A short introduction to quantum symmetry

In quantum theory, symmetries more general than groups are possible. We give a general definition of a quantum symmetry, such that symmetry operations act on the Hilbert space of physical states and notions of unitarity, invariance and covariance are defined. Within this frame, weak quasi quantum groups are described as a natural generalization of group algebras. Consistency with locality distinguishes them from more general quantum symmetries. To find the new kinds of symmetry one should investigate low dimensional quantum systems such as two-dimensional layers.     

A cross-disciplinary introduction to quantum annealing-based algorithms

A central goal in quantum computing is the development of quantum hardware and quantum algorithms in order to analyse challenging scientific and engineering problems. Research in quantum computation involves contributions from both physics and computer science; hence this article presents a concise introduction to basic concepts from both fields that are used in annealing-based quantum computation, an alternative to the more familiar quantum gate model. We introduce some concepts from computer science required to define difficult computational problems and to realise the potential relevance of quantum algorithms to find novel solutions to those problems. We introduce the structure of quantum annealing-based algorithms as well as two examples of this kind of algorithms for solving instances of the max-SAT and Minimum Multicut problems. An overview of the quantum annealing systems manufactured by D-Wave Systems is also presented.     

Non-markovian continuous quantum measurement of retarded observables

We reconsider the non-Markovian time-continuous measurement of a Heisenberg observable x[over ] and show for the first time that it can be realized by an infinite set of entangled von Neumann detectors. The concept of continuous readout is introduced and used to rederive the non-Markovian stochastic Schr?dinger equation. We can prove that, contrary to recent doubts, the resulting non-Markovian quantum trajectories are true single system trajectories and correspond to the continuous measurement of a retarded functional of x[over ].     

量子可积系统和量子反散射方法简介(续)

4 .3　ＸＸＸ模型的本征值问题 ,Ｂｅｔｈｅ假设方程ｔ(ｕ)的本征值问题实际上也是Ｔ(ｕ)的本征值问题 ,它涉及到Ｔ(ｕ)的对角化问题 ,也就是说 ,对辅助空间选什么样的基矢可使Ｔ(ｕ)的表示是对角的 .1 )ＴＮ(ｕ)的矩阵元之间的对易关系在关于谐振子问题的描述中 ,我们已看到算符之间的对易关系的作用 .现在我们需要的是Ｔ(ｕ)的矩阵元之间的对易关系 ,实际上它们已经包含在ＹＢ关系中了 .前面已经指出 ,Ｔ(ｕ)相对于辅助空间为矩阵 ,这涉及到Ｖ的基矢的问题 .在Ｖ中选取自然基 :|ｅ+ 〉 =10 ,|ｅ- 〉 =01 ( 4 0 )则Ｖ Ｖ =Ｃ2 Ｃ2 中的自…     

Generation of spin squeezing via continuous quantum nondemolition measurement

Continuous quantum nondemolition monitoring of a collective atomic spin with an off-resonant laser beam has been performed. Squeezed atomic spin states have thereby been produced with spin noise reduction to 70% below the standard quantum limit expected for a coherent spin state.     

A new and straightforward synthesis route for preparing Cds quantum Dots

In this work, highly stable, soluble and luminescent CdS quantum dots (QDots) with a narrow size distribution were synthesized in ethylene glycol using the polyol process and the solvothermal technique. In this case instead of using a conventional highly toxic sulfur source like H₂S, we use elemental sulfur dissolved in ethylene glycol to perform the reaction. When the solvent reaches its boiling point inside the autoclave, sulfur is reduced to S^?2 and reacts with Cd⁺² ions to form CdS nanocrystals. Analysis of the spectroscopic and TEM measurements showed that 3 nm monodispersed CdS QDots were synthesized and exhibited high photoluminescence (PL) in the blue green region of the spectra when excited with 355 nm.     

A brief introduction to the supersymmetric quantum mechanics (susy QM)

The short introduction (destinated mainly for experimentalists) to the Supersymmetric Quantum Mechanics is given.     

解说量子纠缠理论

量子纠缠态在量子信息处理,如量子隐形传态、量子密集码、量子纠错、量子保密通信、量子计算等过程中起了十分重要的作用.量子纠缠理论主要研究量子态的纠缠刻画、分类及其在量子信息处理中的应用.文章介绍了量子纠缠理论中的一些基本概念和结果,其中包括：量子力学的实在性、局域性的讨论与Bell不等式的联系,Bell不等式与量子态可分性间的关系;纯态和混合态可分性的定义及若干判别准则(包括矩阵正映照方法、部分转置判据、约化判据、重排判据、纠缠见证、协方差判据及局域测不准关系判据);部分纠缠度量的介绍(包括纠缠形成、并发度、相对熵、负度、缠结和纠缠帮助,以及纠缠度量的计算和上下界的估算).     

