Systematically investigating the polarization gradient cooling in an optical molasses of ultracold cesium atoms

We systematically investigate the polarization gradient cooling （PGC） process in an optical molasses of ultracold cesium atoms. The SR mode for changing the cooling laser, which means that the cooling laser frequency is stepped to the setting value while its intensity is ramped, is found to be the best for the PGC, compared with other modes studied. We verify that the heating effect of the cold atoms, which appears when the cooling laser intensity is lower than the saturation intensity, arises from insufficient polarization gradient cooling. Finally, an exponential decay function with a statistical explanation is introduced to explain the dependence of the cold atom temperature on the PGC interaction time.     

Unconventional Energy Bands and Chirality of Ultracold Atoms in Trilayer Honeycomb Lattice

We investigate ultracold fermionic atoms in the trilayer honeycomb lattice. In the low energy approximation, we derive an effective Hamiltonian for pseudospins. The energy spectrum shows a cubic form of the wavevector and is gapless. The quasiparticles and quasiholes are ehiral and show Berry＇s phase π when the wavevector adiabatically evolves along a closed circle, Furthermore, the experimental detection of the energy spectrum is proposed with Bragg scattering techniques.     

Unconventional Landau Levels of Ultracold Fermionic Atoms on Two-Dimensional Honeycomb Lattice

We investigate the unconventional Landau levels of ultracold fermionic atoms on the two-dimensional honeycomb optical lattice subjected to an effective magnetic field, which is created with optical means. In the presence of the effective magnetic field, the energy spectrum of the unconventional Landau levels is calculated. Furthermore, we propose to detect the unconventional Landau levels with Bragg scattering techniques.     

Switching and Fano resonance via exciton–plasmon interaction

We fnrther study theoretically the properties of switching and Fano resonance in a hybrid nanosystem consisting of two quantum dots （QDs） and a metal nanowire via exciton-plasmon interaction. The transmission of the single plasmon can be switched on or off in a wide-frequency region by adjusting the transition frequencies of the QDs and the phase of the propagating plasmon. Specifically, the dynamical mechanism of Fano-type transmission is further revealed and analyzed in detail.     

Dynamics of an Ultracold Bose Gas in Funnel-Shaped Potential

In this paper we develop a variational theory to study the dynamic properties of ultracold Bose gas in a funnel external potential. We obtain one-dimensional nonlinear equation which describes the dynamics of transverse tight confined bosonic gas from three-dimension to one-dimension, and find one-dimensional s-wave scattering length which depends on the shape of transverse confining potential. If the funnel trapping potential is strong enough at zero temperature, all transverse excitations are frozen. We find the dynamic equation which describes the Tonks-Girardeau gas and present a qualitative analysis of the experimental accessibility of the Tonks Girardeau gas with funnel-trapped alkalic atoms.     

Photoassociation of ultracold RbCs molecules into the （2）0- state （v = 189,190） below the 5S1/2 ＋ 6P1/2 dissociation limit

Ultracold polar RbCs molecules are produced via photoassociation in a laser-cooled mixture of 85Rb and 133Cs atoms. The a 3Σ+ state molecules which decay from electronically excited (2)0- state RbCs molecules are detected by resonance-enhanced two-photon ionization. The new rovibrational levels (v = 189, 190) in the (2)0- state are also observed, which exist in theory and have not been observed in experiments yet. The corresponding rotational constants are measured by photoassociation spectroscopy, which are consistent with theoretical calculations using a nonrigid rotor model.     

Effects of interface roughness on photoluminescence full width at half maximum in GaN/AIGaN quantum wells

Low temperature photoluminescence （PL） measurements have been performed for a set of GaN/AlxGal xN quantum wells （QWs）. The experimental results show that the optical full width at half maximum （FWHM） increases relatively rapidly with increasing A1 composition in the AlxGal xN barrier, and increases only slightly with increasing GaN well width. A model considering the interface roughness is used to interpret the experimental results. In the model, the FWHM＇s broadening caused by the interface roughness is calculated based on the triangle potential well approximation. We find that the calculated results accord with the experimental results well.     

Enhancing the precision of phase estimation by weak measurement and quantum measurement reversal

The enhancement of the precision of phase estimation in quantum metrology is investigated by employing weak measurement （WM） and quantum measurement reversal （QMR）. We derive the exact expressions of the optimal quantum Fisher information （QFI） and success probability of phase estimation for an exactly solving model consisting of a qubit interacting with a structured reservoir. We show that the QFI can be obviously enhanced by means of the WM and QMR in different regimes. In addition, we also show that the magnitude of the decoherence involved in the WM and QMR can be a general complex number, which extends the applicable scope of the WM and QMR approach.     

Anisotropy and Magnetic Field Effects on Entanglement of a Two-Spin （1/2,1） Mixed-Spin Heisenberg XY Chain

We study the entanglement in anisotropie （1/2,1） mixed-spin Heisenberg XY model under the presence of an external magnetic field at thermal equilibrium. By adjusting the anisotropic parameter and the magnetic field, one is able to obtain entanglement at higher temperature. We find the evidence of the quantum phase transition in the model and observe that the quantum phase transition point at low temperature moves toward weak magnetic field with the increase of the anisotropic parameter.     

Quantum Tunneling of Two-Species Cold Bosonic Atoms in Optical Lattices

In this letter, we have studied quantum tunneling of two-species cold bosonic atoms in an optical lattices. When the optical lattice is not infinitely long and the spin excitations are not in the long-wavelength limit, quantum tunnelings are presented.     

