High-resolution spectroscopy with laser-cooled and trapped calcium atoms

We report on high-resolution spectroscopy with two different samples of calcium atoms, in a laser-cooled and deflected beam and in a magneto-optical trap. The atomic beam was excited by spatially separated laser fields. For spectroscopy with stored atoms in a magneto-optical trap we used a multiple-pulse excitation scheme. The resolution as low as 2.5 kHz was limited by residual frequency fluctuations of our dye-laser spectrometer. The results should allow to establish a frequency standard with a relative uncertainty below 10^–14.     

Improving Raman velocimetry of laser-cooled cesium atoms by spin-polarization

We study the performances of Raman velocimetry applied to laser-cooled, spin-polarized, cesium atoms. Atoms are optically pumped into the F = 4, m₄=0 ground-state Zeeman sublevel, which is insensitive to magnetic perturbations. High resolution Raman stimulated spectroscopy is shown to produce Fourier-limited lines, allowing, in realistic experimental conditions, atomic velocity selection to one-fiftieth of a recoil velocity.     

Enhancement of Transfer Efficiency of Cold Atoms Using an Optical Guiding Laser Beam

We transfer cold ^87 Rb atoms from a vapour cell chamber to a spatially separated UHV magneto-optical trap （MOT） with the assistance of a red-detuned optical guiding beam and a normal push beam. Efficient optical guiding of the cold atoms is observed within a small detuning window. A pulsed optical guiding beam enhances the transfer efficiency and hence allows us to collect more atoms in UHV MOT in a shorter time, which is favourable for our experiment of achieving Bose-Einstein condensates （BEC）. Besides the easy operation, another advantage of this optical guiding technique is also demonstrated such that slower atomic beams may be efficiently transferred along horizontal direction. This study is a direct application of the optical guiding technique as a powerful tool.     

Adiabatic Passage Based on the Calcium Active Optical Clock

We propose a new application of the optical adiabatic passage effect for the excitation of a thermal atomic beam, which will be used in the calcium active optical clock to produce population inversion. A comparison between the optical adiabatic passage effect and the Rabi π pulse is investigated, 99% of the calcium atoms in the atomic beam that has a wide velocity distribution will be excited to the upper state for population inversion using the adiabatic passage, while 76% at most will be excited to the excited state using the π pulse with suitable parameters.     

Localization of atoms in light fields: Optical molasses,adiabatic compression and squeezing

We present a theoretical study of the localization1 of atoms with an angular momentumJ _g=3 toJ _e=4 transition (e.g., chromium atoms) in quantized optical molasses created by two counterpropagating linearly polarized laser beams. We study the localization as a function of the potential depth, the angle between the polarizations and the interaction time with the molasses in the low-intensity limit, and discuss the possibility of adiabatic compression and squeezing of the atomic distribution.Dedicated to H. Walther on the occasion of his 60th birthday     

VUV spectroscopy of magnetically trapped atomic hydrogen

We discuss the experimental and theoretical aspects of absorption spectroscopy of cold atomic hydrogen gas in a magnetostatic trap using a pulsed narrow-band source (bandwidth 100 MHz) at the Lyman- wavelength (121.6 nm). A careful analysis of the measured absorption spectra enables us to determine non-destructively the temperature and the density of the trapped gas. The development of this diagnostic technique is important for future attempts to reach Bose-Einstein condensation in trapped atomic hydrogen.     

A Zeeman slower based on magnetic dipoles

A transverse Zeeman slower composed of an array of compact discrete neodymium magnets is considered. A simple and precise model of such a slower based on magnetic dipoles is developed. The theory of a general Zeeman slower is modified to include spatial nonuniformity of the slowing laser beam intensity due to its convergence and absorption by slowed atoms. The slower needs no high currents or water cooling and the spatial distribution of its magnetic field can be adjusted. In addition the slower provides a possibility to cool the slowed atoms transversally along the whole length of the slower. Such a slower would be ideal for transportable optical atomic clocks and their future applications in space physics.     

