Quantum Logic Gates and Nuclear Magnetic Resonance Pulse Sequences

There has recently been considerable interest in the use of nuclear magnetic resonance (NMR) as a technology for the implementation of small quantum computers. These computers operate by the laws of quantum mechanics, rather than classical mechanics and can be used to implement new quantum algorithms. Here we describe how NMR in principle can be used to implement all the elements required to build quantum computers, and draw comparisons between the pulse sequences involved and those of more conventional NMR experiments.     

Quantum Physics with Neutrons: From Spinor Symmetry to Kochen-Specker Phenomena

In 1974 perfect crystal interferometry has been developed and immediately afterwards the 4π-symmetry of spinor wave-functions has been verified. The new method opened a new access to the observation of intrinsic quantum phenomena. Spin-superposition, quantum state reconstruction and quantum beat effects are examples of such investigations. In this connection efforts have been made to separate and measure various dynamical and geometrical phases. Non-cyclic and non-adiabatic topological phases have been identified and their stability against various fluctuations and dissipative forces has been investigated by means of ultra-cold neutrons. An entanglement between different degrees of freedom of a single neutron system demonstrated the contextuality feature of quantum mechanics. In its continuation this yields to Kochen-Specker theorem like investigations providing a new basis for information processing and for the understanding of quantum physics in general. All investigations show the equivalence of various phase spaces and show how physical phenomena are correlated by quantum laws. Some curiosa occurred during the experiments and some epistemological aspects will be discussed as well.     

On the Conclusive Tests of Local Realism and Pseudoscalar Mesons

Many beautiful experiments have been addressed to test standard quantum mechanics against local realistic models. Even if a strong evidence favouring standard quantum mechanics is emerged, a conclusive experiment is still lacking, because of low detection efficiencies. Recently, experiments based on pseudoscalar mesons have been proposed as a way for obtaining a conclusive experiment. In this paper, we investigate if this result can effectively be obtained. Our conclusions, based on a careful analysis of the proposed set ups, are that this will not be possible due to intrinsic limitations of these kind of experiments.     

A Spatial Domain Quantum Watermarking Scheme

This paper presents a spatial domain quantum watermarking scheme. For a quantum watermarking scheme, a feasible quantum circuit is a key to achieve it. This paper gives a feasible quantum circuit for the presented scheme. In order to give the quantum circuit, a new quantum multi-control rotation gate, which can be achieved with quantum basic gates, is designed. With this quantum circuit, our scheme can arbitrarily control the embedding position of watermark images on carrier images with the aid of auxiliary qubits. Besides reversely acting the given quantum circuit, the paper gives another watermark extracting algorithm based on quantum measurements. Moreover, this paper also gives a new quantum image scrambling method and its quantum circuit. Differ from other quantum watermarking schemes, all given quantum circuits can be implemented with basic quantum gates. Moreover, the scheme is a spatial domain watermarking scheme, and is not based on any transform algorithm on quantum images. Meanwhile, it can make sure the watermark be secure even though the watermark has been found. With the given quantum circuit, this paper implements simulation experiments for the presented scheme. The experimental result shows that the scheme does well in the visual quality and the embedding capacity.     

Prequantum Classical Statistical Field Theory: Schrödinger Dynamics of Entangled Systems as a Classical Stochastic Process

The idea that quantum randomness can be reduced to randomness of classical fields (fluctuating at time and space scales which are essentially finer than scales approachable in modern quantum experiments) is rather old. Various models have been proposed, e.g., stochastic electrodynamics or the semiclassical model. Recently a new model, so called prequantum classical statistical field theory (PCSFT), was developed. By this model a “quantum system” is just a label for (so to say “prequantum”) classical random field. Quantum averages can be represented as classical field averages. Correlations between observables on subsystems of a composite system can be as well represented as classical correlations. In particular, it can be done for entangled systems. Creation of such classical field representation demystifies quantum entanglement. In this paper we show that quantum dynamics (given by Schrödinger’s equation) of entangled systems can be represented as the stochastic dynamics of classical random fields. The “effect of entanglement” is produced by classical correlations which were present at the initial moment of time, cf. views of Albert Einstein.     

Electric field control of exciton states in quantum dot molecules

Semiconductor nanostructures have attracted considerable interest during the recent years in view of the potential application in quantum information processing. In particular, quantum dots have been suggested to fulfill an essential requirement for quantum computation: controllable interaction that couples two quantum dot qubits. Previous experiments on two vertically aligned quantum dots have demonstrated the formation of coupled exciton states. We show that this coupling between two In_0.60Ga_0.40As/GaAs quantum dots can be tuned by an electric field applied along the molecule axis. This controllable coupling in such a relatively simple configuration could be implemented in a solid-state-based quantum device.     