从光晶格中释放的超冷玻色气体密度-密度关联函数研究

利用量子旋转场理论详细研究了从光晶格中释放的超冷玻色气体的空间密度-密度关联函数.由于量子旋转场理论充分考虑了光晶格中冷原子气体的粒子数涨落和相位效应,该理论能有效应用于具有强相互作用的冷原子系统,从而光晶格处于超流态到绝缘态逐渐过渡过程中的超冷原子气体的关联特性在这一理论体系下都得到了很好的描述.结果表明:随着超冷玻色气体逐渐从绝缘态向超流态过渡,其密度-密度关联图样中连续对角斜线也逐渐向分散的尖峰过渡,理论结果与目前实验观测到的结果符合.除此以外,上述密度-密度关联的结果中还包含了超冷原子系统量子耗散效应,相关结论与目前已有的理论和实验一致.     

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.     

Effective Bose-Hubbard interaction with enhanced nonlinearity in an array of coupled cavities

An array of coupled cavities,each of which contains an N four-level atom,is investigated.When cavity fields dispersively interact with the atoms,an effective Bose-Hubbard model can be achieved.By numerically comparing the full Hamiltonian with the effective one,we find that within the parameters region,the effective Hamiltonian can completely account for the Mott-insulator as well as the phase transition from the similar Mott-insulator to superfluid.Through jointly adjusting the classical Rabi frequency and the detuning,the nonlinearity can be improved.     

Linear and Nonlinear Optical Properties in Spherical Quantum Dots

We calculate the energy eigenvalues and the sate functions of one-electron Quantum Dot （QD） by using a combination of Quantum Genetic Algorithm （QGA） and Hartre-Fock-Roothaan （HFR） method. The linear and the third-order nonlinear optical absorption coefficients for the 1s-1p, 1p-1d, and 1d-1f transitions are examined as a function of the incident photon energy for three different values of the stoichiometric ratio. The results show that the stoichiometric ratio, impurity, relaxation time, and dot size have great influence on the optical absorption coefficients of QDs.     

Ground state of rotating ultracold quantum gases with anisotropic spin orbit coupling and concentrically coupled annular potential

Motivated by recent experimental realization of synthetic spin–orbit coupling in neutral quantum gases, we consider the quasi-two-dimensional rotating two-component Bose–Einstein condensates with anisotropic Rashba spin–orbit coupling subject to concentrically coupled annular potential. For experimentally feasible parameters, the rotating condensate exhibits a variety of rich ground state structures by varying the strengths of the spin–orbit coupling and rotational frequency.Moreover, the phase transitions between different ground state phases induced by the anisotropic spin–orbit coupling are obviously different from the isotropic one.     

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.     

Excitations of optomechanically driven Bose Einstein condensates in a cavity: Photodetection measurements

We present a detailed study to analyze the Dicke quantum phase transition within the thermodynamic limit for an optomechanically driven Bose-Einstein condensate in a cavity. The photodetection-based quantum optical measurements have been performed to study the dynamics and excitations of this optomechanical Dicke system. For this, we discuss the eigenvalue analysis, fluorescence spectrum and the homodyne spectrum of the system. It has been shown that the normal phase is negligibly affected by the mechanical mode of the mirror while it has a significant effect in the superradiant phase. We have observed that the eigenvalues and the spectra both exhibit distinct features that can be identified with the photonic, atomic and phononic branches. In the fluorescence spectra, we further observe an asymmetric coherent energy exchange between the three degrees of freedom of the system in the superradiant phase arising as a result of optomechanical interaction and Bloch-Siegert shift.     

Peculiar Quantum Phase Transitions and Hidden Supersymmetry in a Lipkin-Meshkov-Glick Model

In this paper we theoretically report an unconventional quantum phase transition of a simple Lipkin- Meshkow-Glick model： an interacting collective spin system without external magnetic field. It is shown that this model with integer-spin can exhibit a flrst-order quantum phase transition between different disordered phases, and more intriguingly, possesses a hidden supersymmetry at the critical point. However, for half-integer spin we predict another flrst-order quantum phase transition between two different long-range-ordered phases with a vanishing energy gap, which is induced by the destructive topological quantum interference between the intanton and anti-instanton tunneling paths and accompanies spontaneously breaking of supersymmetry at the same critical point. We also show that, when the total spin-value varies from half-integer to integer this model can exhibit an abrupt variation of Berry phase from π to zero.     

Phase grating in a doubly degenerate four-level system

In this paper, we suggest a doubly degenerate four-level system, in which the transition takes place between the hyperfine energy 52S1/2 F = 1 and 52P3/2 F = 2 in rubidium 87 D2 line, for studying atomic phase grating based on the cross-Kerr and phase conjugation effects. The phase grating with high efficiency can be obtained by tuning phase shift Ф between the coupling and probe field, when the coupling intensity is much stronger than the strength of probe field. Under different coupling intensities, a high diffraction efficiency can be maintained. A new and simple way of implementing phase grating is presented. However, in such an atomic system, two main limitations must be taken into account. First, the independence between steady state probe susceptibility and the coupling intensity, when the population decay rate is larger than the Rabi frequency of the coupling field, cannot result in diffraction grating; second, the sample to be prepared should not be too long.