用于成像超冷费米原子气的量子显微镜

为捕获于光阱中的冷原子拍照,需要配合使用一个快速“闪光”,将它们照亮。而不幸的是,闪光会使冷原子热起来,并将它们从光阱逐出。两个新实验绕过了这个问题,通过使用一组激光光束,在冷却原子的同时,成功地拍摄到一个个冷原子的空间占位。这类量子气显微镜用于成像费米冷原子,尚属首次。新实验为使用冷原子模拟强关联系统中的电子提供了机会,例如模拟高温超导体和超巨磁电阻材料。在过去的几年里,研究人员已经对光学晶格中的玻色原子实现了量子气显微镜成像。使用光频饴(光学粘胶)和其他技术, 在成像过程中保持原子气冷却。与玻色原子相比,费米原子不容易“上像”。这主要是因为,作为费米原子,如锂-6和钾-40,它们更难被冷却。但现在,两个研究组已经成功地摄取了冷费米原子的图像：Zwierlein和他在麻省理工学院的同事展示了他们针对钾原子气的成像技术;而哈佛大学的Greiner等则是完成了对锂原子的成像。     

Recombination of three ultracold fermionic atoms

Three-body recombination of identical, spin-polarized fermionic atoms in the ultracold limit is investigated using model interactions. The mechanisms for recombination are parametrized by the "scattering volume" V(p) and described in the framework of the adiabatic hyperspherical representation. We have calculated the recombination rate K3 as a function of V(p) and have found K3 proportional, variant |V(p)|(8/3) for small |V(p)|. Recombination near a two-body Feshbach resonance can thus be significant.     

Functional integral for ultracold fermionic atoms

We develop a functional integral formalism for ultracold gases of fermionic atoms. It describes the BEC–BCS crossover and involves both atom and molecule fields. Beyond mean field theory we include the fluctuations of the molecule field by the solution of gap equations. In the BEC limit, we find that the low temperature behavior is described by a Bogoliubov theory for bosons. For a narrow Feshbach resonance these bosons can be associated with microscopic molecules. In contrast, for a broad resonance the interaction between the atoms is approximately pointlike and microscopic molecules are irrelevant. The bosons represent now correlated atom pairs or composite “dressed molecules”. The low temperature results agree with quantum Monte Carlo simulations. Our formalism can treat with general inhomogeneous situations in a trap. For not too strong inhomogeneities the detailed properties of the trap are not needed for the computation of the fluctuation effects—they enter only in the solutions of the field equations.     

Scattering length for fermionic alkali atoms

We examine the use of the WKB approximation to determine the p-wave scatteringlength. For this we solve the p partial wave Schrödingerequation and analyse the validity of adopting the semiclassical solution toevaluate the constant factors in the solution. We also calculate the p-wavescattering lengths of ⁶Li and ⁴⁰K for the a³Σ⁺ _u andX¹Σ⁺ _g states respectively using the variable phase method. Thep-wave scattering lengths of ¹³²Cs and ¹³⁴Cs are also calculated.Based on our calculations, the value of the p-wave scattering lengths of⁶Li and ⁴⁰K are -36a_o and -95a_o respectively.     

Disorder physics in mixtures of fermionic atoms

A mixture of light and heavy fermionic atoms in an optical lattice is considered. Tunneling of the heavy atoms is neglected such that they are only subject to thermal fluctuations. There is a complex interplay between light and heavy atoms which results in an Ising-like distribution of heavy atoms with a first-order transition from homogeneous to staggered order. A second-order transition is possible between an ordered and a disordered phase of heavy atoms. Depending on the phase of heavy atoms, light atoms are scattered around and can propagate, diffuse, and localize. Due to correlations in the disordered phase of heavy atoms, there can be a gap in the spectrum of light atoms, leading to a Mottlike incompressible state.     

Molecules of fermionic atoms in an optical lattice

We create molecules from fermionic atoms in a three-dimensional optical lattice using a Feshbach resonance. In the limit of low tunneling, the individual wells can be regarded as independent three-dimensional harmonic oscillators. The measured binding energies for varying scattering length agree excellently with the theoretical prediction for two interacting atoms in a harmonic oscillator. We demonstrate that the formation of molecules can be used to measure the occupancy of the lattice and perform thermometry.     

High-temperature superfluidity of fermionic atoms in optical lattices

Fermionic atoms confined in a potential created by standing wave light can undergo a phase transition to a superfluid state at a dramatically increased transition temperature. Depending upon carefully controlled parameters, a transition to a superfluid state of Cooper pairs, antiferromagnetic states or d-wave pairing states can be induced and probed under realistic experimental conditions. We describe an atomic physics experiment that can provide critical insight into the origin of high-temperature superconductivity in cuprates.     

Resonator-enhanced optical dipole trap for fermionic lithium atoms

We demonstrate a novel optical dipole trap that is based on enhancement of the optical power density of a Nd:YAG laser beam in a resonator. The trap is particularly suited for experiments with ultracold gases, as it combines a potential depth of the order of 1 mK with storage times of several tens of seconds. We study the interactions in a gas of fermionic lithium atoms in our trap and observe the influence of spin-changing collisions and off-resonant photon scattering. A key element in reaching long storage times is the use of an ultralow-noise laser. The dependence of storage time on laser noise is investigated.     

Weakly bound states in heterogeneous waveguides

We study the spectrum of the Helmholtz equation in a two-dimensional infinite waveguide,containing a weak heterogeneity localized at an internal point, and obeying Dirichletboundary conditions at its border. We use the variational theorem to derive the conditionfor which the lowest eigenvalue of the spectrum falls below the continuum threshold and abound state appears, localized at the heterogeneity. We devise a rigorous perturbationscheme and derive the exact expression for the energy to third order in theheterogeneity.     

