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We theoretically explore the possibility to detect weak localization of light in a hot atomic vapor, where one usually expects the fast thermal motion of the atoms to destroy any interference in multiple scattering. To this end, we compute the coherent backscattering peak, assuming high temperature and taking into account the quantum level structure of the atomic scatterers. It is found that the decoherence due to thermal motion can be partially counterbalanced by working at large laser detuning and using small atomic cells with an elongated geometry. Under these conditions, our estimates suggest that weak localization in a hot vapor should be within reach of experimental detection., Comment: 11 pages, 5 figures

We study the widths of interspecies Feshbach resonances in a mixture of the fermionic quantum gases $^{6}\mathrm{Li}$ and $^{40}\mathrm{K}$. We develop a model to calculate the width and position of all available Feshbach resonances for a system. Using the model, we select the optimal resonance to study the $^{6}\mathrm{Li}/^{40}\mathrm{K}$ mixture. Experimentally, we obtain the asymmetric Fano line shape of the interspecies elastic cross section by measuring the distillation rate of $^{6}\mathrm{Li}$ atoms from a potassium-rich $^{6}\mathrm{Li}/^{40}\mathrm{K}$ mixture as a function of magnetic field. This provides us with the first experimental determination of the width of a resonance in this mixture, $\ensuremath{\Delta}B=1.5(5)\text{ }\text{ }\mathrm{G}$. Our results offer good perspectives for the observation of universal crossover physics using this mass-imbalanced fermionic mixture.

We report on the manipulation of the center-of-mass motion (`sloshing') of a Bose Einstein condensate in a time-averaged orbiting potential (TOP) trap. We start with a condensate at rest in the center of a static trapping potential. When suddenly replacing the static trap with a TOP trap centered about the same position, the condensate starts to slosh with an amplitude much larger than the TOP micromotion. We show, both theoretically and experimentally, that the direction of sloshing is related to the initial phase of the rotating magnetic field of the TOP. We show further that the sloshing can be quenched by applying a carefully timed and sized jump in the phase of the rotating field., 11 pages, 9 figures

We study the widths of interspecies Feshbach resonances in a mixture of the fermionic quantum gases 6Li and 40K. We develop a model to calculate the width and position of all available Feshbach resonances for a system. Using the model, we select the optimal resonance to study the {6}Li/{40}K mixture. Experimentally, we obtain the asymmetric Fano line shape of the interspecies elastic cross section by measuring the distillation rate of 6Li atoms from a potassium-rich 6Li/{40}K mixture as a function of magnetic field. This provides us with the first experimental determination of the width of a resonance in this mixture, DeltaB=1.5(5) G. Our results offer good perspectives for the observation of universal crossover physics using this mass-imbalanced fermionic mixture.

We report on the observation of Feshbach resonances in an ultracold mixture of two fermionic species, 6Li and 40K. The experimental data are interpreted using a simple asymptotic bound state model and full coupled channels calculations. This unambiguously assigns the observed resonances in terms of various s- and p-wave molecular states and fully characterizes the ground-state scattering properties in any combination of spin states.

We report on the observation of Feshbach resonances in an ultracold mixture of two fermionic species, (6)Li and (40)K. The experimental data are interpreted using a simple asymptotic bound state model and full coupled channels calculations. This unambiguously assigns the observed resonances in terms of various s- and p-wave molecular states and fully characterizes the ground-state scattering properties in any combination of spin states.

We demonstrate an interference method to determine the low-energy elastic scattering amplitudes of a quantum gas. We linearly accelerate two ultracold atomic clouds up to energies of 1.2 mK and observe the collision halo by direct imaging in free space. From the interference between s- and d- partial waves in the differential scattering pattern we extract the corresponding phase shifts. The method does not require knowledge of the atomic density. This allows us to infer accurate values for the s- and d-wave scattering amplitudes from the zero-energy limit up to the first Ramsauer minimum using only the van der Waals C6 coefficient as theoretical input. For the 87Rb triplet potential, the method reproduces the scattering length with an accuracy of 6%.

