Quantum limits for minimum-uncertainty (MU) angular momentum polarization squeezed states

It is shown that there are certain quantum limits to angular momentum minimum-uncertainty (MU) obtained by polarization squeezing Hamiltonians, beyond which the MU cannot be realized. The analysis is made for two cases: (a) when a strong coherent state is in one mode and a squeezed vacuum is in the other mode. (b) For the interference between two optical modes which are amplitude squeezed, with equal squeezing and intensities’ magnitudes.     

Quantum routing of surface plasmons by two quantum dots

The quantum routing of single surface plasmons in a system with two metal nanowires coupled to a pair of quantum dots is investigated theoretically. Real-space Hamiltonians are utilized to obtain the surface-plasmon routing probabilities scattered by two quantum dots in four ports of two nanowire waveguides. Numerical results show that the routing capability of the surface plasmons transmitted from the input channel into another channel can be significantly enhanced, by properly adjusting the interdot distance and the dot-plasmon couplings. Moreover, multi-peak Fano-like resonances are exhibited in the scattering spectra due to the quadratic dispersion relations of the nanowire waveguides. Therefore, the proposed double-dot configuration may provide potential applications in controlling the surface-plasmon routing and Fano-like resonance.     

Quantum entanglement criteria

Abstract

We review the main criteria used to detect entanglement in quantum systems. The main properties of each criteria are summarized depending on whether the criteria provides a sufficient or necessary condition, whether it involves density matrix or operators, or if the criteria is phase sensitive. We show that several criteria have much in common and they could be related mathematically. We also discuss the features of entanglement which are useful in quantum information technology.     

量子关联成像及其雷达应用研究

介绍了量子关联成像的物理原理,阐述了量子关联成像灵敏度高、抗干扰能力强、信息获取效率高、能够实现单像素探测成像和无透镜成像的技术特点;探讨了量子关联成像雷达的运动物体成像问题和大气影响应对问题,指出可通过提高采样频率、升级跟踪手段、优化成像策略等方式提升量子关联成像雷达的应用性能;展望了量子关联成像雷达在侦察、预警等领域的发展方向,提出未来通过发展极弱光条件下的成像技术、优化设计照明方式、建立多基站协同体系、发展人工智能算法和多维信息智能融合算法等方式,进一步提升量子关联成像雷达的发现概率、跟踪精度、判别准确度、有效作用距离。     

Engineering the spectral and temporal properties of a GHz-bandwidth heralded single-photon source interfaced with an on-demand,broadband quantum memory

Photonics offers a route to fast and distributed quantum computing in ambient conditions, provided that photon sources and logic gates can be operated deterministically. Quantum memories, capable of storing and re-emitting photons on demand, enable quasi-deterministic operations by synchronizing stochastic events. Interfaced source–memory systems are thus a key building block in photonics-based quantum information processors. We discuss the design of the single-photon source in this type of light–matter interface and present an experimental system based on a Raman-type quantum memory. In addition to the spectral purity of the produced heralded single photons, we find that their temporal distinguishability also becomes important due to the implicit temporal binning derived from photon storage in the memory. When aiming to operate the source–memory system at high repetition rates, a practical compromise between both of these requirements needs to be found. Our implemented photon source system demonstrates such a solution and enables passive stability, high brightness in a single-pass configuration, high purity as well as good mode matching to our Raman memory.     

Implementation and testing of Lanczos-based algorithms for Random-Phase Approximation eigenproblems

The treatment of the Random-Phase Approximation Hamiltonians, encountered in different frameworks, like time-dependent density functional theory or Bethe–Salpeter equation, is complicated by their non-Hermicity. Compared to their Hermitian Hamiltonian counterparts, computational methods for the treatment of non-Hermitian Hamiltonians are often less efficient and less stable, sometimes leading to the breakdown of the method. Recently [Grüning et al. Nano Lett. 8 (2009) 2820], we have identified that such Hamiltonians are usually pseudo-Hermitian. Exploiting this property, we have implemented an algorithm of the Lanczos type for Random-Phase Approximation Hamiltonians that benefits from the same stability and computational load as its Hermitian counterpart, and applied it to the study of the optical response of carbon nanotubes. We present here the related theoretical grounds and technical details, and study the performance of the algorithm for the calculation of the optical absorption of a molecule within the Bethe-Salpeter equation framework.     

Entanglement dynamics and quasi-periodicity in discrete quantum walks

We study the entanglement dynamics of discrete time quantum walks acting on bounded finite sized graphs. We demonstrate that, depending on system parameters, the dynamics may be monotonic, oscillatory but highly regular, or quasi-periodic. While the dynamics of the system are not chaotic since the system comprises linear evolution, the dynamics often exhibit some features similar to chaos such as high sensitivity to the system's parameters, irregularity and infinite periodicity. Our observations are of interest for entanglement generation, which is one primary use for the quantum walk formalism. Furthermore, we show that the systems we model can easily be mapped to optical beamsplitter networks, rendering experimental observation of quasi-periodic dynamics within reach.     

