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arXiv:1411.4730 · Physical Review A

Radio-frequency spectroscopy of a linear array of Bose-Einstein condensates in a magnetic lattice

P. Surendran, Smitha Jose, Y. Wang, I. Herrera, Hui Hu, Xia-Ji Liu, S. Whitlock, Russell McLean, Andrei I. Sidorov, Peter Hannaford

Published Feb 6, 2015• 9 citations
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Abstract

We report site-resolved radio-frequency spectroscopy measurements of Bose-Einstein condensates of $^{87}\mathrm{Rb}$ atoms in about 100 sites of a one-dimensional (1D) 10-$\ensuremath{\mu}\mathrm{m}$-period magnetic lattice produced by a grooved magnetic film plus bias fields. Site-to-site variations of the trap bottom, atom temperature, condensate fraction, and chemical potential indicate that the magnetic lattice is remarkably uniform, with variations in the trap bottoms of only $\ifmmode\pm\else\textpm\fi{}\phantom{\rule{4pt}{0ex}}0.4$ mG. At the lowest trap frequencies (radial and axial frequencies of 1.5 kHz and 260 Hz, respectively), temperatures down to 0.$16\ensuremath{\mu}\mathrm{K}$ are achieved in the magnetic lattice, and at the smallest trap depths (50 kHz) condensate fractions up to 80% are observed. With increasing radial trap frequency (up to 20 kHz, or aspect ratio up to $\ensuremath{\sim}80)$ large condensate fractions persist, and the highly elongated clouds approach the quasi-1D Bose gas regime. The temperature estimated from analysis of the spectra is found to increase by a factor of about 5, which may be due to suppression of rethermalizing collisions in the quasi-1D Bose gas. Measurements for different holding times in the lattice indicate a decay of the atom number with a half-life of about 0.9 s due to three-body losses and the appearance of a high-temperature $(\ensuremath{\sim}1.5$ $\ensuremath{\mu}\mathrm{K})$ component which is attributed to atoms that have acquired energy through collisions with energetic three-body decay products.

DOI
10.1103/physreva.91.023605

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We report site-resolved radio-frequency spectroscopy measurements of Bose-Einstein condensates of 87Rb^{87}\mathrm{Rb}87Rb atoms in about 100 sites of a one-dimensional (1D) 10-\ensuremathμm\ensuremath{\mu}\mathrm{m}\ensuremathμm-period magnetic lattice produced by a grooved magnetic film plus bias fields. Site-to-site variations of the trap bottom, atom temperature, condensate fraction, and chemical potential indicate that the magnetic lattice is remarkably uniform, with variations in the trap bottoms of only \ifmmode±\else\textpm\fi0.4\ifmmode\pm\else\textpm\fi{}\phantom{\rule{4pt}{0ex}}0.4\ifmmode±\else\textpm\fi0.4 mG. At the lowest trap frequencies (radial and axial frequencies of 1.5 kHz and 260 Hz, respectively), temperatures down to 0.16\ensuremathμK16\ensuremath{\mu}\mathrm{K}16\ensuremathμK are achieved in the magnetic lattice, and at the smallest trap depths (50 kHz) condensate fractions up to 80% are observed. With increasing radial trap frequency (up to 20 kHz, or aspect ratio up to \ensuremath∼80)\ensuremath{\sim}80)\ensuremath∼80) large condensate fractions persist, and the highly elongated clouds approach the quasi-1D Bose gas regime. The temperature estimated from analysis of the spectra is found to increase by a factor of about 5, which may be due to suppression of rethermalizing collisions in the quasi-1D Bose gas. Measurements for different holding times in the lattice indicate a decay of the atom number with a half-life of about 0.9 s due to three-body losses and the appearance of a high-temperature (\ensuremath∼1.5(\ensuremath{\sim}1.5(\ensuremath∼1.5 \ensuremathμK)\ensuremath{\mu}\mathrm{K})\ensuremathμK) component which is attributed to atoms that have acquired energy through collisions with energetic three-body decay products.
We report the realization of a periodic array of Bose-Einstein condensates (BECs) of 87Rb^{87}\mathrm{Rb}87Rb F=1F=1F=1 atoms trapped in a one-dimensional magnetic lattice close to the surface of an atom chip. A clear signature for the onset of BEC in the magnetic lattice is provided by in situ site-resolved radio-frequency spectra, which exhibit a pronounced bimodal distribution consisting of a narrow component characteristic of a BEC together with a broad thermal cloud component. Similar bimodal distributions are found for various sites across the magnetic lattice. The realization of a periodic array of BECs in a magnetic lattice represents a major step towards the implementation of magnetic lattices for quantum simulation of many-body condensed matter phenomena in lattices of complex geometry and arbitrary period.
This thesis reports the realization of Bose-Einstein condensation (BEC) of 87Rb F=1 atoms in multiple sites of a one-dimensional 10 μm-periods magnetic lattice. The magnetic microtraps are created by a perpendicularly magnetized, multi-layered structure of TbGdFeCo film integrated on to an atom chip, together with uniform bias magnetic fields. Clear signatures for the onset of quantum degeneracy in multiple sites of the the magnetic lattice are provided by in-situ site-resolved radio-frequency spectroscopy
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 a proof-of-principle experiment on a fully permanent magnet atom chip. We study ultracold atoms and produce a Bose-Einstein condensate. The magnetic trap is loaded efficiently by adiabatic transport of a magnetic trap via the application of uniform external fields. Radio frequency spectroscopy is used for in-trap analysis and to determine the temperature of the atomic cloud. The formation of a Bose-Einstein condensate is observed in time-of-flight images and as a narrow peak appearing in the radio frequency spectrum.
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.
We report the efficient production of a large Bose-Einstein condensate in 87Rb^{87}\mathrm{Rb}87Rb atoms. This is achieved by quickly reducing the radio frequency of the magnetic field at a rate of $\ensuremath{-}96.8\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}∕\mathrm{s}$ during the final stage of evaporative cooling, and we have produced a condensate with $(2.2\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms against a serious three-body recombination loss. We observed the dependence of the cooling efficiency on the rate at which the truncated energy changes by measuring the condensate growth for three kinds of radio-frequency sweep. The experimental results quantitatively agree with calculations based on the quantum kinetic theory of a Bose gas.