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arXiv:1801.09461 · Journal of Modern Optics

On loading of a magneto-optical trap on an atom-chip with U-wire quadrupole field

Vivek Singh, Vibhuti Bhushan Tiwari, K. A. P. Singh, Satya Ram Mishra

Published Aug 21, 2018• 14 citations
arXivHTMLPDF

Abstract

The role of U-wire generated quadrupole-like magnetic field profile on loading of cold 87Rb atoms in a magneto-optical trap (MOT) on atom-chip has been investigated. It is shown by simulations that the quadrupole field profile and the centre of the quadrupole field vary depending upon the U-wire current and bias field values. Moreover, the resemblance of generated field profile to the ideal quadrupole field critically depends on the values of current in U-wire and bias magnetic field. The observed number of trapped atoms in the MOT and its position has also been found to depend on the quadrupole field profile. The number of trapped atoms in the MOT reaches maximum when the field profile approaches close to an ideal quadrupole field. These studies are, in general, useful in optimizing the number in the MOT before transfer of these cold atoms to the microtraps of an atom-chip.

DOI
10.1080/09500340.2018.1508772

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Chinese Physics B•Mar 1, 2017•3 citations
The role of U-wire generated quadrupole-like magnetic field profile on loading of cold 87Rb atoms in a magneto-optical trap (MOT) on atom-chip has been investigated. It is shown by simulations that the quadrupole field profile and the centre of the quadrupole field vary depending upon the U-wire current and bias field values. Moreover, the resemblance of generated field profile to the ideal quadrupole field critically depends on the values of current in U-wire and bias magnetic field. The observed number of trapped atoms in the MOT and its position has also been found to depend on the quadrupole field profile. The number of trapped atoms in the MOT reaches maximum when the field profile approaches close to an ideal quadrupole field. These studies are, in general, useful in optimizing the number in the MOT before transfer of these cold atoms to the microtraps of an atom-chip.
In this paper, we have developed a combined magnetic field system to explore an alternative technical route for the generation of Bose–Einstein condensates containing numerous atoms on an atom chip. The system is characterized by the fact that the quadrupole magnetic field required by the magneto-optic trap is generated by U-shaped current-carrying wires combined with bias magnetic fields, whereas the quadrupole magnetic field adopted for the magnetic trap is generated by anti-Helmholtz coils. By fine-tuning the bias magnetic fields, the collection of atoms in the mirror MOT and the loading of the quadrupole magnetic trap can be optimized. The initial number of [Formula: see text]Rb atoms in the magnetic trap is approximately [Formula: see text], and the lifetime of the atoms is over 30 s. This scheme can facilitate the chip-surface design and is suitable for the precise manipulation of Bose–Einstein condensates near the chip surface.
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.
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.
With a movable quadrupole magnetic trap (QMT) generated by two pairs of partly-overlapped magneto-optical trap (MOT) and transfer coils, a highly efficient transport of cold87Rb atoms to the atom chip surface is realized. By the QMT movable means of linearly increasing the current in the transfer coils but simultaneously keeping the current in the MOT coils constant, the velocity profile of the moving trap is Blackman-type-like. With this QMT movable scheme, the cold atoms are transferred to the atom chip surface from the MOT center. In the transfer process, the cold atom temperature is increased by about 30 μK, and the transfer efficiency is above 90%. This system provides an appropriate cold atoms source for the atom chip interferometer and also can be used to investigate the interaction between atom and chip surface. ? 2016, Chinese Lasers Press. All right reserved.
Magneto-optical traps on atom chips are usually restricted to small atomic samples due to a limited capture volume caused primarily by distorted field configurations. Here we present a magneto-optical trap based on a millimeter-sized wire structure which generates a magnetic field with minimized distortions. Together with the loading from a high-flux two-dimensional magneto-optical trap, we achieve a loading rate of $8.4\ifmmode\times\else\texttimes\fi{}{10}^{10}$ atoms/s and maximum number of $8.7\ifmmode\times\else\texttimes\fi{}{10}^{9}$ captured atoms. The wire structure is placed outside of the vacuum to enable a further adaptation to new scientific objectives. Since all magnetic fields are applied locally without the need for external bias fields, the presented setup will facilitate parallel generation of Bose-Einstein condensates on a conveyor belt with a cycle rate above 111 Hz.
We demonstrate the direct loading of cold atoms into a microchip 2-mm Z-trap, where the evaporative cooling can be performed efficiently, from a macroscopic quadrupole magnetic trap with a high loading efficiency. The macroscopic quadrupole magnetic trap potential is designed to be moveable by controlling the currents of the two pairs of anti-Helmholtz coils. The cold atoms are initially prepared in a standard six-beam magneto-optical trap and loaded into the macroscopic quadrupole magnetic trap, and then transported to the atom chip surface by moving the macroscopic trap potential. By means of a three-dimensional absorption imaging system, we are able to optimize the position alignment of the atom cloud in the macroscopic trap and the microchip Z-shaped wire. Consequently, with a proper magnetic transfer scheme, we load the cold atoms into the microchip Z-trap directly and efficiently. The loading efficiency is measured to be about 50%. This approach can be used to generate appropriate ultracold atoms sources, for example, for a magnetically guided atom interferometer based on atom chip.