力的关系.     

Hybridization of electronic states in quantum dots through photon emission

The self-assembly of semiconductor quantum dots has opened up new opportunities in photonics. Quantum dots are usually described as 'artificial atoms', because electron and hole confinement gives rise to discrete energy levels. This picture can be justified from the shell structure observed as a quantum dot is filled either with excitons (bound electron-hole pairs) or with electrons. The discrete energy levels have been most spectacularly exploited in single photon sources that use a single quantum dot as emitter. At low temperatures, the artificial atom picture is strengthened by the long coherence times of excitons in quantum dots, motivating the application of quantum dots in quantum optics and quantum information processing. In this context, excitons in quantum dots have already been manipulated coherently. We show here that quantum dots can also possess electronic states that go far beyond the artificial atom model. These states are a coherent hybridization of localized quantum dot states and extended continuum states: they have no analogue in atomic physics. The states are generated by the emission of a photon from a quantum dot. We show how a new version of the Anderson model that describes interactions between localized and extended states can account for the observed hybridization.     

退相干和量子力学的诠释

近二十年来,针对量子力学围绕其概念基础产生的哲学争议,产生了不同于“哥本哈根诠释”和“正统”解释的可供选择的解诠方案。在解决测量问题上,退相干单独不能解决测量问题,但是退相干把环境相互作用包括进来不仅提出了新的观念问题,而且暗示了基本问题的解决方案。     

第Ⅰ类三态叠加多模叠加态光场高次和压缩特性

构造了由多模相干态|{Zj}〉q、多模真空态|{0j}〉q和多模相干态的相反态|{-Zj}〉q线性叠加组成的第Ⅰ类三态叠加多模叠加态光场|ψ(3)1〉q,利用多模压缩态理论,研究了态|ψ(3)1〉q中广义电场分量的等幂次高次和压缩特性.结果表明,在一定条件下,态|ψ(3)1〉q的广义电场分量(即第二正交相位分量)可分别呈现出周期性变化的等幂次奇数模与偶数次、偶数模与奇数次、偶数模与偶数次、奇数模与奇数次的高次和压缩效应.     

Dynamical coupling of wind and ocean waves through wave-induced air flow

Understanding the physical mechanisms behind the generation of ocean waves by wind has been a longstanding challenge. Previous studies have assumed that ocean waves induce fluctuations in velocity and pressure of the overlying air that are synchronized with the waves, and numerical models have supported this assumption. In a complex feedback, these fluctuations provide the energy for wave generation. The spatial and temporal structure of the wave-induced airflow therefore holds the key to the physics of wind-wave coupling, but detailed observations have proved difficult. Here we present an analysis of wind velocities and ocean surface elevations observed over the open ocean. We use a linear filter to identify the wave-induced air flow from the measurements and find that its structure is in agreement with 'critical-layer' theory. Considering that the wave-induced momentum flux is then controlled by the wave spectrum and that it varies considerably in vertical direction, a simple parameterization of the total air-sea momentum flux is unlikely to exist.     

Applying Gaussian quantum discord to quantum key distribution

In this paper, we theoretically prove that the Gaussian quantum discord state of optical field can be used to complete continuous variable （CV） quantum key distri- bution （QKD）. The calculation shows that secret key can be distilled with a Gaussian quantum discord state against entangling cloner attack. Secret key rate is increased with the increasing of quantum discord for CV QKD with the Gaussian quantum discord state. Although the calculated results point out that secret key rate using the Gaussian quantum discord state is lower than that using squeezed state and coherent state at the same energy level, we demonstrate that the Gaussian quantum discord, which only involving quantum correlation without the existence of entanglement, may provide a new resource for realizing CV QKD.     

均质储层球向渗流模型及其Laplace空间解

针对均质储层中球面径向渗流模型,利用Laplace变换,获得了在考虑井筒储集和表皮系数的井底条件和各种外边界(定压、封闭、无穷大)条件下的无量纲储层压力和井底压力的Laplace空间解,并剖析了解的相似结构和相似核函数的构成和固有特征.相似核函数仅与渗流基本微分方程的2个线性无关的解及外边界条件有关,而与内边界条件无关...