An optical conveyor belt for single neutral atoms

Using optical dipole forces we have realized controlled transport of a single or any desired small number of neutral atoms over a distance of a centimeter with sub-micrometer precision. A standing wave dipole trap is loaded with a prescribed number of cesium atoms from a magneto-optical trap. Mutual detuning of the counter-propagating laser beams moves the interference pattern, allowing us to accelerate and stop the atoms at preselected points along the standing wave. The transportation efficiency is close to 100%. This optical ‘single-atom conveyor belt’ represents a versatile tool for future experiments requiring deterministic delivery of a prescribed number of atoms on demand. Received: 6 July 2001 / Published online: 23 November 2001     

2D dark optical surface lattice with an efficient intensity-gradient cooling of atoms by evanescent wave interference

We propose a novel scheme to form a 2D dark optical surface lattice (DOSL) for cold atoms on the surface of the dense flint glass by using two sets of blue-detuned evanescent wave interference fields and a blue-detuned evanescent wave field. In the 2D DOSL, cold atoms will be trapped in the vicinity of minimum intensity and suffered the minimal light shift as well as the lowest coherence loss. The total potential and trap-depth of the individual optical micro-trap in the 2D DOSL are high enough to trap cold atoms (T = 120 μK) released from the standard magneto-optical trap (MOT), and atoms trapped in the 2D DOSL can be cooled to several μK with the efficient intensity-gradient Sisyphus cooling. The lattice constant of the DOSL can be controllable by changing the incident angles of lights.     

Cold atoms: A new medium for quantum optics

Laser-cooled and trapped cesium atoms have been used as a nonlinear medium in a nearly resonant cavity. A study of the semiclassical dynamics of the system was performed, showing bistability and instabilities. In the quantum domain, squeezing in a probe beam having interacted with this system was demonstrated.Dedicated to H. Walther on the occasion of his 60th birthday     

The dynamic Kingdon trap: A novel design for the storage and crystallization of laser-cooled ions

On the basis of analytical pseudo-potential calculations and extensive numerical simulations it is proved that the dynamic Kingdon trap is a working design for the permanent storage and the crystallization of laser-cooled ions.Dedicated to H. Walther on the occasion of his 60th birthday     

Nonlinear resonances and phase transitions of two-ion Coulomb clusters in a Paul trap: Calculations without laser cooling

The mechanism responsible for transitions of laser-cooled trapped ions from an ordered crystal state to an irregular cloud state has been discussed controversially. A numeric and analytic study of the relative motion of two trapped ions without laser cooling is performed and compared with the results of previous simulations involving the laser. It turns out that the system without laser, in spite of its simplicity, already exhibits a non-monotonic dependence of crystal stability on trap parameters, which is linked to the presence of low-order nonlinear resonances.     

Coherent excitation of Rydberg atoms in an ultracold gas

We demonstrate two schemes for the coherent excitation of Rydberg atoms in an ultracold gas of rubidium atoms employing the three-level ladder system 5S_1/2-5P_3/2-n?_j. In the first approach rapid adiabatic passage with pulsed laser fields yields Rydberg excitation probabilities of 90% in the center of the laser focus. In a second experiment two-photon Rydberg excitation with continuous-wave fields is applied which results in Rabi oscillations between the ground and Rydberg state. The experiments represent a prerequisite for the control of interactions in ultracold Rydberg gases and the application of ultracold Rydberg gases for quantum information processing.     

Three-dimensional spatial diffusion in optical molasses

We have studied the expansion of a small cloud of⁸⁵Rb atoms in three-dimensional optical molasses (lin lin and ⁺ – ^– configurations) and observed diffusive motion. We determined the spatial-diffusion coefficients for various laser intensities and detunings, and compared them (in the case of lin lin molasses) to values calculated from friction and momentum-diffusion coefficients of a one-dimensional (1D) theory of laser cooling. The predicted variations of the spatial-diffusion coefficient with laser intensity and detuning are in good qualitative agreement with the experimental data. We found that the minimal value observed experimentally, 6 × 10^–4 cm²/s, lies within a factor of 3 of the 1D theoretical minimum, , 26/M, whereM is the atomic mass.Dedicated to H. Walther on the occassion of his 60th birthday     

