Quantum phase transitions in matrix product states of one-dimensional spin-1 chains

We present a new model of quantum phase transitions in matrix product systems of one-dimensional spin-1 chains and study the phases coexistence phenomenon. We find that in the thermodynamic limit the proposed system has three different quantum phases and by adjusting the control parameters we are able to realize any phase, any two phases equal coexistence and the three phases equM coexistence. At every critical point the physical quantities including the entanglement are not discontinuous and the matrix product system has long-range correlation and N-spin maximal entanglement. We believe that our work is helpful for having a comprehensive understanding of quantum phase transitions in matrix product states of one-dimensional spin chains and of certain directive significance to the preparation and control of one-dimensional spin lattice models with stable coherence and N-spin maximal entanglement.     

Quantum Theory of Electronic Double-Slit Diffraction

Phenomena of electron, neutron, atomic and molecular diffraction have been studied in many experiments, and these experiments have been explained by some theoretical works. We study electronic double-slit diffraction with a quantum mechanical approach and obtain the following results： （1） When the ratio of d ＋ a /a=n （n = 1,2, 3,...）, orders n, 2n, 3n,... are missing in diffraction pattern. （2） When the ratio of d＋a/a≠ n （n =1,2, 3,...）, there is a not missing order in diffraction pattern. （3） The slit thickness c has a large affect on the electronic diffraction pattern, which is a new quantum effect. We believe that all the predictions in our work can be tested by the electronic double slit diffraction experiment.     

Ground State of a Two-Electron Quantum Dot with a Gaussian Confining Potential

We investigate the ground-state properties of a two-dimensional two-electron quantum dot with a Gaussian confining potential under the influence of perpendicular homogeneous magnetic field. Calculations are carried out by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. A ground-state behaviour （singlet→triplet state transitions） as a function of the strength of a magnetic field has been found. It is found that the dot radius R of the Gaussian potential is important for the ground-state transition and the feature of ground-state for the Gaussian potential quantum dot （QD）, and the parabolic potential QDs are similar when R is larger. The larger the quantum dot radius, the smaller the magnetic field for the singlet-triplet transition of the ground-state of two interacting electrons in the Gaussian quantum dot.     

Quantum phase transitions in matrix product states of one-dimensional spin-1/2 chains

For the matrix product system of a one-dimensional spin-1/2 chain, we present a new model of quantum2 phase transitions and find that in the thermodynamic limit, both sides of the critical point are respectively described by phases ｜Ψa 〉=｜1··· 1 representing all particles spin up and ｜Ψb 〉=｜0··· 0 representing all particles spin down, while the phase transition point is an isolated intermediate-coupling point where√ the two phases coexist equally, which is2 described by the so-called N-qubit maximally entangled GHZ state ｜Ψpt =√2/2（｜1··· 1 ＋｜0··· 0）. At the critical point,2the physical quantities including the entanglement are not discontinuous and the matrix product system has longrange correlation and N-qubit maximal entanglement. We believe that our work is helpful for having a comprehensive understanding of quantum phase transitions in matrix product states of one-dimensional spin chains and of potential directive significance to the preparation and control of one-dimensional spin lattice models with stable coherence and N-qubit maximal entanglement.     

The Topological Structure of the SU（2） Chern-Simons Topological Current in the Four-Dimensional Quantum Hall Effect

In the light of the decomposition of the SU（2） gauge potential for I = 1/2, we obtain the SU（2） Chern-Simons current over S4, i.e. the vortex current in the effective field for the four-dimensional quantum Hall effect. Similar to the vortex excitations in the two-dimensional quantum Hall effect （2D FQH） which are generated from the zero points of the complex scalar field, in the 4D FQH, we show that the SU（2） Chern-Simons vortices are generated from the zero points of the two-component wave functions ψ, and their topological charges are quantized in terms of the Hopf indices and Brouwer degrees of Ф-mapping under the condition that the zero points of field ψ are regular points.     

Quantum metrology

The statistical error is ineluctable in any measurement. Quantum techniques, especially with the development of quantum information, can help us squeeze the statistical error and enhance the precision of measurement. In a quantum system, there are some quantum parameters, such as the quantum state, quantum operator, and quantum dimension, which have no classical counterparts. So quantum metrology deals with not only the traditional parameters, but also the quantum parameters. Quantum metrology includes two important parts： measuring the physical parameters with a precision beating the classical physics limit and measuring the quantum parameters precisely. In this review, we will introduce how quantum characters （e.g., squeezed state and quantum entanglement） yield a higher precision, what the research areas are scientists most interesting in, and what the development status of quantum metrology and its perspectives are.     

Quantum phase transitions in matrix product states of one-dimensional spin-1 chains

We present a new model of quantum phase transitions in matrix product systems of one-dimensional spin-1 chains and study the phases coexistence phenomenon. We find that in the thermodynamic limit the proposed system has three different quantum phases and by adjusting the control parameters we are able to realize any phase, any two phases equal coexistence and the three phases equal coexistence. At every critical point the physical quantities including the entanglement are not discontinuous and the matrix product system has long-range correlation and N-spin maximal entanglement. We believe that our work is helpful for having a comprehensive understanding of quantum phase transitions in matrix product states of one-dimensional spin chains and of certain directive significance to the preparation and control of one-dimensional spin lattice models with stable coherence and N-spin maximal entanglement.     

Multiparty Quantum Secret Report

A multiparty quantum secret report scheme is proposed with quantum encryption. The boss Alice and her M agents first share a sequence of （M ＋ 1）-particle Greenberger-Horne-Zeilinger （GHZ） states that only Alice knows which state each （M ＋ 1）-particle quantum system is in. Each agent exploits a controlled-not （CNot） gate to encrypt the travelling particle by using the particle in the GHZ state as the control qubit. The boss Alice decrypts the travelling particle with a CNot gate after performing a aσ∞ operation on her particle in the GHZ state or not. After the GHZ states （the quantum key） are used up, the parties check whether there is a vicious eavesdropper, say Eve, monitoring the quantum line, by picking out some samples from the GHZ states shared and measuring them with two measuring bases. After confirming the security of the quantum key, they use the remaining GHZ states repeatedly for the next round of quantum communication. This scheme has the advantage of high intrinsic efficiency for the qubits and total efficiency.     

Quantum phase distribution and the number—phase Wigner function of the generalized squeezed vacuum states associated with solvable quantum systems

The quantum phase properties of the generalized squeezed vacuum states associated with solvable quantum systems are studied by using the Pegg-Barnett formalism.Then,two nonclassical features,i.e.,squeezing in the number and phase operators,as well as the number-phase Wigner function of the generalized squeezed states are investigated.Due to some actual physical situations,the present approach is applied to two classes of generalized squeezed states:solvable quantum systems with discrete spectra and nonlinear squeezed states with particular nonlinear functions.Finally,the time evolution of the nonclassical properties of the considered systems has been numerically investigated.     

Tuning Photoluminescence Energy and Fine Structure Splitting in Single Quantum Dots by Uniaxial Stress

We report a photoluminescence （PL） energy red-shift of single quantum dots （QDs） by applying an in-plane compressive uniaxial stress along the [110] direction at a liquid nitrogen temperature. Uniaxial stress has an effect not only on the confinement potential in the growth direction which results in the PL shift, but also on the cylindrical symmetry of QDs which can be reflected by the change of the full width at half maximum of PL peak. This implies that uniaxial stress has an important role in tuning PL energy and fine structure splitting of QDs.