We analyze a method of compressing a cloud of cold atoms by dynamic control of a far off-resonance optical lattice. We show that by reducing the lattice spacing either continuously or in discrete steps while cooling the atoms with optical molasses large compression factors can be achieved. Particle motion in the time-dependent lattice is studied numerically using a three-dimensional semiclassical model. Two experimentally realistic models are analyzed. In the first we continuously vary the lattice beam angles to compress atoms initially in a Gaussian distributed cloud with standard deviation of 250 µm into a single site of a two-dimensional lattice of area A ∼ 35 × 35λ2, with λ the wavelength of the lattice beams. This results in an optical depth for an on-resonant probe beam >80 which is an increase by a factor of about 1800 compared to the uncompressed cloud. In the second approach we use a discrete set of lattice beam angles to decrease the spatial scale of the cloud by a factor of 500, and localize a few atoms to a single lattice site with an area A ≲ λ2.
We analyze a method of compressing a cloud of cold atoms by dynamic control of a far off-resonance optical lattice. We show that by reducing the lattice spacing either continuously or in discrete steps while cooling the atoms with optical molasses large compression factors can be achieved. Particle motion in the time-dependent lattice is studied numerically using a three-dimensional semiclassical model. Two experimentally realistic models are analyzed. In the first we continuously vary the lattice beam angles to compress atoms initially in a Gaussian distributed cloud with standard deviation of 250 µm into a single site of a two-dimensional lattice of area A ∼ 35 × 35λ2, with λ the wavelength of the lattice beams. This results in an optical depth for an on-resonant probe beam >80 which is an increase by a factor of about 1800 compared to the uncompressed cloud. In the second approach we use a discrete set of lattice beam angles to decrease the spatial scale of the cloud by a factor of 500, and localize a few atoms to a single lattice site with an area A ≲ λ2.
We model the efficiency of loading atoms of various species into a one-dimensional optical lattice from a cold ensemble, taking into account the initial cloud temperature and size, the lattice laser properties affecting the trapping potential, and the atomic parameters. Stochastic sampling and dynamical evolution are used to simulate the transfer, leading to estimates of transfer efficiency for varying trap depth and profile. Tracing the motion of the atoms also enables the evaluation of the equilibrium temperature and site occupancy in the lattice. The simulation compares favorably against a number of experimental results and is used to compute an optimum lattice-waist-to-cloud-radius ratio for a given optical power.
We analyze the light-induced atom–atom interactions in optically thick atomic clouds and show that, when the laser frequency is on-resonance with the atomic transition, they become attractive. On the basis of this analysis we propose and demonstrate a novel scheme to compress a cold and dense atomic cloud with a short on-resonance laser pulse. The compression force arises from attenuation of the laser light by the atomic cloud. The following free propagation of the atoms shows a lenslike behavior that yields a transient density increase at the focal time, where neither laser nor magnetic field perturbations exist. A cooling pulse, which is applied at the focal time of this lens, restores the initial temperature of atoms, and hence the phase space density is increased. Finally, we adopt our compression scheme to a quasi-steady-state mode by temporally chopping it with the cooling and trapping beams of a magnet-optical trap.
We propose a mechanism for coherent densification in a tilted optical lattice. The starting point is a well-known property of Wannier-Stark ladders: a homogeneous resonant coupling between neighboring Wannier-Stark states restores coherent translation. When the coupling amplitude is position-dependent and has a node, the local transport velocity vanishes at the node. A single wave packet is thereby transported toward this point, while a cloud distributed around the node undergoes a reduction of its spatial extent. The aim is to provide a reversible density-redistribution tool in a lattice of fixed period, complementary to matter-wave lenses, accordion lattices, box traps, and coherent-transport protocols. We introduce a nearest-neighbor effective model in which the coupling varies linearly with the site index. This model admits an exact analytic solution via a generating function and the method of characteristics; in the continuum limit it predicts exponential contraction of distances to the node. We discuss a primary implementation based on a spatially shaped Raman or Bragg coupling, and an alternative based on temporal modulation of the lattice depth. Estimates for cesium indicate that densification times ranging from a few tens to a few hundreds of milliseconds are achievable with realistic parameters. The mechanism is not dissipative cooling, but a coherent compression step that can be halted, reversed, or followed by a controlled expansion stage.