We study hydrodynamic behavior in expanding thermal clouds of Rb-87 released from an elongated trap. At our highest densities the mean free path is smaller than the radial size of the cloud. After release the clouds expand anisotropically. The cloud temperature drops by as much as 30%. This is attributed to isentropic cooling during the early stages of the expansion. We present an analytical model to describe the expansion and to estimate the cooling. Important consequences for time-of-flight thermometry are discussed., Comment: 7 pages with 2 figures

We analyze the collisionless motion of atoms in magnetic traps in relation to evaporative cooling. For the example of a long Ioffe quadrupole trap we investigate both the regular and stochastic regimes of motion. We emphasize a strong influence of the regime of collisionless motion on the process of evaporative cooling. For evaporation of atoms across an axial potential barrier the stochastic motion of atoms at energies above the barrier enables three-dimensional evaporation, i.e., particles acquiring in elastic collisions a total enery higher than the barrier height ${\mathit{E}}_{0}$ escape from the trap. The regular motion leads to axial evaporation: only atoms which due to collisions acquire an axial energy higher than ${\mathit{E}}_{0}$ leave the trap. The rate of axial evaporation is smaller by a factor \ensuremath{\sim}\ensuremath{\eta}=${\mathit{E}}_{0}$/T\ensuremath{\gg}1, where T is the gas temperature. This has important consequences for evaporative cooling, which we discuss in relation to trapped atomic hydrogen. \textcopyright{}1996 The American Physical Society.

We analytically study the atom-dimer scattering problem in the near-integrable limit when the oscillator length l0 of the transverse confinement is smaller than the dimer size, ∼l02/|a|, where a

Atomic hydrogen is an important model system for the investigation of the properties of ultra‐cold gases. We summarize the current experimental situation for magnetically confined hydrogen, putting emphasis on recent optical experiments with Lyman‐α radiation. As the research is aimed at gas‐phase properties, the samples are studied at densities where interatomic collisions are frequent and internal thermal equilibrium may be assumed. Interestingly, although optically thick, the samples still may be cooled with light. We discuss both Doppler cooling and light induced evaporative cooling.

We demonstrate a two-dimensional magneto-optical trap (2D MOT) as a beam source for cold $^{6}\text{L}\text{i}$ atoms. The source is side loaded from an oven operated at temperatures in the range $600\ensuremath{\lesssim}T\ensuremath{\lesssim}700\text{ }\text{K}$. The performance is analyzed by loading the atoms into a three-dimensional MOT located 220 mm downstream from the source. The maximum recapture rate of $\ensuremath{\sim}{10}^{9}\text{ }{\text{s}}^{\ensuremath{-}1}$ is obtained for $T\ensuremath{\approx}700\text{ }\text{K}$ and results in a total of up to ${10}^{10}$ trapped atoms. The recaptured fraction is estimated to be $30\ifmmode\pm\else\textpm\fi{}10%$ and limited by beam divergence. The most-probable velocity in the beam $({\ensuremath{\alpha}}_{z})$ is varied from 18 to 70 m/s by increasing the intensity of a push beam. The source is quite monochromatic with a full width at half maximum velocity spread of 11 m/s at ${\ensuremath{\alpha}}_{z}=36\text{ }\text{m}/\text{s}$, demonstrating that side loading completely eliminates beam contamination by hot vapor from the oven. We identify depletion of the low-velocity tail of the oven flux as the limiting loss mechanism. Our approach is suitable for other atomic species.

Liquid helium covered surfaces are indispensable in experiments with atomic hydrogen gas (H) at low temperature (T ≲ 1 K). Helium is used because H does not dissolve in this liquid, it is chemically inert and offers the weakest concievable atom-surface interation to the H atoms. These properties enable confinement of H gas while surface adsorption is unimportant except at the lowest temperatures. Therefore helium films are very effective in supressing massive surface catalized recombination in which solid molecular hydrogen is formed.

Hydrogen atoms are very well suited to study elastic and inelastic collisions at very low energies. Atom‐atom collisions are being studied in the gas phase, where the lowest temperatures are achieved in magnetic traps. Atom‐surface collisions are studied by observing the reflection of a beam of H‐atoms from the surface of liquid Helium.

