Squeezing and entanglement in a Bose-Einstein condensate

Entanglement, a key feature of quantum mechanics, is a resource that allows the improvement of precision measurements beyond the conventional bound attainable by classical means. This results in the standard quantum limit, which is reached in today's best available sensors of various quantities such as time and position. Many of these sensors are interferometers in which the standard quantum limit can be overcome by using quantum-entangled states (in particular spin squeezed states) at the two input ports. Bose-Einstein condensates of ultracold atoms are considered good candidates to provide such states involving a large number of particles. Here we demonstrate spin squeezed states suitable for atomic interferometry by splitting a condensate into a few parts using a lattice potential. Site-resolved detection of the atoms allows the measurement of the atom number difference and relative phase, which are conjugate variables. The observed fluctuations imply entanglement between the particles, a resource that would allow a precision gain of 3.8 dB over the standard quantum limit for interferometric measurements.     

量子霍耳效应的半经典分析

霍耳效应的解释是以电场、磁场等基本经典概念为基础；而量子霍耳效应的本质是一种量子力学效应．本文避开量子力学高深的数学语言，以经典分析的手法对量子霍耳效应的一些基本概念及实验结果给出较园满的解释．     

Experimental non-classicality of an indivisible quantum system

In contrast to classical physics, quantum theory demands that not all properties can be simultaneously well defined; the Heisenberg uncertainty principle is a manifestation of this fact. Alternatives have been explored--notably theories relying on joint probability distributions or non-contextual hidden-variable models, in which the properties of a system are defined independently of their own measurement and any other measurements that are made. Various deep theoretical results imply that such theories are in conflict with quantum mechanics. Simpler cases demonstrating this conflict have been found and tested experimentally with pairs of quantum bits (qubits). Recently, an inequality satisfied by non-contextual hidden-variable models and violated by quantum mechanics for all states of two qubits was introduced and tested experimentally. A single three-state system (a qutrit) is the simplest system in which such a contradiction is possible; moreover, the contradiction cannot result from entanglement between subsystems, because such a three-state system is indivisible. Here we report an experiment with single photonic qutrits which provides evidence that no joint probability distribution describing the outcomes of all possible measurements--and, therefore, no non-contextual theory--can exist. Specifically, we observe a violation of the Bell-type inequality found by Klyachko, Can, Binicio?lu and Shumovsky. Our results illustrate a deep incompatibility between quantum mechanics and classical physics that cannot in any way result from entanglement.     

Wave-particle duality of C(60) molecules

Quantum superposition lies at the heart of quantum mechanics and gives rise to many of its paradoxes. Superposition of de Broglie matter waves' has been observed for massive particles such as electrons, atoms and dimers, small van der Waals clusters, and neutrons. But matter wave interferometry with larger objects has remained experimentally challenging, despite the development of powerful atom interferometric techniques for experiments in fundamental quantum mechanics, metrology and lithography. Here we report the observation of de Broglie wave interference of C(60) molecules by diffraction at a material absorption grating. This molecule is the most massive and complex object in which wave behaviour has been observed. Of particular interest is the fact that C(60) is almost a classical body, because of its many excited internal degrees of freedom and their possible couplings to the environment. Such couplings are essential for the appearance of decoherence, suggesting that interference experiments with large molecules should facilitate detailed studies of this process.     

受非线性作用的微观粒子的运动特性及其非线性量子力学理论(Ⅰ)

与线性场的情况相比，微观粒子在非线性作用下的运动特性和本性发生很大变化，这说明在线性作用和非线性场中微观粒子的性质是明显不同的．这启示我们必须建立微观粒子在非线性体系或非线性场中运动规律的新理论．为此研究了与线性量子力学描写的微观量子效应迥然不同的宏观量子效应与非线性作用下的孤立子运动的紧密关系．结合现代孤立子理论和超导与超流理论，首先提出了非线性量子力学的基本原理及在此基础上建立了系统、完整的非线性量子力学理论体系，以及由此理论得出的一些新结论．最后还论证这个理论的正确性和自洽性，理论的运用范围及它的重大意义．     