Confinement and superfluidity in one-dimensional degenerate fermionic cold atoms

The physical properties of arbitrary half-integer spins F = N - (1/2) fermionic cold atoms trapped in a one-dimensional optical lattice are investigated by means of a low-energy approach. Two different superfluid phases are found for F > or = (3/2) depending on whether a discrete symmetry is spontaneously broken or not: an unconfined BCS pairing phase and a confined molecular-superfluid instability made of 2N fermions. We propose an experimental distinction between these phases for a gas trapped in an annular geometry. The confined-unconfined transition is shown to belong to the Z(N) generalized Ising universality class. We discuss the possible Mott phases at (1/2) filling.     

Mixtures of fermionic atoms with mass imbalance in optical lattices

We study ground-state properties of ultracold fermionic mixtures with strong mass imbalance in one and two-dimensional optical lattices through large scale numerical simulations of the attractive Falicov-Kimball model in harmonic confining potentials. In the one-dimensional case, we observe a formation of insulating atomic-density-wave domains at low particle fillings and a coexistence of insulating and metallic domains at intermediate and large particle fillings. Moreover, we show how the formation of metallic regions is reflected in the momentum distribution of the light atoms. In two dimensions, we find a rich spectrum of density-wave patterns including the homogeneous distributions, the axial striped distributions, the labyrinthine phases as well as the segregated phases.      

费米环境中两个三能级原子系统的纠缠演化

利用负性纠缠度(negativity)研究了两个三能级原子系统在费米环境中的纠缠演化问题-结果表明,两个三能级原子系统的纠缠演化不仅依赖于系统和环境的相互作用强度,而且还依赖于系统所处的具体量子态-通过例子发现,系统和环境相互作用强度越大,纠缠衰减越快;对于纯态,仅当时间趋于无穷时纠缠才被完全破坏;对于混态,则在有限的时间内纠缠即被彻底破坏-通过一般的分析找到了一类免退相干的量子子空间-在这些子空间中,量子态不受环境的影响,故其纠缠不变-研究有助于理解费米环境造成的退相干对玻色系统纠缠的影响- 关键词： 费米环境 纠缠演化 两个三能级原子     

Fiftyfold improvement in the number of quantum degenerate fermionic atoms

We have produced a quantum degenerate 6Li Fermi gas with up to 7 x 10(7) atoms, an improvement by a factor of 50 over all previous experiments with degenerate Fermi gases. This was achieved by sympathetic cooling with bosonic 23Na in the F=2, upper hyperfine ground state. We have also achieved Bose-Einstein condensation of F=2 sodium atoms by direct evaporation.     

Ground state of alkaline-earth fermionic atoms in one-dimensional optical lattices

The properties of the wave transmission in the lossy single-negative (SNG) metamaterials are experimentally investigated. The model structure of lossy epsilon-negative (ENG) monolayer and SNG bilayer consisting of a lossy ENG material and a mu-negative (MNG) are considered in this work. Simulation and experimental results show that the transmittance of the lossy SNG materials can be enhanced by two approaches, increasing dissipation coefficient and increasing the thickness of the lossy SNG. The lossy ENG material is physically fabricated by using composite right- and left-handed transmission lines grafted with radiation unit cell. The experimental results are in very good agreement with the previous theoretical analysis.     

Condensation of pairs of fermionic atoms near a Feshbach resonance

We have observed Bose-Einstein condensation of pairs of fermionic atoms in an ultracold 6Li gas at magnetic fields above a Feshbach resonance, where no stable 6Li2 molecules would exist in vacuum. We accurately determined the position of the resonance to be 822+/-3 G. Molecular Bose-Einstein condensates were detected after a fast magnetic field ramp, which transferred pairs of atoms at close distances into bound molecules. Condensate fractions as high as 80% were obtained. The large condensate fractions are interpreted in terms of preexisting molecules which are quasistable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.     

Pure gas of optically trapped molecules created from fermionic atoms

We report on the production of a pure sample of up to 3 x 10(5) optically trapped molecules from a Fermi gas of 6Li atoms. The dimers are formed by three-body recombination near a Feshbach resonance. For purification, a Stern-Gerlach selection technique is used that efficiently removes all trapped atoms from the atom-molecule mixture. The behavior of the purified molecular sample shows a striking dependence on the applied magnetic field. For very weakly bound molecules near the Feshbach resonance, the gas exhibits a remarkable stability with respect to collisional decay.     

Dissipation-induced d-wave pairing of fermionic atoms in an optical lattice

We show how dissipative dynamics can give rise to pairing for two-component fermions on a lattice. In particular, we construct a parent Liouvillian operator so that a BCS-type state of a given symmetry, e.g., a d-wave state, is reached for arbitrary initial states in the absence of conservative forces. The system-bath couplings describe single-particle, number-conserving and quasilocal processes. The pairing mechanism crucially relies on Fermi statistics. We show how such Liouvillians can be realized via reservoir engineering with cold atoms representing a driven dissipative dynamics.     

Mott transition of fermionic atoms in a three-dimensional optical trap

We study theoretically the Mott metal-insulator transition for a system of fermionic atoms confined in a three-dimensional optical lattice and a harmonic trap. We describe an inhomogeneous system of several thousand sites using an adaptation of dynamical mean-field theory solved efficiently with the numerical renormalization group method. Above a critical value of the on-site interaction, a Mott-insulating phase appears in the system. We investigate signatures of the Mott phase in the density profile and in time-of-flight experiments.