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Chinese Physics B

Direct loading of atoms from a macroscopic quadrupole magnetic trap into a microchip trap

Jun Jie Cheng, Jingfang Zhang, Xinping Xu, Haichao Zhang, Yuzhu Wang

Published Mar 1, 2017• 3 citations
Publisher

Abstract

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.

DOI
10.1088/1674-1056/26/3/033701

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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.
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.
We demonstrate efficient loading of neutral atoms into a magnetic waveguide produced by the magnetic fields of microfabricated current-carrying conductors. The lithographically patterned conductors on this ``atom chip'' can be used to make a variety of guiding and trapping structures for manipulating cold atoms and Bose-Einstein condensates. A three-chamber vacuum apparatus collects atoms in a magneto-optical trap, precools them via evaporative cooling, and delivers them to the final chamber containing the atom chip. We describe in detail how the precooled atomic cloud is transferred from a macroscopic magnetic Ioffe-Pritchard trap to the microscopic magnetic waveguide on the atom chip 21cm21\phantom{\rule{0.3em}{0ex}}\mathrm{cm}21cm away. Permanent magnets provide a confining two-dimensional quadrupole field to guide the atoms between the two chambers while longitudinally the cloud is allowed to freely expand during the transfer. Strategically placed coils are used to control the longitudinal size and speed of the atomic cloud as it is loaded on the atom chip.
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.
We report the transport of magnetically trapped cold atoms over a large distance. Atoms are captured in a magneto-optical trap (MOT) and loaded into a magnetic quadrupole trap. The quadrupole potential is then moved over a distance of 33 cm into an ultra-high-vacuum (UHV) chamber using a chain of quadrupole coils. By running suitable currents through the quadrupole coil pairs the trapping geometry of the potential is maintained during the transport process, thus minimizing heating of the trapped atom cloud. Magnetically trapped 87Rb{}^{87}\mathrm{Rb}87Rb atoms at an initial temperature of $T=125 \ensuremath{\mu}\mathrm{K}$ were heated by less than $20 \ensuremath{\mu}\mathrm{K}$ during the transport process, and up to 109{10}^{9}109 atoms were transferred into the UHV chamber. Due to the spatial separation of the final magnetic trap and the MOT we have been able to capture atoms in the MOT, while storing magnetically trapped atoms in the UHV chamber.
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.
We experimentally demonstrated the transport of cold atoms in a magnetic quadrupole trap in the nonadiabatic regime on the order of the trap period. The trap potential velocity was controlled by varying the currents of the two pairs of anti-Helmholtz coils. We efficiently transported cold atoms with two velocity regimes. For example, with a potential velocity of 18 (28 cm/s), the final velocity of a cold atom cloud is 0.6±2.7 (58.6±1.5 cm/s). The trap motion with proper velocity could expel energetic atoms from the trap, resulting in a decline in temperature of high residual velocity atoms.