Pang′s孤立子的量子力学的特性研究

从量子理论的基本原理和公式推导出了原来由半经典半量子理论推导出的Pang’s孤立子所满足的方程,使这个描述生物能量和生物信息传输的理论,建立在可靠的量子理论基础上,并得出了与原来特性有所不同的这种孤立子的一些特性。     

二维谐振子量子运动的新探讨

二维谐振子的一子力学描述在经典极限下过渡到经典统计力学而不是经典力学，给出二维谐振子量运动的双波描述，这双波描述在经典极限下过渡到经典力学，量子力学描述是这双波描述的一种统计情况。     

Strong dipolar effects in a quantum ferrofluid

Symmetry-breaking interactions have a crucial role in many areas of physics, ranging from classical ferrofluids to superfluid (3)He and d-wave superconductivity. For superfluid quantum gases, a variety of new physical phenomena arising from the symmetry-breaking interaction between electric or magnetic dipoles are expected. Novel quantum phases in optical lattices, such as chequerboard or supersolid phases, are predicted for dipolar bosons. Dipolar interactions can also enrich considerably the physics of quantum gases with internal degrees of freedom. Arrays of dipolar particles could be used for efficient quantum information processing. Here we report the realization of a chromium Bose-Einstein condensate with strong dipolar interactions. By using a Feshbach resonance, we reduce the usual isotropic contact interaction, such that the anisotropic magnetic dipole-dipole interaction between 52Cr atoms becomes comparable in strength. This induces a change of the aspect ratio of the atom cloud; for strong dipolar interactions, the inversion of ellipticity during expansion (the usual 'smoking gun' evidence for a Bose-Einstein condensate) can be suppressed. These effects are accounted for by taking into account the dipolar interaction in the superfluid hydrodynamic equations governing the dynamics of the gas, in the same way as classical ferrofluids can be described by including dipolar terms in the classical hydrodynamic equations. Our results are a first step in the exploration of the unique properties of quantum ferrofluids.     

Quantum mirages formed by coherent projection of electronic structure

Image projection relies on classical wave mechanics and the use of natural or engineered structures such as lenses or resonant cavities. Well-known examples include the bending of light to create mirages in the atmosphere, and the focusing of sound by whispering galleries. However, the observation of analogous phenomena in condensed matter systems is a more recent development, facilitated by advances in nanofabrication. Here we report the projection of the electronic structure surrounding a magnetic Co atom to a remote location on the surface of a Cu crystal; electron partial waves scattered from the real Co atom are coherently refocused to form a spectral image or 'quantum mirage'. The focusing device is an elliptical quantum corral, assembled on the Cu surface. The corral acts as a quantum mechanical resonator, while the two-dimensional Cu surface-state electrons form the projection medium. When placed on the surface, Co atoms display a distinctive spectroscopic signature, known as the many-particle Kondo resonance, which arises from their magnetic moment. By positioning a Co atom at one focus of the ellipse, we detect a strong Kondo signature not only at the atom, but also at the empty focus. This behaviour contrasts with the usual spatially-decreasing response of an electron gas to a localized perturbation.     

Creation of a six-atom 'Schrödinger cat' state

Among the classes of highly entangled states of multiple quantum systems, the so-called 'Schr?dinger cat' states are particularly useful. Cat states are equal superpositions of two maximally different quantum states. They are a fundamental resource in fault-tolerant quantum computing and quantum communication, where they can enable protocols such as open-destination teleportation and secret sharing. They play a role in fundamental tests of quantum mechanics and enable improved signal-to-noise ratios in interferometry. Cat states are very sensitive to decoherence, and as a result their preparation is challenging and can serve as a demonstration of good quantum control. Here we report the creation of cat states of up to six atomic qubits. Each qubit's state space is defined by two hyperfine ground states of a beryllium ion; the cat state corresponds to an entangled equal superposition of all the atoms in one hyperfine state and all atoms in the other hyperfine state. In our experiments, the cat states are prepared in a three-step process, irrespective of the number of entangled atoms. Together with entangled states of a different class created in Innsbruck, this work represents the current state-of-the-art for large entangled states in any qubit system.     