Nanoscale focusing of atoms with a pulsed standing wave

We have theoretically and experimentally investigated the focusing properties of a detuned pulsed standing wave onto a beam of neutral atoms. In close analogy to the continuous-wave situation the dipole force leads to a periodic focusing of atoms with a period of λ/2, provided an adiabatic condition is fulfilled. Pulsed laser light is conveniently converted to short wavelengths and hence offers advantages in the application of atom lithography with elements of technological interest having blue or UV resonance lines. Received: 6 October 1999 / Revised version: 3 February 2000 / Published online: 5 April 2000     

One- and Two-Dimensional Arrays of Double-Well Optical Traps for Cold Atoms or Molecules

We propose a novel scheme to form one- and two-dimensional arrays of double-well optical dipole traps for cold atoms (or molecules) by using an optical system composed of a binary π-phase grating and a lens illuminated by a plane light wave, and study the relationship between the maximum intensity Imax of each optical well (or the maximum trapping potential Umax for ^85Rb atoms) and the relative aperture β (= α/f) of the lens. We also calculate the intensity gradients of each optical well and their curvatures, and estimate the spontaneous photon-scattering rate of trapped atom in each well, including Rayleigh and Raman scattering rates. Our study shows that the proposed 1D and 2D arrays of double-well traps can be used to prepare 1D and 2D novel optical lattices with cold atoms (or molecules), or form an all-optically integrated atom optical chip, or even to realize an array of all-optical double-well atomic (or molecular) Bose-Einstein condensates by optical-potential evaporative cooling, and so on.     

An Enriched ^40K Source for Atomic Cooling

We have developed an enriched ⁴⁰K source used in ⁴⁰K--⁸⁷Rb atomic mixture cooling experiment. The enriched ⁴⁰K source is a home-made dispenser which releases ⁴⁰K atoms by the redox reaction between ⁴⁰K enriched KCl and calcium. It is efficient and easy to be made and used. We collect 10⁷ ～10^{8 40}K atoms in collection magneto-optical trap. With this dispenser, we perform a quantum degenerate Fermi gas experiment.     

Controllable Photonic Band Gap and Defect Mode in a 1D CO2-Laser Optical Lattice

We propose a new method to form a novel controfiable photonic crystal with cold atoms and study the photonic band gap （PBG） of an infinite 1D CO2-laser optical lattice of SSRb atoms under the condition of quantum coherence. A significant gap generated near the resonant frequency of the atom is founded and its dependence on physical parameters is also discussed. Using the eigenquation of defect mode, we calculate the defect mode when a defect is introduced into such a lattice. Our study shows that the proposed new method can be used to optically probe optical lattice in situ and to design some novel and controllable photonic crystals.     

System for precise measurement in inverse Raman spectroscopy

A system for precise measurement was developed for inverse Raman spectroscopy, using a cw argon laser and a pulsed dye laser pumped by a YAG laser. The frequency accuracy was assured by monitoring the frequency of both of the lasers with an iodine fluorescence cell or a Fabry-Perot etalon dynamically calibrated by a stabilized HeNe laser. The spectrometer system employed a digitally controlled mechanism to handle the complicated measurement procedure and hence to reduce the overall measurement time. The performance of the system was tested by measuring the CH₄ v₁ lines and the H₂ Q(1) line.     

Stability of two-ion crystals in the Paul trap: A comparison of exact and pseudopotential calculations

The stability of two-ion crystals in a Paul trap with a dc component in the quadrupole potential has been studied with the use of the monodromy matrix. The pseudopotential model predicts crystals with the ions at rest either along the trap axis or in the radial plane. The solutions of the full equations of motion disagree with the predictions of the pseudopotential model when the radial and axial secular frequencies are nearly degenerate: the crystal is either unstable (as first noted by Emmertet al.) or exists in a previously unanticipated configuration in which the ions lie at an angle to the trap axes. A bifurcation diagram near the edge of the crystalline stability range does not support a frequency-doubling route to chaos.Dedicated to H. Walther on the occasion of his 60th birthday