Zhang Hai-Chao

Archimedean-Spiral-Based Microchip Ring Waveguide for Cold Atoms

We present a scheme for generating a ring magnetic waveguide on a single-layer atom chip. The wire layout consists of two interleaved Archimedean spirals of the same size. The waveguide avoids the trapping perturbation caused by the input and output ports, resulting in an enclosed guiding loop for neutral atoms in weak-field seeking states. Such a configuration can create a tight and deep trap potential with a small current. Taking the |F = 2, mF = 2〉 state of 87Rb as an example, the trap frequency and depth are estimated to be 18 kHz and 335 μK, respectively, with a dc current of 2 A.

Optimal transport of cold atoms by modulating the velocity of traps

This work experimentally demonstrates a new method of optimizing the transport of cold atoms via modulating the velocity profile imposed on a magnetic quadrupole trap. The trap velocity and corresponding modulation are controlled by varying the currents of two pairs of anti-Helmholtz coils. Cold 87Rb atoms are transported in a non-adiabatic regime over 22 mm in 200 ms. For the transported atoms their final-vibration amplitude dependences of modulation period number, depth, and initial phase are investigated. With modulation period n=5, modulation depth K=0.55, and initial phase =0, cold atom clouds with more atom numbers, smaller final-vibration amplitude, and lower temperature are efficiently transported. Theoretical analysis and numerical simulation are also provided, which are in good agreement with experimental results.

Energy Spectrum of Ground State and Excitation Spectrum of Quasi-particle for Hard-Core Boson in Optical Lattices

We investigate the energy spectrum of ground state and quasi-particle excitation spectrum of hard-core bosons, which behave very much like spinless noninteracting fermions, in optical lattices by means of the perturbation expansion and Bogoliubov approach. The results show that the energy spectrum has a single band structure, and the energy is lower near zero momentum; the excitation spectrum gives corresponding energy gap, and the system is in Mott-insulating state at Tonks limit. The analytic result of energy spectrum is in good agreement with that calculated in terms of Greens function at strong correlation limit.

Monte Carlo Simulation of Cooling Induced by Parametric Resonance

We demonstrate that the parametric resonance in a magnetic quadrupole trap can be exploited to cool atoms by using Birds method. In our programme the parametric resonance was realized by anisotropically modulating the trap potential. The modulation frequency dependences of temperature and fraction of the trapped atoms are explored. Furthermore, the temperature after the modulation as functions of the modulation amplitude and the mean elastic collision time are also studied. These results are valuable for the experiment of parametric resonance in a quadrupole trap.

Existence of the transverse relaxation time in optically excited bulk semiconductors

Two basic types of depolarization mechanisms, carrier-carrier (CC) and carrier-phonon (CP) scattering, are investigated in optically excited bulk semiconductors (3D), in which the existence of the transverse relaxation time is proven based on the vector property of the interband transition matrix elements. The dephasing rates for both CC and CP scattering are determined to be equal to one half of the total scattering-rate-integrals weighted by the factors (1 - cos chi), where chi are the scattering angles. Analytical expressions of the polarization dephasing due to CC scattering are established by using an uncertainty broadening approach, and analytical ones due to both the polar optical-phonon and non-polar deformation potential scattering (including inter-valley scattering) are also presented by using the sharp spectral functions in the dephasing rate calculations. These formulas, which reveal the trivial role of the Coulomb screening effect in the depolarization processes, are used to explain the experimental results at hand and provide a clear physical picture that is difficult to extract from numerical treatments.

Trapping of neutral atoms with a radio-frequency field

Based on the dressed-atom approach, we discuss a two-dimensional (2D) radio-frequency trap for neutral atoms, in which the trap potential derives from the magnetic-dipole transition among the hyperfine Zeeman sublevels. By adjusting the detuning of the radiation from resonance, the trapping states will be changed predominantly from the bare states Of m(FgF) > 0 to other states of m(FgF) 0. In comparison to the optical field, the radio-frequency trap eliminates the spontaneous emission heating of the atoms. Unlike other oscillating traps reported in the e literature, the configuration of the radio-frequency trap is suitable for realization of a miniature magnetic guide.

Bichromatic Coherent Population Trapping Under Hanle Configuration

We report the observation of bichromatic coherent population trapping under the transition (87)RbFg = 1 --> F-e = 0 with two linearly polarized laser beams by means of a Hanle effect configuration. The mechanism behind this effect is identified, and numerical solutions for the pump rates equations are presented.