Frequency-coded quantum key distribution

We report an intrinsically stable quantum key distribution scheme based on genuine frequency-coded quantum states. The qubits are efficiently processed without fiber interferometers by fully exploiting the nonlinear interaction occurring in electro-optic phase modulators. The system requires only integrated off-the-shelf devices and could be used with a true single-photon source. Preliminary experiments have been performed with weak laser pulses and have demonstrated the feasibility of this new setup.     

An enhanced refractive index with suppressed absorption in a graphene nanostructure under external magnetic field

The dispersive-absorptive optical properties of a weak probe field are investigated based on quantum coherence and interference in a Landau-quantized graphene structure. It is found that an enhanced refractive index with vanishing absorption can be obtained in this structure through proper adjusting the controlling parameters of the system. The switching between superluminal and subluminal light propagation is also discussed. Our scheme can be employed in real experiments to develop new types of nanoelectronic devices for realizing all-optical switching process and can have practical application in dispersion compensation and solid-state quantum communication.     

多量子比特核磁共振体系的实验操控技术

随着量子信息与量子计算科学的发展,量子信息处理器被广泛地用于量子计算、量子模拟、量子度量等方面的研究.为了能在实验上实现这些日益复杂的方案,将量子计算机的潜能转化成现实,需要不断提高可操控的量子体系比特位数,实现更复杂的量子操控.核磁共振自旋体系作为一个优秀的量子实验测试平台,提供了丰富而又精密的量子操控手段.近几年来在此平台上进行了不少的多量子比特实验,发展并积累了一系列的多量子比特实验技术.本文首先阐述了核磁共振体系多量子比特实验中的实验困难,然后结合7量子比特标记赝纯态制备以及其他有关实验,对多比特实验过程中应用到的实验技术进行介绍.最后对核磁共振体系多量子比特实验技术方向的进一步研究进行了总结和展望.     

Application of non-Hermitian Hamiltonian model in open quantum optical systems

Non-Hermitian systems have observed numerous novel phenomena and might lead to various applications. Unlike standard quantum physics, the conservation of energy guaranteed by the closed system is broken in the non-Hermitian system, and the energy can be exchanged between the system and the environment. Here we present a scheme for simulating the dissipative phase transition with an open quantum optical system. The competition between the coherent interaction and dissipation leads to the second-order phase transition. Furthermore, the quantum correlation in terms of squeezing is studied around the critical point. Our work may provide a new route to explore the non-Hermitian quantum physics with feasible techniques in experiments.     

Macroscopic tunnel splittings in superconducting phase qubits

Prototype Josephson-junction based qubit coherence times are too short for quantum computing. Recent experiments probing superconducting phase qubits have revealed previously unseen fine splittings in the transition energy spectra. These splittings have been attributed to new microscopic degrees of freedom (microresonators), a previously unknown source of decoherence. We show that macroscopic resonant tunneling in the extremely asymmetric double-well potential of the phase qubit can have observational consequences that are strikingly similar to the observed data.     

Non-classical neutron beams for fundamental and solid state research

The curious dual nature of the neutron, sometimes a particle, sometimes a wave, is wonderfully manifested in the various non-local interference and quantum contextuality effects observed in neutron interferometry. Non-classical states may become useful for novel fundamental and solid state research. Here we discuss unavoidable quantum losses as they appear in neutron phase-echo and spin rotation experiments and we show how entanglement effects in a single particle system demonstrate quantum contextuality. In all cases of interactions, parasitic beams are produced which cannot be recombined completely with the original beam. This means that a complete reconstruction of the original state would, in principle, be impossible which causes a kind of intrinsic irreversibility. Even small interaction potentials can have huge effects when they are applied in quantum Zeno-like experiments. Recently, it has been shown that an entanglement between external and internal degrees of freedom exists even in single particle systems. This contextuality phenomenon also shows that a quantum system carries much more information than usually extracted. The path towards advanced neutron quantum optics will be discussed.      

Unsharp spin observables, non-locality and Fry, Walther and Li experiment

Recently it has been demonstrated that Bell inequalities for spin 1/2 particles must be modified if unsharp spin observables are considered, and furthermore, the modified Bell inequalities may not be violated by quantum mechanics if the observables are sufficiently unsharp. In case of massive particles there may be more imperfection than seems to appear in the photon EPR experiments. So the experiment proposed by Fry, Walther and Li can place experimental limits on the unsharpness of spin variables. It sheds new light on the much debated issues like non-local correlations in quantum mechanics.     

