Xiaoji Zhou

Magic wavelengths for terahertz clock transitions

Magic wavelengths for laser trapping of boson isotopes of alkaline-earth metal atoms Sr, Ca, and Mg are investigated while considering terahertz clock transitions between the {sup 3}P{sub 0}, {sup 3}P{sub 1}, and {sup 3}P{sub 2} metastable triplet states. Our calculation shows that magic wavelengths for laser trapping do exist. This result is important because those metastable states have already been used to make accurate clocks in the terahertz frequency domain. Detailed discussions for magic wavelengths for terahertz clock transitions are given in this article.

Frequency-stabilized diode laser at 780 nm with a continuously locked time over 100 h

Two extended-cavity diode lasers at 780 nm which are longtime frequency-stabilized to Rb87 saturated absorption signals are reported. A high-performance frequency-locking circuit module using a first-harmonic detection technique is designed and achieved. Two lasers are continuously frequency-stabilized for over 100 h in conventional laboratory condition. The Allan standard deviation of either laser is estimated to be 1.3×10 -11 at an integration time of 25 s. The system environment temperature drift is demonstrated to be the main factor affecting long-term stability of the stabilized lasers based on our correlation study between beat frequency and system environment temperature.

Resonant sequential scattering in two-frequency-pumping superradiance from a Bose-Einstein condensate

We study sequential scattering in superradiance from a Bose-Einstein condensate pumped by a two-frequency laser beam. We find that the distribution of atomic side modes presents highly different patterns for various frequency difference between the two pump components. A novel distribution is observed, with a frequency difference of eight times the recoil frequency. These observations reveal that the frequency overlap between the end-fire modes related to different side modes plays an essential role in the dynamics of sequential superradiant scattering. The numerical results from a semiclassical model qualitatively agree with our observations.

Effective preparation and collisional decay of atomic condensates in excited bands of an optical lattice

We present a method for the effective preparation of a Bose-Einstein condensate (BEC) into the excited bands of an optical lattice via a standing-wave pulse sequence. With our method, the BEC can be prepared in either a single Bloch state in an excited band or a coherent superposition of states in different bands. Our scheme is experimentally demonstrated by preparing a 87 Rb BEC into the d band and the superposition of s -a ndd-band states of a one-dimensional optical lattice, within a few tens of microseconds. We further measure the decay of the BEC in the d-band state and carry an analytical calculation for the collisional decay of atoms in the excited-band states. Our theoretical and experimental results agree well.

Critical correlations in an ultra-cold Bose gas revealed by means of a temporal Talbot–Lau interferometer

The transition from a thermal cloud to a Bose-Einstein condensate (BEC) in an interacting ultra-cold Bose gas is a prototype in a universality class of diverse phase transitions. For a trapped ultra-cold Bose gas, we were able to study the critical regime both above and below the critical temperature with a Talbot-Lau interferometer, observing a peak in the correlation length. From this peak, we managed to determine the universal critical exponents for this phase transition as well as the finite-size and interaction corrections to the critical temperature. The results are all in quantitative agreement with theory. This work demonstrates the potential application of the Talbot-Lau interferometer to a wide range of critical phase transitions in ultra-cold atomic gases.

Observation of diffraction phases in matter-wave scattering

We study the diffraction phase of different orders via the Dyson expansion series, for ultracold atomic gases scattered by a standing-wave pulse. As these diffraction phases are not observable in a single pulse scattering process, a temporal Talbot-Lau interferometer consisting of two standing-wave pulses is demonstrated experimentally with a Bose-Einstein condensate to explore this physical effect. The role of the diffraction phases is clearly shown by the second standing-wave pulse in the relative population of different momentum states. Our experiments demonstrate obvious effects beyond the Raman-Nath method, while agree well with our theory by including the diffraction phases. In particular, the observed asymmetry in the dependence of the relative population on the interval between two standing-wave pulses reflects the diffraction phase differences. The role of interatomic interaction in the Talbot-Lau interferometer is also discussed.

Rapid nonadiabatic loading in an optical lattice

We present a scheme for nonadiabatically loading a Bose-Einstein condensate into the ground state of a one-dimensional optical lattice within a few tens of microseconds, i.e., typically in less than half the Talbot period. This technique of coherent control is based on sequences of pulsed perturbations, and the experimental results demonstrate its feasibility and effectiveness. As the loading process is much shorter than the traditional adiabatic loading time scale, this method may find many applications.

Detection of saturated absorption spectroscopy at high sensitivity with displaced crossovers

We present an unconventional experimental approach for detecting saturated absorption spectroscopy. Using this approach, crossover peaks are displaced, leaving out peaks corresponding to an atoms natural resonant frequencies. Sensitivity of detection can also be enhanced. Consequently, the spectrum could reflect the energy structure of atoms more explicitly. Without harmful influence from crossovers, the locking range of the error signal is significantly increased and the symmetry of the dispersion line shape is perfectly preserved, so reliability of frequency stabilization can be improved.

Imprinting Light Phase on Matter Wave Gratings in Superradiance Scattering

Superradiance scattering from a Bose-Einstein condensate is studied with a two-frequency pump- ing beam. We demonstrate the possibility of fully tuning the backward mode population as a function of the locked initial relative phase between the two frequency components of the pump- ing beam. This result comes from an imprinting of this initial relative phase on two matter wave gratings, formed by the forward mode or backward mode condensate plus the condensate at rest, so that cooperative scattering is affected. A numerical simulation using a semiclassical model agrees with our observations.

Cooperative atomic scattering of light from a laser with a colored noise spectrum

The collective atomic recoil lasing is studied for an ultracold and collisionless atomic gas in a partially coherent pump with a colored noise. Compared to white noise, correlations in colored noise are found to be able to greatly enhance or suppress the growth rate above or below a critical detuning. Effects on cooperative scattering of light for noise correlation time, noise intensity, and pump-probe detuning are discussed. This result is consistent with our simulation and linear analysis about the evolution equations in the regions of instability.