Cross-gain modulation in closely spaced energy states quantum dash semiconductor optical amplifiers

A detailed small-signal analysis of cross-gain modulation is performed for closely spaced energy state quantum dash (QDsh) semiconductor optical amplifier (SOA). The analysis takes into account the carrier transition in all electron and hole states, the gain dispersion of the active layer, the effect of the energy detuning between the probe and pump signals and the effect of doping on the characteristics of cross-gain wavelength conversion. Our analysis reveals that broadband conversion efficiency can be obtained in QDsh SOA when the energy of the pump signal is at − 20 meV below the ground state. Also we find that large 3 dB bandwidth can be achieved when the energy of the pump and the probe signals is at + 30 meV. Our analysis shows that doping the dashes with P-type concentration can enhance the efficiency and the intrinsic 3 dB bandwidth of cross-gain wavelength conversion.     

Quenching of the Haldane gap in LiVSi<Subscript>2</Subscript>O<Subscript>6</Subscript> and related compounds

We report results of susceptibility χ and ⁷Li NMR measurements on LiVSi₂O₆. The temperature dependence of the magnetic susceptibility χ(T) exhibits a broad maximum, typical for low-dimensional magnetic systems. Quantitatively it is in agreement with the expectation for an S=1 spin chain, represented by the structural arrangement of V ions. The NMR results indicate antiferromagnetic ordering below T_N=24 K. The intra- and interchain coupling J and J_p for LiVSi₂O₆, and also for its sister compounds LiVGe₂O₆, NaVSi₂O₆ and NaVGe₂O₆, are obtained via a modified random phase approximation which takes into account results of quantum Monte Carlo calculations. While J_p is almost constant across the series, J varies by a factor of 5, decreasing with increasing lattice constant along the chain direction. The comparison between experimental and theoretical susceptibility data suggests the presence of an easy-axis magnetic anisotropy, which explains the formation of an energy gap in the magnetic excitation spectrum below T_N, indicated by the variation of the NMR spin-lattice relaxation rate at T≪T_N.     

Dynamic Entanglement and Separability Criteria for Quantum Computing Bit States

A theoretical framework is demonstrated to evaluate the degree of entanglement of bit states in quantum computing. Separability of general superposition of Hilbert space unit vectors is discussed, and criteria in amplitude as well as in phase are derived. By these criteria the possibility of different quantum gates such as controlled not (CN), controlled controlled not (CCN), controlled rotation (CR), and controlled phase shift (CPS), to create the entanglement is examined. Furthermore, the selection of measurement mode external to the quantum system is incorporated in the formula using Kronecker delta (δ _kx ), introducing the concept of dynamic entanglement. With this the process of wavefunction collapse upon measurement is understood as the result of the activation of the dynamic entanglement. A firefly in a box model is used to show a pure state of ontological uncertainty, which is in a dynamically entangled state in Hilbert space.     

Low- x QCD physics from RHIC and HERA to the LHC

We present a summary of the physics of gluon saturation and non-linear QCD evolution at small values of the parton momentum fraction x in the proton and nucleus in the context of recent experimental results at HERA and RHIC. The rich physics potential of low-x studies at the LHC, especially in the forward region, is discussed and some benchmark measurements in pp, pA and AA collisions are introduced.     

量子算法与量子计算实验

从量子体系的基本特性出发 ,介绍了量子计算的基本概念和物理背景 ,系统阐述了几种主要的量子算法以及量子计算在实验方面的发展现状。对比经典计算机 ,讨论了量子计算机的优越性、实现量子计算的困难和以期克服的途径。     

Donor impurity states in coupled quantum well wires under hydrostatic pressure and applied electric field

In this work we study the binding energy of the ground state for a hydrogenic donor impurity in laterally coupled GaAs/Ga_1−xAl_xAs quantum well wires, considering the simultaneous effects of hydrostatic pressure and applied electric field. We have used a variational method and the effective mass and parabolic band approximations. The low dimensional structure consists of two quantum well wires with rectangular transverse section coupled by a central Ga_1−xAl_xAs barrier. Our results are reported for several sizes of the structure and we have taken into account variations of the impurity position along the growth direction of the heterostructure.     

Donor impurity states in zinc-blende GaN/AlGaN quantum well: Quantum confinement and laser-dressed effects

Within the framework of effective-mass approximation, the effects of a laser field on the ground-state donor binding energy in zinc-blende (ZB) GaN/AlGaN quantum well (QW) have been investigated variationally. Numerical results show that the donor binding energy is highly dependent on QW structure parameters and Al composition in ZB GaN/AlGaN QW. The laser field effects are more noticeable on the donor binding energy of an impurity localized inside the QW with small well width and low Al composition. However, for the impurity located in the vicinity of the well edge of the QW, the donor binding energy is insensible to the variation of Al composition, well width and laser field intensity in ZB GaN/AlGaN QW. In particular, the competition effects between laser field and quantum confinement on impurity states have also been investigated in this paper.     

Fractional revival and association schemes

Fractional revival occurs between two vertices in a graph if a continuous-time quantum walk unitarily maps the characteristic vector of one vertex to a superposition of the characteristic vectors of the two vertices. This phenomenon is relevant in quantum information in particular for entanglement generation in spin networks. We study fractional revival in graphs whose adjacency matrices belong to the Bose–Mesner algebra of association schemes. A specific focus is a characterization of balanced fractional revival (which corresponds to maximal entanglement) in graphs that belong to the Hamming scheme. Our proofs exploit the intimate connections between algebraic combinatorics and orthogonal polynomials.