Reality in neutron interference experiments

The wave-particle dualism becomes very obvious in matter wave interference experiments. Neutron interferometers based on wave front and amplitude division have been developed in the past. Most experiments have been performed with the perfect crystal neutron interferometer, which provides widely separated coherent beams allowing new experiments in the field of fundamental, nuclear, and solid-state physics. A nondispersive sample arrangement and the difference of stochastic and deterministic absorption have been investigated. In case of a deterministic absorption process the attenuation of the interference pattern is proportional to the beam attenuation, whereas in case of stochastic absorption it is proportional to the square root of the attenuation. This permits the formulation of Bell-like inequalities which will be discussed in detail. The verification of the4 symmetry of spinors and of the quantum mechanical spin-superposition experiment on a macroscopic scale are typical examples of interferometry in spin space. These experiments were continued with two resonance coils in the beams, where the results showed that coherence persists, even if an energy exchange between the neutron and the resonator system occurs with certainty. A quantum beat effect was observed when slightly different resonance frequencies were applied to both beams. In this case, the extremely high energy sensitivity of2.7×10 ^–19 eV was achieved. This effect can be interpreted as a magnetic Josephson-effect analog. Phase echo systems and experiments with pulsed beams show how interference phenomena can be made visible by a proper beam handling inside and behind the interferometer. All the results obtained until now are in agreement with the formalism of quantum mechanics but stimulate the discussion about the interpretation of this basic theory.     

基于提高精度的量子雷达方案及其进展

定位测量一直是人们关注的问题,将量子技术应用于定位任务有望展示出经典定位方案无 法达到的优势。目前,已有部分量子雷达方案被提出,展示了科研工作者从量子信息角度对定位 测量的新思考。本文总结了部分已有的量子雷达方案,介绍了相关方案的概念和分类,并着重对 量子定位、量子照明和三维增强雷达三种方案的基本原理进行论述,同时分析了各方案的优势以 及亟待解决的问题。     

Quantum computation and simulation with vibrational modes of trapped ions

Vibrational degrees of freedom in trapped-ion systems have recently been gaining attention as a quantum resource,beyond the role as a mediator for entangling quantum operations on internal degrees of freedom, because of the large available Hilbert space. The vibrational modes can be represented as quantum harmonic oscillators and thus offer a Hilbert space with infinite dimensions. Here we review recent theoretical and experimental progress in the coherent manipulation of the vibrational modes, including bosonic encoding schemes in quantum information, reliable and efficient measurement techniques, and quantum operations that allow various quantum simulations and quantum computation algorithms. We describe experiments using the vibrational modes, including the preparation of non-classical states, molecular vibronic sampling, and applications in quantum thermodynamics. We finally discuss the potential prospects and challenges of trapped-ion vibrational-mode quantum information processing.     

Kapitza-Dirac效应的GAC理论及其应用

从发现光电效应以来,人们对光与物质的相互作用的研究越来越多,越来越深入。尤其在激光发明以后,光场的强度越来越大,光束的准直性越来越好,人们可以利用它做大量的实验,从而发现了很多新的实验现象,例如阈上电离[1],Kapitza＿Dirac效应[2],高阶谐波效应[3]等。这些现象用原有的光电理论已无法解释,从而发展了一些多光子电离的理论,其中以美国SouthernUniversity的郭东升教授及其合作者所发展的非微扰理论较为引人关注。这篇文章将详细介绍郭教授的工作以及相关的实验结果与理论研究的比较,希望人们对多光子领域的研究有更深一步的了解。文章首先介绍光与物质相互作用的理论基础,即Volkov解的得出及其讨论;文章第二部分将对多光子过程进行描述,并用第一部分结果进行一些定性的讨论;第三部分介绍Kapiza＿Dirac效应的实验结果与理论研究的比较;第四部分对整个理论体系予以综合评论,以及对强激光和物质作用的理论和实际应用的前景展望。     

Kapitza-Dirac效应的GAC理论及其应用

从发现光电效应以来,人们对光与物质的相互作用的研究越来越多,越来越深入。尤其在激光发明以后,光场的强度越来越大,光束的准直性越来越好,人们可以利用它做大量的实验,从而发现了很多新的实验现象,例如阈上电离[1],Kapitza＿Dirac效应[2],高阶谐波效应[3]等。这些现象用原有的光电理论已无法解释,从而发展了一些多光子电离的理论,其中以美国SouthernUniversity的郭东升教授及其合作者所发展的非微扰理论较为引人关注。这篇文章将详细介绍郭教授的工作以及相关的实验结果与理论研究的比较,希望人们对多光子领域的研究有更深一步的了解。文章首先介绍光与物质相互作用的理论基础,即Volkov解的得出及其讨论;文章第二部分将对多光子过程进行描述,并用第一部分结果进行一些定性的讨论;第三部分介绍Kapiza＿Dirac效应的实验结果与理论研究的比较;第四部分对整个理论体系予以综合评论,以及对强激光和物质作用的理论和实际应用的前景展望。     

Quantum study of information delay in electromagnetically induced transparency

Using electromagnetically induced transparency (EIT), it is possible to delay and store light in atomic ensembles. Theoretical modeling and recent experiments have suggested that the EIT storage mechanism can be used as a memory for quantum information. We present experiments that quantify the noise performance of an EIT system for conjugate amplitude and phase quadratures. It is shown that our EIT system adds excess noise to the delayed light that has not hitherto been predicted by published theoretical modeling. In analogy with other continuous-variable quantum information systems, the performance of our EIT system is characterized in terms of conditional variance and signal transfer.