A complete laser cooling setup was built, with focus on threedimensional near-resonant optical lattices for cesium. These consist of regularly ordered micropotentials, created by the interference of four laser beams. One key feature of optical lattices is an inherent ”Sisyphus cooling” process. It efficiently extracts kinetic energy from the atoms, leading to equilibrium temperatures of a few µK. The corresponding kinetic energy is lower than the depth of the potential wells, so that atoms can be trapped. We performed detailed studies of the cooling processes in optical lattices by using the time-of-flight and absorption-imaging techniques. We investigated the dependence of the equilibrium temperature on the optical lattice parameters, such as detuning, optical potential and lattice geometry. The presence of neighbouring transitions in the cesium hyperfine level structure was used to break symmetries in order to identify, which role “red” and “blue” transitions play in the cooling. We also examined the limits for the cooling process in optical lattices, and the possible difference in steady-state velocity distributions for different directions. Moreover, in collaboration with ´Ecole Normale Sup´erieure in Paris, numerical simulations were performed in order to get more insight in the cooling dynamics of optical lattices. Optical lattices can keep atoms almost perfectly isolated from the environment and have therefore been suggested as a platform for a host of possible experiments aimed at coherent quantum manipulations, such as spin-squeezing and the implementation of quantum logic-gates. We developed a novel way to trap two different cesium ground states in two distinct, interpenetrating optical lattices, and to change the distance between sites of one lattice relative to sites of the other lattice. This is a first step towards the implementation of quantum simulation schemes in optical lattices.
An innovative and practical scheme of building far-detuned optical lattice for 87Rb atoms is proposed.The disposals of aligning the lattice beams,tuning the lattice frequency and controlling the tapered amplifier for output are described in detail.Alignment of optical lattices is quite difficult in principle,for several beams are required to hit the same atomic cloud.For the relatively near-detuned one-and two-dimensional lattices,the coarse alignment is accomplished by tuning the lattice laser onto resonance with the magnetic-optic trap(MOT) frequency,and then blowing away the MOT in real time.A more precision alignment is implemented at the end of the MOT loading,the atoms are first pumped into the lower hyperfine level by turning off the repumping for some time;then,the pulsed lattice beams are turned on for a short time at some reasonably large detuning.Finally,a fluorescent image of the MOT is taken without repumping,in order to detect only those atoms which are repumped by the lattice laser.For the purpose of controlling the detuning of the lattice easily and accurately,a home-made grating wavemeter with a resolution better than 1 GHz is used.This way allows the laser to be locked at any frequency by using a software PID and is experimentally simple to implement.The intensity of the lattice is controlled directly by pulsing the current through the tapered amplifier using a function generator and a laser diode driver.This technique has already been demonstrated before by Prof.M.Kasevich's group at Stanford. Our experiment starts with a MOT capturing approximately 4107 atoms in 200 ms.The lattice loading is overlap with the end of polarization gradient cooling(PGC),after that,the molasses laser beams are extinguished, and the adiabatic expansion is accomplished in the same time by a decrease in the lattice light intensity according to release function.On the basis of MOT and PGC,the dependences of atomic loading on such parameters as the intensity and frequency detuning of optical lattice are investigated experimentally.The vibration frequency is measured by intentionally modulating the trap intensity.Experimental results show that the lattice structure facilitates the cooling with the temperature of atoms cloud being reduced to 1/3 compared with free space polarization gradient cooling.The system design,experimental results and conclusions are of definite significance and can serve as a fine reference for other kinds of lattices designs or alkali atomic plans.
We characterize the semiclassical dynamics of dilute thermal atom clouds located in three-dimensional optical lattices generated by stationary optical Bessel beams. The dynamics of the cold atoms is explored in the quasi-Hamiltonian regime that arises using laser beams with far-off resonance detuning. Although the transverse structure of Bessel beams exhibits a complex topological structure, it is found that the longitudinal motion along the main propagation axis of the beam is the detonator of a high sensitivity of the atoms' motion to the initial conditions. This effect would not be properly described by bidimensional models. We show that an experimental implementation can be highly simplified by an analysis of the behaviour of the dynamical system under scale transformations. Experimentally feasible signatures of the chaotic dynamics of the atom clouds are also identified.