Number-resolved master equation approach to quantum measurement and quantum transport

In addition to the well-known Landauer–Büttiker scattering theory and the nonequilibrium Green’s function technique for mesoscopic transports, an alternative (and very useful) scheme is quantum master equation approach. In this article, we review the particle-number (n)-resolved master equation (n-ME) approach and its systematic applications in quantum measurement and quantum transport problems. The n-ME contains rich dynamical information, allowing efficient study of topics such as shot noise and full counting statistics analysis. Moreover, we also review a newly developed master equation approach (and its n-resolved version) under self-consistent Born approximation. The application potential of this new approach is critically examined via its ability to recover the exact results for noninteracting systems under arbitrary voltage and in presence of strong quantum interference, and the challenging non-equilibrium Kondo effect.     

Using quantum theory to improve measurement precision

Progress in science is inextricably linked with how well we can observe the world around us. It is by making increasingly better measurements that scientific theories are tested and refined. In this article we address the question of what the ultimate limit to measurement precision is and how it could be achieved in the laboratory. We focus on how, by making use of quantum theory, it is possible to make better measurements than anything that can be achieved with classical techniques. This opens the door to an array of new technologies and could help answer some of science's most engaging questions.     

Experimental approaches to the quantum measurement paradox

I examine the question of how far experiments that look for the effects of superposition of macroscopically distinct states are relevant to the classic measurement paradox of quantum mechanics. Existing experiments on superconducting devices confirm the predictions of the quantum formalism extrapolated to the macroscopic level, and to that extent provide strong circumstantial evidence for its validity at this level, but do not directly test the principle of superposition of macrostates. A more ambitious experiment, not obviously infeasible with current technology, could provide a direct test between quantum mechanics and a whole class of theories embodying the postulate of realism at the macroscopic level.     

Generalized quantum measurement

We overcome one of Bell's objections to ‘quantum measurement’ by generalizing the definition to include systems outside the laboratory. According to this definition, a generalized quantum measurement takes place when the value of a classical variable is influenced significantly by an earlier state of a quantum system. A generalized quantum measurement can then take place in equilibrium systems, provided the classical motion is chaotic. This Letter deals with this classical aspect of quantum measurement, assuming that the Heisenberg cut between the quantum dynamics and the classical dynamics is made at a very small scale. For simplicity, a gas with collisions is modelled by an “Arnold gas”.     

An introduction to quantum information processing: applications and realizations

The ideas behind Quantum Information Processing (QIP) began to appear about twenty years ago and theoretical developments have been appearing ever since. Over the last decade quantum experiments have begun to realize QIP and point the way towards the routes for future quantum information technology. This review is a simple introduction to QIP for non-experts. It discusses the basic ideas of and the underpinning quantum theory needed to understand them, as well as various applications and algorithms, and some of the main candidate routes for realizing QIP.     

Boolean transfer from coherent quantum logics to quantum logics with continuous superselection rules

Quantum logics with continuous superselection rules are shown to be Booleanvalued coherent quantum logics. Since modern set theory provides a transfer principle from standard mathematics to Boolean-valued mathematics, this makes it possible to transfer automatically well-known results on coherent quantum logics to quantum logics with continuous superselection rules. Many illustrations are given.     

A model of quantum measurement in Josephson junctions

A model for the quantum measurement of the electronic current in a Josephson junction is presented and analyzed. The model is similar to a Stern-Gerlach apparatus, relying on the deflection of a spin-polarized particle beam by the magnetic field created by the Josephson current. The aim is (1) to explore, with the help of a simple model, some general ideas about the nature of the information which can be obtained by measurements upon a quantum system and (2) to find new approaches for obtaining information about the nature of the states of a macroscopic quantum system. In the case of sufficiently strong coupling between the system and the apparatus, we find that the model provides in principle a standard ideal measurement of the value of the instantaneous Josephson current. In the case of weak coupling, where the measurement is not ideal, we show that the scattering of neutrons from a junction can in principle be used to measure the average value of the Josephson current, thereby allowing an experimental distinction to be made between an eigenstate of relative phase and one of relative Cooper pair number. The possibility of the latter type of measurement suggests an experimental approach to answer a question of fundamental interest, namely whether two isolated superconductors (or superfluids) possess a definite relative phase or a definite relative number of superconducting (or super/lowing) particles.     

Linking quantum discord to entanglement in a measurement

We show that a von Neumann measurement on a part of a composite quantum system unavoidably creates distillable entanglement between the measurement apparatus and the system if the state has nonzero quantum discord. The minimal distillable entanglement is equal to the one-way information deficit. The quantum discord is shown to be equal to the minimal partial distillable entanglement that is the part of entanglement which is lost, when we ignore the subsystem which is not measured. We then show that any entanglement measure corresponds to some measure of quantum correlations. This powerful correspondence also yields necessary properties for quantum correlations. We generalize the results to multipartite measurements on a part of the system and on the total system.