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Europhysics Letters (EPL)

Steep magnetic trap for ultra cold atoms

Vladan Vuletić, Theodor W. Hänsch, Claus Zimmermann

Published Nov 10, 1996• 29 citations
Publisher

Abstract

A combination of current coils and ferromagnetic pole shoes is used to construct a magnetic quadrupole trap for ultra cold neutral atoms. Field gradients of up to 3000 G/cm have been achieved continuously with an electric power consumption of only 10 watts. A maximum field gradient of 1 T/cm is calculated for a current that can be applied for several seconds. The trap has been loaded with 8 × 10 6 lithium atoms ( 7 Li) from a magneto-optical trap and the energy distribution determined by stepwise reduction of the trapping potential. Finally, magnetic trapping of fermionic 6 Li has been demonstrated.

DOI
10.1209/epl/i1996-00234-2

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A combination of current coils and ferromagnetic pole shoes is used to construct a magnetic quadrupole trap for ultra cold neutral atoms. Field gradients of up to 3000 G/cm have been achieved continuously with an electric power consumption of only 10 watts. A maximum field gradient of 1 T/cm is calculated for a current that can be applied for several seconds. The trap has been loaded with 8 × 10 6 lithium atoms ( 7 Li) from a magneto-optical trap and the energy distribution determined by stepwise reduction of the trapping potential. Finally, magnetic trapping of fermionic 6 Li has been demonstrated.
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.
A microscopic magnetic quadrupole trap for neutral atoms has been realized with a combination of permanent magnets, coils, and ferromagnetic pole pieces. The attainable magnetic field gradients of $3\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0ex}{0ex}}\mathrm{G}/\mathrm{cm}$ infer a spatial extension of the ground state much smaller than the wavelength of optical transitions. The field gradient can be varied over a wide range which allows for efficient loading of $4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ lithium atoms from a shallow potential by adiabatic transport and compression. During compression the observed 275-fold density increase implies a collision rate increase by a factor of 1700.
For the first time ultra-cold thulium atoms were trapped in a magnetic quadrupole trap with a small field gradient (20 Gs cm -1 ). The atoms were loaded from a cloud containing 4 × 10 5 atoms that were preliminarily cooled in a magneto-optical trap to the sub-Doppler temperature of 80 μK. As many as 4 × 10 4 atoms were trapped in the magnetic trap at the temperature of 40 μK. By the character of trap population decay the lifetime of atoms was determined (0.5 s) and an upper estimate was obtained for the rate constant of inelastic binary collisions for spin-polarised thulium atoms in the ground state (g in < 10 -11 cm 3 s -1 ).
We propose two double-well magnetic traps for cold neutral atoms that use current-carrying wires. These traps can be used to trap two different atomic species. The spatial distributions of the magnetic fields, gradients, and curvatures from these two wire configurations were calculated and analyzed. Our study shows that the double-well traps proposed here can be continuously changed into single-well traps by reduction of the current in a wire (or a coil) and vice versa and that a maximum field gradient greater than 5×104 G/cm and a maximum field curvature (at each trap center) greater than 2.5×107 G/cm2 can be generated in our double-well traps and used to achieve two-species Bose–Einstein condensations and study the properties of double-well Bose–Einstein condensations or to achieve sympathetic cooling between two atomic samples or even to achieve two-species magneto-optical traps and study cold collisions between two atomic samples.
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.
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.