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arXiv:cond-mat/0608370 · Physical Review A

Atom trapping with a thin magnetic film

Micah Boyd, Erik W. Streed, Patrick Medley, Gretchen K. Campbell, Jongchul Mun, Wolfgang Ketterle, David E. Pritchard

Published Oct 22, 2007• 27 citations
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Abstract

We have created a $^{87}\mathrm{Rb}$ Bose-Einstein condensate in a magnetic trapping potential produced by a hard disk platter written with a periodic pattern. Cold atoms were loaded from an optical dipole trap and then cooled to Bose-Einstein condensation on the surface with radio-frequency evaporation. Fragmentation of the atomic cloud due to imperfections in the magnetic structure was observed at distances closer than $40\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ from the surface. Attempts to use the disk as an atom mirror showed dispersive effects after reflection.

DOI
10.1103/physreva.76.043624

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•Jan 1, 2007•5 citations
We have created a 87Rb^{87}\mathrm{Rb}87Rb Bose-Einstein condensate in a magnetic trapping potential produced by a hard disk platter written with a periodic pattern. Cold atoms were loaded from an optical dipole trap and then cooled to Bose-Einstein condensation on the surface with radio-frequency evaporation. Fragmentation of the atomic cloud due to imperfections in the magnetic structure was observed at distances closer than 40\ensuremathμm40\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}40\ensuremathμm from the surface. Attempts to use the disk as an atom mirror showed dispersive effects after reflection.
A combination of radio frequency radiation and magnetic field gradients was used to\ntrap atoms in dressed states. In a magnetic field with a quadrupole minimum, RF\nfields resonant with the (|F,mf i) |1,−1i ! |1, 0i transition trapped the atoms on\nthe surface of a sphere, and gravity caused the atoms to pool at the bottom of the\nsphere. BECs were transferred into this dressed Zeeman trap with 100% efficiency,\nwith lifetimes of up to 30 s, and trapping frequencies of up to 250 Hz were measured.\nA hard disk platter with a specially written magnetic pattern was used to generate\nmagnetic fields to confine atoms tightly. Detrimental interactions with the surface\nwere avoided by using an extremely thin film with a large magnetic remnant. BECs\nof up to 5×104 atoms were produced in cigar shaped traps 40 µm above the surface,\nand trap frequencies up to 5 kHz were measured. After evaporation, condensed clouds\nmoved closer to the surface to probe imperfections in the magnetic potential, revealing\ndefects at distances closer than 35 µm. Finally, BECs were dropped from a height of\n350 µm in an attempt to achieve specular reflection, but a large amount of dispersion\nwas observed.\nFinally, BECs were loaded into a three-dimensional optical lattice, and a quantum\nphase transition from a superfluid to a Mott insulator was observed. Using microwave\nspectroscopy, the density dependent “clock shift” was was found to depend on the\noccupation number of the wells. The singly occupied lattice sites were then investigated\nas an atomic clock system with no density shift. Linewidths as small as 1 Hz\nFWHM out of 6.8 GHz are comparable to current atomic frequency standards.
We describe an apparatus for quickly and simply producing 87Rb^{87}\text{R}\text{b}87Rb Bose-Einstein condensates. It is based on a magnetic quadrupole trap and a red-detuned optical dipole trap. We collect atoms in a magneto-optical trap (MOT) and then capture the atoms in a magnetic quadrupole trap and force rf evaporation. We then transfer the resulting cold dense cloud into a spatially mode-matched optical dipole trap by lowering the quadrupole field gradient to below gravity. This technique combines the efficient capture of atoms from a MOT into a magnetic trap with the rapid evaporation of optical dipole traps; the approach is insensitive to the peak quadrupole gradient and the precise trapping beam waist. Our system reliably produces a condensate with $N\ensuremath{\approx}2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms every 16 s.
Multiple bounces of cold rubidium atoms have been observed for times up to one second in a trap formed by gravity and a 2 cm-diameter spherical mirror made from a sinusoidally magnetized floppy disk. We have studied the dynamics of the atoms bouncing in this trap from several different heights up to 40.5 mm and we conclude that the atoms are reflected specularly and with reflectivity 1.01(3). Slight roughness of the mirror is caused by harmonics in the magnetization of the surface and by discontinuities at the boundaries between recorded tracks. As the next step in this atom optics program we propose using a magnetic mirror to create a 2D atomic gas. We discuss how cold atoms can be loaded into the ground state of a static magnetic potential well that exists above the surface of the mirror as a consequence of the intermediate-field Zeeman effect.
