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Chinese Physics Letters

Controllable Magnetic Focusing of Cold Atoms on a Chip

Yang Liu, Yun Min, Yin Jian-Ping

Published Jun 28, 2006• 1 citation
Publisher

Abstract

We propose a new lens scheme to focus cold atoms by using a controllable inhomogeneous magnetic field from a square current-carrying wire fabricated on a chip. The spatial distributions of the magnetic field are calculated, and the results show that the generated magnetic field is a two-dimensional (2D) quadrupole one and can be used to focus cold atoms or a cold atomic beam. The dynamic processes of cold atoms passing through our square wire layout and its focusing properties are studied by using Monte Carlo simulations. Our study shows that the atomic clouds can be focused effectively by our magnetic lens scheme, and the focal length of the atomic lens and its radius of focused spot can be continuously changed by adjusting the current in the wires.

DOI
10.1088/0256-307x/23/7/012

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We propose a new lens scheme to focus cold atoms by using a controllable inhomogeneous magnetic field from a square current-carrying wire fabricated on a chip. The spatial distributions of the magnetic field are calculated, and the results show that the generated magnetic field is a two-dimensional (2D) quadrupole one and can be used to focus cold atoms or a cold atomic beam. The dynamic processes of cold atoms passing through our square wire layout and its focusing properties are studied by using Monte Carlo simulations. Our study shows that the atomic clouds can be focused effectively by our magnetic lens scheme, and the focal length of the atomic lens and its radius of focused spot can be continuously changed by adjusting the current in the wires.
A new scheme to realize a two-dimensional (2D) array of magnetic micro-lenses for a cold atomic beam, formed by an array of square current-carrying wires, is proposed. We calculate the spatial distributions of the magnetic fields from the array of current-carrying wires and the magnetic focusing potential for cold rubidium atoms, and study the dynamic focusing processes of cold atoms passing through the magnetic micro-lens array and its focusing properties by using Monte-Carlo simulations and trajectory tracing method. The result shows that the proposed micro-lens array can be used to focus effectively a cold atomic beam, even to load ultracold atoms or a BEC sample into a 2D optical lattice formed by blue detuned hollow beams.
An array of square current-carrying wires is proposed to construct a two-dimensional (2D) array of surface magneto-optical traps and Ioffe magnetic ones for realizing a 2D lattice of Bose-Einstein condensations (BECs) on an atom chip. Our study shows that when a vertical homogeneous bias magnetic field is only used, the wires will form a 2D array of quadrupole magnetic traps, which can be used to construct 2D lattice of surface magneto-optical micro-traps. While another horizontal homogeneous bias field is added simultaneously, the above 2D array of quadrupole micro-traps will be changed as an array of surface Ioffe micro-traps, which can be used to form a 2D magnetic lattice and then realize a 2D array of BECs. The dynamic loading process of cold (87)Rb atoms from each micro-MOT into Ioffe micro-trap is studied by using Monte-Carlo simulations, and our results show that the scheme can be used to realize a natural and effective loading of cold atoms from 2D array of micro-MOTs into 2D array of Ioffe micro-traps, and the loading efficiency can be reach ~ 65%. Moreover, the positions of each BEC (or MOT) in 2D array of magnetic micro-traps can be controlled by adjusting the currents in the wires or by changing the additional vertical bias field.
We propose a novel controllable double-well magneto-optical trap (MOT) by using a square current-carrying wire and a bias magnetic field. The spatial distributions of the magnetic fields and their gradients are calculated, and the results show that the proposed trap has double magnetic wells with two centres of zero magnetic field and can be continuously evolved as a single-well trap by reducing the current in the wire or increasing the bias field, which can be used to realize a controllable double-well MOT, even to prepare one- and two-dimensional arrays of magneto-optical lattices on the surface of an atom chip. We also estimate the number (~107) and temperature (~260 μK) of trapped cold atoms in our MOT.
A mini-magnetic lens for laser-cooled rubidium atoms is experimentally demonstrated in this paper. The key component of the mini-magnetic lens is a mini-coil with a radius of 2 mm. When the cold atomic clouds are transported in the vicinity of the coil along its axial, they are compressed in the longitudinal direction due to the interaction with the non-uniform magnetic field of the coil. Given a current carrying time of about 10 ms, the atomic clouds tend to be gradually compressed in the axial direction with the increase of the coil current. When the coil current is greater than 0.9 A, the atomic clouds begin to expand. At a threshold value of 0.9 A, the focus length of the mini-magnetic lens is determined to be about 1.3 mm. Compared with the case that no current passes through the mini-coil, the dimension of the focused atom clouds is one order of magnitude smaller. Moreover, the focus length can be controlled by both the coil current and its carrying time. Numerical simulations are also given which are in agreement with the experimental results.
Abstract Atom chips provide flexible technologies for implementing modern concepts in quantum optics, quantum measurement, and quantum information processing. Atom chips are miniature devices that confine, control and manipulate cold atoms using electric, magnetic, and light fields. Due to the shrinkage of scale, arbitrary magnetic traps can be generated from current sources outside the chip’s vacuum compartment, in contrast to a traditional setup. This makes it easy to change trap configurations in an experiment that involves rapid prototyping of quantum states and quantum trajectory designs in free space. In this work, we show relevant parameters needed for transferring a cold atom cloud at the recoil limit from a magneto-optical trap (MOT) to an area close to the atom chip. To create a movable magnetic potential for this transfer, we used the MOT coils and an additional pair of coils in an anti-Helmholtz configuration. The properties of the movable potential were obtained by performing the Computer Simulation Tool (CST EM Studio suite®). Furthermore, an appropriate magnetic trap on a chip is developed, based on the simulation from COMSOL Multiphysics. We used a magnetic field gradient of around 20 G/cm to transport the cold atom with a distance over 20 mm with a temperature gain below 100 micro-Kelvin. The simulation results are based on an atom chip with a size of 2×2 cm2 and a copper wire thickness of 2 mm. The atom chip consists of Z, U and I-shaped wires that generate a quadrupole magnetic field. The resulting field minimum can be made at least 7 mm away from the chip surface.
A novel controllable cold neutral atom guiding scheme based on parallel current-carrying wires and bias magnetic field is proposed.Magnetic field distribution in cross-section at different transverse bias field B_(bx) is calculated.The centre of magnetic traps is discussed.Single-channel and double-channel magnetic guiding can be realized.Based on this scheme,an atom beam splitter,which has adjustable splitting ratio, is designed by adding a longitudinal magnetic bias field B_(by).The transverse distribution of magnetic trap is calculated,and the magnetic contour plane of the splitter is shown.By using the classical Monte-Carlo method,the splitting process is simulated,and this scheme is validated by the simulation results.Splitting ratio between left and right guide changes from 0.9/0.1 to 0.1/0.9 when longitudinal magnetic bias field B_(by) changes from -0.005 G to 0.005 G.