Visibility of ghost imaging in a two-arm microscope imaging system

Ghost imaging with a classical thermal source is investigated in a two-arm microscope imaging system. The dependence of the imaging visibility on the aperture of the reference lens is discussed. It is shown that by using large apertures, good visibility as well as enhancing resolution can be obtained. The effects from the distance the object is moved away from the original plane are also studied, and one can obtain good visibility with a well-resolved image by changing the distance.     

Exactly and quasi-exactly solvable 'discrete' quantum mechanics

A brief introduction to discrete quantum mechanics is given together with the main results on various exactly solvable systems. Namely, the intertwining relations, shape invariance, Heisenberg operator solutions, annihilation/creation operators and dynamical symmetry algebras, including the q-oscillator algebra and the Askey-Wilson algebra. A simple recipe to construct exactly and quasi-exactly solvable (QES) Hamiltonians in one-dimensional 'discrete' quantum mechanics is presented. It reproduces all the known Hamiltonians whose eigenfunctions consist of the Askey scheme of hypergeometric orthogonal polynomials of a continuous or a discrete variable. Several new exactly and QES Hamiltonians are constructed. The sinusoidal coordinate plays an essential role.     

基于量子关联的显微成像研究进展

简要回顾了量子关联成像的基本原理和发展历程,从量子光源和经典光源的角度详细介绍了量子关联成像在显微成像中的研究进展。做出了基于经典源的量子关联成像因易于实施、成本较低,在显微成像中更具应用前景的判断。     

On the quantum core of an optical vortex

An optical vortex is a line around which the phase increases by an integer multiple of 2π. It follows that the phase on the line itself is undefined and hence the field must have zero amplitude there. Berry and Dennis have suggested that this line of darkness is smoothed by a ‘quantum core’ with a radius proportional to ?^1/2 and have illustrated this idea by considering the competition between stimulated and spontaneous emission by an excited atom placed in the vicinity of the vortex. We show here that a similar phenomenon may be seen in absorption when the quantum state of motion of the absorbing atom is taken into consideration. There is, however, an underlying quantum singularity in which the absorption events for an atom centred on the vortex core can take place only if accompanied by a transfer of angular momentum to the atomic motion. The nature of this singularity relies on the evolution of an entangled state between the electronic and motional degrees of freedom of the trapped atom. We comment briefly on the effects of field quantisation on this quantum core of the optical vortex.     

Nonlocality for continuous variable system with local symplectic operation

We study Bell's theorem for two-mode squeezed state with realizable operations in experiment. For the purpose, we suggest the Bell–CHSH operator with photon presence measurement using symplectic operation and displacement. The symplectic operation can be decomposed into phase shifter and squeezing operation in single mode. These operations are realizable experimentally in quantum optics. As a result, we obtain a larger degree of quantum nonlocality by local symplectic operation and displacement.     

Coherence area profiling in multi-spatial-mode squeezed states

The presence of multiple bipartite entangled modes in squeezed states generated by four-wave mixing enables ultra-trace sensing, imaging, and metrology applications that are impossible to achieve with single-spatial-mode squeezed states. For Gaussian seed beams, the spatial distribution of these bipartite entangled modes, or coherence areas, across each beam is largely dependent on the spatial modes present in the pump beam, but it has proven difficult to map the distribution of these coherence areas in frequency and space. We demonstrate an accessible method to map the distribution of the coherence areas within these twin beams. We also show that the pump shape can impart different noise properties to each coherence area, and that it is possible to select and detect coherence areas with optimal squeezing with this approach.     

Time Evolution of Stable Squeezed States

Abstract

It is shown using a straightforward approach that, for a single mode of an electromagnetic field, only quadratic Hamiltonians can preserve squeezed states for all time. The time-evolution equations for the parameters of a stable squeezed state are given for arbitrary quadratic Hamiltonians. It is also pointed out that there is no single Hamiltonian that preserves minimum uncertainty for all squeezed minimum-uncertainty states.     

Quantum trapping conditions for three-level atom flying through bimodal cavity field

A new trapping effect of a three-level atom in interaction with a bimodal cavity field is proposed. This problem consists of the possibility for realization of initially separated states of an atom and an electromagnetic field after interaction. The quantum properties of a bimodal field, which satisfy the reversible conditions for the atom flying through the cavity, were studied.     

From Flux Quantization to Superconducting Quantum Bits

One of the important predictions of the early phenomenological theories of superconductivity such as the London and Ginzburg–Landau (GL) theory is the quantization of magnetic flux in multiply connected superconductors, which is one of the first demonstrations of a quantum effect on a macroscopic scale. In this paper, which is devoted to Vitalij Lazarevich Ginzburg on the occasion of his 90th birthday, we analyze a superconducting cylinder acting as a flux box as well as a superconducting disk acting as a Cooper pair box in the framework of GL theory. We extend this analysis to leaky flux and Cooper pair boxes which are obtained by introducing weak links allowing for the entry and exit of flux quanta and Cooper pairs from the respective boxes at finite rates. Flux and Cooper pair slippage processes by coherent quantum tunneling result in effective two-level quantum systems forming the basis for flux and charge quantum bits presently considered for the solid-state implementation of quantum information processing. We show that the corresponding Hamiltonians describing the leaky flux and Cooper pair box can be transformed into each other by a canonical transformation. PACS numbers: 74.20.De, 74.25.Bt, 74.25.Ha, 74.50.+r