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Physical Review Letters

Microscopic Magnetic Quadrupole Trap for Neutral Atoms with Extreme Adiabatic Compression

Vladan Vuletić, Thomas H. Fischer, Matthew F. Praeger, Theodor W. Hänsch, Claus Zimmermann

Published Feb 23, 1998• 48 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.80.1634

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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×\else\texttimes\fi105G/cm3\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0ex}{0ex}}\mathrm{G}/\mathrm{cm}3\ifmmode×\else\texttimes\fi105G/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×\else\texttimes\fi1054\ifmmode\times\else\texttimes\fi{}{10}^{5}4\ifmmode×\else\texttimes\fi105 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.
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 describe planar and mostly planar current geometries for constructing microscopic magnetic traps for neutral atoms. These geometries are well-suited for fabrication from superconductors using standard microfabrication techniques. Magnetic-field gradients greater than 5\ifmmode\times\else\texttimes\fi{}105{10}^{5}105 G/cm, and field curvatures greater than 108{10}^{8}108 G/cm2{\mathrm{cm}}^{2}cm2 can be produced in microscopic traps. Trap loading could be accomplished by constructing a nested series of traps, and compressing atom clouds from larger traps into smaller ones. In the smallest magnetic microtraps the motional ground-state energy of atoms in the trap can exceed the recoil energy from resonant photons, which may allow direct laser cooling to the trap ground state. If a number of atoms can be simultaneously cooled to the ground state, the resulting ``Bose clusters'' should exhibit interesting optical behavior, since their spatial extent would be less than the resonant light wavelength.
We report the trapping of neutral atoms in a permanent magnet trap. Approximately 1\ifmmode\times\else\texttimes\fi{}108{10}^{8}108 ground-state lithium atoms have been confined in a nonzero magnetic-field minimum produced by six permanent magnets in an Ioffe configuration. These atoms have a kinetic temperature of 1.1 mK and a peak density of approximately 3\ifmmode\times\else\texttimes\fi{}109{10}^{9}109 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The trapped-atom lifetime is 240 s, limited by collisions with background gas. This trap provides an environment in which quantum-statistical effects, atomic collisions, and other ultralow-temperature phenomena can be studied.
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.
Trapping particles is a key starting point for a range of physics experiments. The ability to confine, manipulate and interrogate a trapped species enables an invaluable degree of control and precision. Each trap offers different advantages and possibilities, and emerging experiments often demand different methods of trapping. This thesis describes a new three-dimensional microwave trap for atoms and molecules. Like an optical dipole trap, it can trap ground state polar molecules, but its depth and volume are larger by factors of 10^3 and 10^9 respectively. The experiment consists of a lithium oven, Zeeman slower, magneto-optical trap (MOT), a moving magnetic trap, and a high-quality Fabry-Perot cavity which serves as the microwave trap. Starting from a MOT of 1×10^8 lithium-7 atoms, the atomic cloud is cooled from a temperature of around 1mK to 41(1)μK with the recently developed Raman grey molasses method. Next, the population is optically pumped to a magnetically trappable state, and up to 85(2)% are loaded into a quadrupole magnetic trap. These atoms are then transferred to a moving magnetic trap and translated 600mm to the centre of the microwave cavity. After optimizing the transport to minimize atom heating and loss, we can produce samples of 2×10^7 atoms in the microwave chamber, at a phase space density of 3.58(15)×10^7. Finally, loading of the microwave trap is demonstrated and its basic properties studied. When the input power to the cavity is 610W, the electric field amplitude at the centre reaches roughly 20kV/cm. At this input power, centre-of-mass oscillation frequencies in the microwave trap were measured as 28.55(5)Hz along the axis of the cavity, and 8.81(8)Hz along the orthogonal direction. The observed lifetime was 1.76(12)s, limited by the vacuum quality. The atomic cloud expands and cools to 22(3)μK when loaded into the microwave trap, with a phase space density of 4(1)×10^7. This is consistent with adiabatic loading from the magnetic trap.
We demonstrate the loading of a miniaturized magnetic guide for neutral atoms made of small-size current conductors. Cold rubidium atoms are initially stored in a shallow spherical quadrupole field and subsequently transferred into the magnetic guide by gradually transforming the trapping potential. Density, temperature, and atom number have been monitored in situ by absorption imaging with a charge-coupled device camera. A fraction of 1414%14 of the initially trapped atoms has been transferred into the linear microtrap at a temperature of $39\ensuremath{\mu}\mathrm{K}$.