We studied the low-energy limit of atomsurface scattering and the phenomenon of quantum reflection by using hydrogen atoms and the surface of liquid helium. Measuring the specular reflectivity of a liquid-helium-covered hemispheric mirror for a beam of hydrogen atoms we obtained information on elastic and inelastic scattering phenomena. At 100 mK, reflectivities larger than 90% were achieved; this can be used to demonstrate atomic beam focusing. Approximately three-fourths of the atoms escaping from a small orifice in a cell were reflected back into the cell. The dominant inelastic processes at work here are surface absorption and direct inelastic scattering over small angles under excitation of surface ripplons (quantized capillary waves). Helium-covered surfaces maybe useful for atomic-beam manipulation of atomic hydrogen.

We have used a heater and bolometer to create and detect ballistic heat pulses of spin-polarized hydrogen atoms (Harrow-down). Energy can be effectively coupled from helium-covered walls to the Harrow-down gas without recombination. The energy loss coefficient of Harrow-down on /sup 4/He has been measured to be 0.2 +- 0.1. This technique can be considered as the generation of very-low-velocity atomic beams of Harrow-down, down to 200 mK.

Pumping experiments were carried out in atomic hydrogen using an ESR technique. A model of the decay of the four hyperfine states in the presence of a moderately strong resonant microwave field is described and compared with experimental results. The results indicate that because of spin-exchange relaxation, it is difficult to influence the nuclear polarization of the lower hyperfine states by means of ESR pumping. The model is applied to the design of an ESR-operated beam source of spin-polarized atomic hydrogen. We estimate that in our experiments the maximum flux of particles ejected from the source was 5\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$.

The possibilities for observation and the properties are discussed for two-dimensional spin waves in a nondegenerate spin-polarized atomic hydrogen gas adsorbed on a superfluid helium film. We present results for the spin-transport parameters D0 and µ, based on two-dimensional effective-range theory. The spin-wave quality factor is an order of magnitude smaller than in the volume case.

We have calculated the spin-exchange two-body rate constants associated with the population dynamics of the hyperfine levels of atomic deuterium as a function of magnetic field in the Boltzmann zero-temperature limit. Results indicate that a gas of low-field--seeking deuterium atoms trapped in a static magnetic field minimum decays rapidly into an ultrastable gas of doubly spin-polarized deuterium. We also discuss the temperature dependence of various effects.

Atomic hydrogen (H) has recently been created in a long‐lived state referred to as spin‐polarized atomic hydrogen (H↓). A moderate density gas (n≃1017 atoms/ cc) has been produced at temperatures T≃400 mK. H↓ is predicted to be the only atomic substance that remains gaseous to T = 0 K; current experiments are consistent with this picture. Since the H↓ atoms behave as composite bosons the gas is expected to have a Bose‐Einstein condensation (BEC) and related superfluidity at a sufficiently high density or low temperature. H↓ is expected to display spectacular static and dynamic properties. Since the gas is polarized, the dominant electronic magnetization, Me, is proportional to the atomic density. In an inhomogeneous magnetic field Me will also be highly inhomogeneous. BEC will be spatially localized and the static Me should be an identifying feature of this state. The low‐lying excitation spectrum should not only feature electronic and nuclear spin‐transitions, but should also have magnetic translational modes of the gas. The experimental technique for stabilization of H↓ as well as many of the predicted magnetic properties are discussed.

An extensive description is given of a special ESR spectrometer designed to perform electron-spin resonance in the (160--200)-GHz frequency range in spin-polarized atomic hydrogen in high magnetic fields. Results obtained in this experiment clearly demonstrate how a state of nuclear polarization is established by recombination under conditions of slow nuclear relaxation. A new method to measure the ratio K/sub a/a/K/sub a/b of two-body recombination-rate constants is introduced, yielding the result 2

We analyze and reinterpret experiments with electron- and nuclear-spin (''doubly'')-polarized hydrogen (Harrow-downX) in a cell with a very low concentration of magnetic impurities. The destabilizing magnetic relaxation due to remaining impurities (G/sub i//sup s/) is found to be suppressed by increasing the thickness of the helium film covering the walls of the cell. Using recent information on surface three-body recombination, we obtain good quantitative agreement with more recent experiments if we assume thermal gradients, up to ..delta..T/Tapprox. =0.1. We extract both the value and field dependence for the three-body surface recombination (L/sub 3//sup s/ = 1.5(2) x 10/sup -24/ cm/sup 4/ s/sup -1/ at B = 8 T). We also extract the magnetic field dependence of the impurity relaxation.

