A cesium gas strongly confined in one dimension : sideband cooling and collisional properties

We study one-dimensional sideband cooling of Cesium atoms strongly confined in a far-detuned optical lattice. The Lamb-Dicke regime is achieved in the lattice direction whereas the transverse confinement is much weaker. The employed sideband cooling method, first studied by Vuletic et al.\cite{Vule98}, uses Raman transitions between Zeeman levels and produces a spin-polarized sample. We present a detailed study of this cooling method and investigate the role of elastic collisions in the system. We accumulate $83(5)$ of the atoms in the vibrational ground state of the strongly confined motion, and elastic collisions cool the transverse motion to a temperature of $2.8 \mu$K=$0.7 \hbar\omega_{\rm osc}/k_{\rm B}$, where $\omega_{\rm osc}$ is the oscillation frequency in the strongly confined direction. The sample then approaches the regime of a quasi-2D cold gas. We analyze the limits of this cooling method and propose a dynamical change of the trapping potential as a mean of cooling the atomic sample to still lower temperatures. Measurements of the rate of thermalization between the weakly and strongly confined degrees of freedom are compatible with the zero energy scattering resonance observed previously in weak 3D traps. For the explored temperature range the measurements agree with recent calculations of quasi-2D collisions\cite{Petr01}. Transparent analytical models reproduce the expected behavior for $k_{\rm B}T \gg \hbar \omega_{\rm osc}$ and also for $k_{\rm B}T \ll \hbar \omega_{\rm osc}$ where the 2D features are prominent.Comment: 18 pages, 12 figure

Doping Dependence of Spin Dynamics in Electron-Doped Ba(Fe1-xCox)2As2

The spin dynamics in single crystal, electron-doped Ba(Fe1-xCox)2As2 has been investigated by inelastic neutron scattering over the full range from undoped to the overdoped regime. We observe damped magnetic fluctuations in the normal state of the optimally doped compound (x=0.06) that share a remarkable similarity with those in the paramagnetic state of the parent compound (x=0). In the overdoped superconducting compound (x=0.14), magnetic excitations show a gap-like behavior, possibly related to a topological change in the hole Fermi surface (Lifshitz transition), while the imaginary part of the spin susceptibility prominently resembles that of the overdoped cuprates. For the heavily overdoped, non-superconducting compound (x=0.24) the magnetic scattering disappears, which could be attributed to the absence of a hole Fermi-surface pocket observed by photoemission.Comment: 6 pages, 5 figures, published versio

Atomic matter wave scanner

We report on the experimental realization of an atom optical device, that allows scanning of an atomic beam. We used a time-modulated evanescent wave field above a glass surface to diffract a continuous beam of metastable Neon atoms at grazing incidence. The diffraction angles and efficiencies were controlled by the frequency and form of modulation, respectively. With an optimized shape, obtained from a numerical simulation, we were able to transfer more than 50% of the atoms into the first order beam, which we were able to move over a range of 8 mrad.Comment: 4 pages, 4 figure

Doping-Dependent and Orbital-Dependent Band Renormalization in Ba(Fe_1-xCo_x)_2As_2 Superconductors

Angle resolved photoemission spectroscopy of Ba(Fe1-xCox)2As2 (x = 0.06, 0.14, and 0.24) shows that the width of the Fe 3d yz/zx hole band depends on the doping level. In contrast, the Fe 3d x^2-y^2 and 3z^2-r^2 bands are rigid and shifted by the Co doping. The Fe 3d yz/zx hole band is flattened at the optimal doping level x = 0.06, indicating that the band renormalization of the Fe 3d yz/zx band correlates with the enhancement of the superconducting transition temperature. The orbital-dependent and doping-dependent band renormalization indicates that the fluctuations responsible for the superconductivity is deeply related to the Fe 3d orbital degeneracy.Comment: 5 pages, 4 figure