Magnetically and optically confined Bose-Einstein condensates were studied near a microfabricated surface. Condensate fragmentation observed in microfabricated magnetic traps was not observed in optical dipole traps at the same location. The measured condensate lifetime was >or=20 s and independent of the atom-surface separation under both magnetic and optical confinement. Radio-frequency spin-flip transitions driven by technical noise were directly observed for optically confined condensates and could limit the condensate lifetime in microfabricated magnetic traps.
A rapid atomic beam of rubidium (87Rb) is produced by two-dimensional magneto-optical trap (2D MOT), and then trapped by three-dimensional magneto-optical trap (3D MOT) with high vacuum for further cooling. After a process of optical molasses cooling, atoms are reloaded into a magnetic trap, where radio frequency (RF) evaporation cooling is implemented. The precooled atoms in the magnetic trap are then transferred into a far detuning optical dipole trap, where Bose-Einstein condensate (BEC) appears by further evaporation cooling. The 3D MOT is loaded to its maximum within 25 s and then BEC is prepared in 16 s. Due to the linear intensity of magnetic trap, the frequency can be scanned fast in the RF evaporation cooling process. In our experiment, the frequency scans from 39 MHz to 15 MHz in 6 s and then scans to 2 MHz in 5 s. The number of atoms in 3D MOT is about 11010, and there are 5105 atoms in the BEC after a succession of cooling processes. To optimize the performances of 2D MOT, a special light path is constructed. And prisms with high reflectivity are used to reduce the imbalance between opposite propagating cooling +beams. Furthermore, quarter-wave plates are used to keep the polarization state of the cooling beam when reflected by prisms or mirrors. The atoms are cooled to a temperature about 15 K in the magnetic trap by RF evaporation. In such a low temperature, the loss of magnetic trap (Majorana loss) will prevent the atoms from reaching a high density, and the atoms cannot be cooled further. To reduce the loss rate of the magnetic trap, the far blue detuning light (532 nm, 18 W) is added to plug the zero point of the magnetic trap. In the optically plugged magnetic trap, atoms with high density are cooled down enough, which gives a good start for the loading of optical dipole trap.
Atom chips are devices used to magnetically trap and manipulate ultracold atoms and Bose-Einstein condensates near a surface. In particular, permanent magnetic film atom chips can allow very tight confinement and intricate magnetic field designs while circumventing technical current noise. Research described in this thesis is focused on the development of a magnetic film atom chip, the production of Bose-Einstein condensates near the film surface, the characterisation of the associated magnetic potentials using rf spectroscopy of ultracold atoms and the realisation of a precision sensor based on splitting Bose-Einstein condensates in a double-well potential. The atom chip itself combines the edge of a perpendicularly magnetised GdTbFeCo film with a machined silver wire structure. A mirror magneto-optical trap collects up to 5 x 108 87Rb atoms beneath the chip surface. The current-carrying wires are then used to transfer the cloud of atoms to the magnetic film microtrap and radio frequency evaporative cooling is applied to produce Bose-Einstein condensates consisting of 1 x 105 atoms. We have identified small spatial magnetic field variations near the film surface that fragment the ultracold atom cloud. These variations originate from inhomogeneity in the film magnetisation and are characterised using a novel technique based on spatially resolved radio frequency spectroscopy of the atoms to map the magnetic field landscape over a large area. The observations agree with an analytic model for the spatial decay of random magnetic fields from the film surface. Bose-Einstein condensates in our unique potential landscape have been used as a precision sensor for potential gradients. We transfer the atoms to the central region of the chip which produces a double-well potential. A single BEC is formed far from the surface and is then dynamically split in two by moving the trap closer to the surface. After splitting, the population of atoms in each well is extremely sensitive to the asymmetry of the potential and can be used to sense tiny magnetic field gradients or changes in gravity on a small spatial scale.