Zhou Xiao-Ji

Microwave Atomic Clock in the Optical Lattice with Specific Frequency

A scheme for a microwave atomic clock is proposed for Cs or Rb atoms trapped in a blue detuned optical lattice. The ac Stark shift of the clock transition due to a trapping laser is calculated. We analyze it at some specific laser wavelength. Compared with the case of the fountain clock, the cavity related shifts, the collision shift and the Doppler effect are eliminated or suppressed dramatically in an atomic lattice clock. By analyzing various sources of clock uncertainty, a microwave atomic lattice clock with a high accuracy and small volume is feasible.

Ultra-Stable Rubidium-Stabilized External-Cavity Diode Laser Based on the Modulation Transfer Spectroscopy Technique

We construct an ultra-stable external-cavity diode laser via modulation transfer spectroscopy referencing on a hyperfine component of the 87Rb D2 lines at 780nm. The Doppler-free dispersion-like modulation transfer signal is obtained with high signal-to-noise-ratio. The instability of the laser frequency is measured by beating with an optical frequency comb which is phase-locked to an ultra-stable oven controlled crystal oscillator. The Allan deviation is 3.9 × 10−13 at 1s averaging time and 9.8 × 10−14 at 90s averaging time.

Magic Wavelength for Caesium Transition Line 6S1/2 – 6P3/2

We investigate the magic wavelengths of the trapping laser for 6S1/2 – 6P3/2 of the Cs atom in a region where the optical shift between two different states can be eliminated. For fine levels and linear polarized laser they are 930.4 nm and 937.2 nm. The magic wavelengths range from 927.7 nm to 945.0 nm for circle-polarized perturbing laser. Effects of nuclear spin, the hyper-fine Zeeman levels, and the polarization of the light, which generate different magic wavelengths, are further discussed.

An Empirical Formula for the Measurement of Spatial Coherence of Trapped Bose Gases

An empirical formula, which demonstrates the interference fringe visibility as a function of the slit separation and the temperature of thermal atoms, is obtained by analysing a recent experiment [Bloch et al. Nature 403 (2000) 166] that refers to the spatial correlation function of a trapped Bose gas below the critical temperature. We find that the decay rate of the coherence function for the non-condensed component is about two orders larger than that of the pure condensed component. The changes of the interference fringe visibility versus the falling time for different temperatures and slit separations are also discussed.

A Scheme for Realizing the Continuous Wave Atom Laser

We propose a scheme for realizing the long-term continuous output of an atom laser. In this scheme the cold atoms in several subsidiary Bose-Einstein condensates (BEC) will be coupled one by one in turn into one main BEC which serves for the continuous output of a coherent atomic beam. We discuss the process of the phase unification of the newly entered BEC atoms with the remaining atoms in the main BEC trap. An ideal dynamical equilibrium can be established when the transition rate of the uncondensed atoms transforming into the condensate and the decay rate of coupling output satisfy some conditions. The coupling method from the subsidiary BEC to the main BEC and the time rates of all different processes are considered.

Strong Outcoupling from Spin-2 87Rb Bose?Einstein Condensates

A pulsed atom laser is experimentally demonstrated by means of outcoupling coherent atoms from 87Rb Bose–Einstein condensates in magnetic trap via radio-frequency pulses. To study the strong outcoupling dynamics of the atom laser, the original |F = 2,mF = 2 condensate and the coupled |F = 2,mF = 1 component, both of which overlap in space usually, are separated spatially by collective oscillations. The number of atoms in three of the five Zeeman states are measured and compared with the theoretical results.

The Weak-Coupling of Bose-Einstein Condensates

The coherent characteristics of four trapped Bose-Einstein condensates (BEC) conjunct one by one in a ring shape which is divided by two far off-resonant lasers, are studied. Four coupled Gross-Pitaevskii equations are used to describe the dynamics of the system. Two kinds of self-trapping effects are discussed in the coupled BECs, and the phase diagrams for different initial conditions and different coupling strengths are discussed. This study can be used to determine interaction parameters between atoms in BEC.

The Study of the Phase of Bose–Einstein Condensate*

We first propose to study the phase of Bose-Einstein condensate in the phase space. The mean value of the phase and the phase fluctuation of Bose-Einstein condensate are considered, and their explicit expressions are given with the Thomas-Fermi approximation. For a finite atom number, we find that the phase of condensate is determined by the oscillation frequency of the harmonic confining potential at certain time. The effects of the atom number and time on the phase of condensate are also discussed for the same kinds of atoms.

Cooperative scattering measurement of coherence in a spatially modulated Bose gas

Correlations of a Bose gas released from an optical lattice are measured using superradiant scattering. Conditions are chosen so that, after initial incident light pumping at the Bragg angle for diffraction, superradiant scattering into the Bragg diffracted mode is preponderant due to matter-wave amplification and mode competition. A temporal analysis of the superradiant scattering gain reveals periodical oscillations and damping due to the initial lack of coherence between lattice sites. Such damping is used for characterizing first-order spatial correlations in our system with a precision of one lattice period.

Phase-Dependent Effects in Stern–Gerlach Experiments

In the frame of quantum mechanics, we consider an ensemble of spin-½ neutral particles passing through a Stern–Gerlach apparatus and explore how their motions depend on the initial phase difference between two internal spin states. Assuming the particles moving along y-axis, due to the initial phase difference between spin states, they not only split along the longitudinal direction (z-axis) but also separate along the lateral direction (x-axis). The dependence of the lateral displacement on the initial phase difference reminds one of the picture of a quantum interference. This generalized interference provides an alternative approach to measuring the initial phase difference. The experimental realization with ultracold atoms or Bose–Einstein condensates is also discussed.