Ground state laser cooling using electromagnetically induced transparency

A laser cooling method for trapped atoms is described which achieves ground state cooling by exploiting quantum interference in a driven Lambda-shaped arrangement of atomic levels. The scheme is technically simpler than existing methods of sideband cooling, yet it can be significantly more efficient, in particular when several motional modes are involved, and it does not impose restrictions on the transition linewidth. We study the full quantum mechanical model of the cooling process for one motional degree of freedom and show that a rate equation provides a good approximation.Comment: 4 pages, 3 figures; v2: minor modifications to abstract, text and figure captions; v3: few references added and rearranged; v4: One part significantly changed, 1 figure removed, new equations; v5: typos corrected, to appear in PR

Bose-Einstein condensation in quasi2D trapped gases

We discuss BEC in (quasi)2D trapped gases and find that well below the transition temperature $T_c$ the equilibrium state is a true condensate, whereas at intermediate temperatures $T<T_c$ one has a quasicondensate (condensate with fluctuating phase). The mean-field interaction in a quasi2D gas is sensitive to the frequency $\omega_0$ of the (tight) confinement in the "frozen" direction, and one can switch the sign of the interaction by changing $\omega_0$. Variation of $\omega_0$ can also reduce the rates of inelastic processes, which opens prospects for tunable BEC in trapped quasi2D gases.Comment: 4 revtex pages, 1 figure, text is revised, figure improve

Reconstruction of motional states of neutral atoms via MaxEnt principle

We present a scheme for a reconstruction of states of quantum systems from incomplete tomographic-like data. The proposed scheme is based on the Jaynes principle of Maximum Entropy. We apply our algorithm for a reconstruction of motional quantum states of neutral atoms. As an example we analyze the experimental data obtained by the group of C. Salomon at the ENS in Paris and we reconstruct Wigner functions of motional quantum states of Cs atoms trapped in an optical lattice

Optics with an Atom Laser Beam

We report on the atom optical manipulation of an atom laser beam. Reflection, focusing and its storage in a resonator are demonstrated. Precise and versatile mechanical control over an atom laser beam propagating in an inhomogeneous magnetic field is achieved by optically inducing spin-flips between atomic ground states with different magnetic moment. The magnetic force acting on the atoms can thereby be effectively switched on and off. The surface of the atom optical element is determined by the resonance condition for the spin-flip in the inhomogeneous magnetic field. A mirror reflectivity of more than 98% is measured

Wave Packet Echoes in the Motion of Trapped Atoms

We experimentally demonstrate and systematically study the stimulated revival (echo) of motional wave packet oscillations. For this purpose, we prepare wave packets in an optical lattice by non-adiabatically shifting the potential and stimulate their reoccurence by a second shift after a variable time delay. This technique, analogous to spin echoes, enables one even in the presence of strong dephasing to determine the coherence time of the wave packets. We find that for strongly bound atoms it is comparable to the cooling time and much longer than the inverse of the photon scattering rate

Cooling atomic motion with quantum interference

We theoretically investigate the quantum dynamics of the center of mass of trapped atoms, whose internal degrees of freedom are driven in a $\Lambda$-shaped configuration with the lasers tuned at two-photon resonance. In the Lamb-Dicke regime, when the motional wave packet is well localized over the laser wavelenght, transient coherent population trapping occurs, cancelling transitions at the laser frequency. In this limit the motion can be efficiently cooled to the ground state of the trapping potential. We derive an equation for the center-of-mass motion by adiabatically eliminating the internal degrees of freedom. This treatment provides the theoretical background of the scheme presented in [G. Morigi {\it et al}, Phys. Rev. Lett. {\bf 85}, 4458 (2000)] and implemented in [C.F. Roos {\it et al}, Phys. Rev. Lett. {\bf 85}, 5547 (2000)]. We discuss the physical mechanisms determining the dynamics and identify new parameters regimes, where cooling is efficient. We discuss implementations of the scheme to cases where the trapping potential is not harmonic.Comment: 11 pages, 3 figure