Quantum information processing with atoms and photons

Quantum information processors exploit the quantum features of superposition and entanglement for applications not possible in classical devices, offering the potential for significant improvements in the communication and processing of information. Experimental realization of large-scale quantum information processors remains a long-term vision, as the required nearly pure quantum behaviour is observed only in exotic hardware such as individual laser-cooled atoms and isolated photons. But recent theoretical and experimental advances suggest that cold atoms and individual photons may lead the way towards bigger and better quantum information processors, effectively building mesoscopic versions of 'Schr?dinger's cat' from the bottom up.     

量子力学和经典力学的关系

讨论了量子力学和经典力学之间的关系,由不确定性原理推出了量子力学过渡到经典力学的宏观过渡条件.     

Generation of Fock states in a superconducting quantum circuit

Spin systems and harmonic oscillators comprise two archetypes in quantum mechanics. The spin-1/2 system, with two quantum energy levels, is essentially the most nonlinear system found in nature, whereas the harmonic oscillator represents the most linear, with an infinite number of evenly spaced quantum levels. A significant difference between these systems is that a two-level spin can be prepared in an arbitrary quantum state using classical excitations, whereas classical excitations applied to an oscillator generate a coherent state, nearly indistinguishable from a classical state. Quantum behaviour in an oscillator is most obvious in Fock states, which are states with specific numbers of energy quanta, but such states are hard to create. Here we demonstrate the controlled generation of multi-photon Fock states in a solid-state system. We use a superconducting phase qubit, which is a close approximation to a two-level spin system, coupled to a microwave resonator, which acts as a harmonic oscillator, to prepare and analyse pure Fock states with up to six photons. We contrast the Fock states with coherent states generated using classical pulses applied directly to the resonator.     

带电粒子穿越半无限均匀电磁场的双波描述

用双波量子理论描述带电粒子穿越半无限均匀电磁场的运动.在一级WKB近似条件下,所得结果类似于经典力学.     

量子和经典力学描述核磁共振机理的比较

核磁共振测井是目前发展最快的测井方法，简要介绍了核磁共振测井方法和技术发展情况．核磁共振技术是核磁共振测井的基础，由于它涉及较深的微观理论，着重从量子力学和经典电动力学两个方面介绍核磁共振机理推导过程，并进行了研究和对比，说明两者的区别和联系，以明确核磁共振测井技术研究中的一些基本概念和术语．     

光散射的量子理论概述

概述了光散射现象和几种基本的光散射类型。采用量子力学观点,把光散射看作光和物质原子的相互作用,给出了光散射的量子图像。基于量子力学理论介绍了康普顿散射、瑞利散射和拉曼散射,对它们进行了理论分析,给出了相关公式。给出了光散射的基本理论,深入讨论了这三种重要光散射现象的区别和联系,有助于了解原子或分子的相关结构。     

Experimental demonstration of quantum memory for light

The information carrier of today's communications, a weak pulse of light, is an intrinsically quantum object. As a consequence, complete information about the pulse cannot be perfectly recorded in a classical memory, even in principle. In the field of quantum information, this has led to the long-standing challenge of how to achieve a high-fidelity transfer of an independently prepared quantum state of light onto an atomic quantum state. Here we propose and experimentally demonstrate a protocol for such a quantum memory based on atomic ensembles. Recording of an externally provided quantum state of light onto the atomic quantum memory is achieved with 70 per cent fidelity, significantly higher than the limit for classical recording. Quantum storage of light is achieved in three steps: first, interaction of the input pulse and an entangling field with spin-polarized caesium atoms; second, subsequent measurement of the transmitted light; and third, feedback onto the atoms using a radio-frequency magnetic pulse conditioned on the measurement result. The density of recorded states is 33 per cent higher than the best classical recording of light onto atoms, with a quantum memory lifetime of up to 4 milliseconds.     

介观物质系统的统计性质及量子涨落

从统计物理角度分析了介观系统与宏观系统在统计属性方面的不同,如自平均效应缺失、涨落影响显著、系综均值差异等,因此其量子涨落效应会对介观自组织结构的形成和消失及其稳定性产生重要影响.本文还从有源LC回路的运动方程出发,讨论了在真空态下介观LC电路中电荷电流的量子涨落关系,得出了与不确定关系相似的结构形式.