We have calculated the magnetic dipolar spin-relaxation rates of magnetically trapped doubly-spin-polarized atomic deuterium gas. The results are expressed in a simple closed formula describing the dipolar-relaxation-limited stability in the Boltzmann ($T\ensuremath{\gg}{T}_{F}$), the degeneracy ($T\ensuremath{\ll}{T}_{F}$), and the crossover ($T\ensuremath{\simeq}{T}_{F}$) regimes. In the degeneracy regime we typically find lifetimes of some hours. We also discuss the dipolar decay of magnetically trapped spin-polarized fermionic alkali-metal atoms.

By means of ESR in a high magnetic field the hyperfine states of a gas of spin-polarized atomic hydrogen are directly probed. This allows a direct determination of the spin-state populations and nuclear polarization. The unusual ESR line shape is attributed to field inhomogeneities. The temperature of the gas was determined by use of ESR with no indication of nonequilibrium due to Kapitza resistance. Recombination rates between hyperfine states could be determined.

We studied the scattering of low-energetic neutral hydrogen atoms from the surface of liquid helium by a capillary flow method at sub-Kelvin temperatures. With decreasing temperature a decreasing flow impedance is observed, which may be interpreted as arising from an increasing probability of elastic surface scattering. This behavior is compared with a model for atom-surface scattering. Emphasis is put on the quantum nature of the scattering process.

Publisher Summary This chapter reviews the properties of new quantum gases and properties of spin-polarized tritium. The hydrogen (H 2 ) atom with its single electron and proton, bearing a spinoff is the simplest and most abundant atom in the universe. The atomic species in a discharge is short lived and is recombined to form H 2 . The hydrogen maser, which operates on the zero-field hyperfine transitions of the hydrogen atom, has become the most stable time and frequency source in existence. The chapter discusses the properties of new quantum gases and some of the properties of spin-polarized tritium (Ti). These new quantum gases promise to have many new and exciting properties in the low-temperature, high-density regime of quantum degeneracy. The chapter also describes the theory of decay. Magnetic properties of the many-body gas concentrates on many-body phenomena on surfaces or in two-dimension. In the condensed state, hydrogen and its isotope deuterium provide physics with the two fundamental many-body quantum systems of nature, boson- and fermion-fluids. Because of its very light mass and weak interactions, a many-particle system of electron spin-polarized hydrogen is predicted to have the unique property that it will remain in the gaseous state at the absolute zero of temperature.

We developed a sensitive device to detect atomic hydrogen fluxes. The device is based on a measurement of the heating due to forced recombination of the atoms in the flux. We found that 60±10 % of the recombination heat is collected by the detector. The DC noise equivalent (1σ) flux Φ is 6×1010 at/s at a bandwidth of 0.4 Hz and a dynamic range of 1000 (Φmax=6×1013 at/s). The device may be used at ambient temperatures below 450 mK. We discuss possible improvements of the design.

We report on a calculation of the spin-exchange two-body rate constants associated with the population dynamics of the hyperfine levels of atomic deuterium as a function of magnetic field in the Boltzmann zero temperature limit. We find that a gas of low field seeking deuterium atoms trapped in a static magnetic field minimum decays rapidly into an ultra stable gas of doubly spin-polarized deuterium.

We have determined the adsorption energy of H\ensuremath{\downarrow} on $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ mixtures to be $\frac{{\ensuremath{\epsilon}}_{a}}{k}=0.34\ifmmode\pm\else\textpm\fi{}0.03$ K, essentially independent of $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ concentration. We observe a pure second-order decay of the density due to surface recombination with rate constant $\frac{{K}_{s}}{{T}^{\frac{1}{2}}}=1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ ${\mathrm{cm}}^{2}$ ${\mathrm{K}}^{\ensuremath{-}1/2}$ ${\mathrm{S}}^{\ensuremath{-}1}$ at $B=8$ T. The nuclear relaxation time ${T}_{1}$, predicted to be of order hours, is found to be of order seconds by an indirect measurement. H\ensuremath{\downarrow} remained gaseous down to $T=0.085$ K, the lowest temperature measured.