Surface effects in magnetic microtraps

Abstract

We have investigated Bose-Einstein condensates and ultra cold atoms in the vicinity of a surface of a magnetic microtrap. The atoms are prepared along copper conductors at distances to the surface between 300 µm and 20 µm. In this range, the lifetime decreases from 20 s to 0.7 s showing a linear dependence on the distance to the surface. The atoms manifest a weak thermal coupling to the surface, with measured heating rates remaining below 500 nK/s. In addition, we observe a periodic fragmentation of the condensate and thermal clouds when the surface is approached.PACS numbers: 03.75. Fi, 03.75.Be, 34.50.Dy, Micropotentials have proven to be a powerful tool to manipulate and structure the shape of a Bose-Einstein condensate [1] on a length scale shorter than the coherence length of the condensate. Besides the manipulation with light [2,3,4,5,6], current carrying microstructures [7] are particularly interesting since they can be tailored in an arbitrary way, providing a variety of potential geometries. In previous experiments with magnetic microtraps the work was mainly focused on the demonstration of different trapping geometries, loading schemes and guiding principles [8,9,10,11,12,13,14]. The recent realization of Bose-Einstein condensates in magnetic microtraps [15,16] however provides new possibilities to control coherent matter on the micrometer scale. Coherent beam splitters, on-chip interferometers or quantum dots may become feasible. In current experimental setups, the dimensions of the conductors vary from 1 µm to 100 µm and the distance between the trap minimum and the surface is typically of the same size. At such small distances the atoms are affected by the nearby surface. For experiments with coherent matter waves or even single atoms in microfabricated traps an understanding of the mutual influences of the atoms and the surface is highly desirable.In this letter, we describe three effects on ultra cold atoms which appear in the vicinity of the surface of a magnetic microtrap. We observe a decrease of the lifetime of the atomic cloud which scales roughly linearly with the distance to the surface. At 20 µm, the lifetime is reduced to less than 1 s, which has to be compared to the "far distance" value of 100 s. Simultaneously, an increased heating rate is observed which, however, does not exceed 500 nK/s. Furthermore, a periodic fragmentation of both, the thermal cloud and the condensate occurs when the surface is approached at distances of about 250 µm. This gives strong evidence for additional potentials arising from the nearby surface.In our experiment, the microtrap is generated by a microstructure which consists of seven parallel copper conductors with widths of 3 µm, 11 µm and 30 µm, a height of 2.5 µm and a length of 25 mm [17]. The conductors are electroplated on an Al 2 O 3 ceramic substrate.An additional copper wire with a circular diameter of 90 µm is mounted parallel to the microstructure, allowing for reference measurements. The free surface of the wire is in the plane of the fabricated c...

Effect of magnetization inhomogeneity on magnetic microtraps for atoms

“…Firstly, the amplitude and structure of the corrugated potential are constant from day to day; however the amplitude increases as the trap is positioned closer to the surface, with an approximate power law dependence given by y −a , where a = 1.8 ± 0.3. For y 0 > 100 ñm the potential has a characteristic period of about 390 ñm, significantly longer than that commonly observed near electroplated wires [17,31]. Closer to the film, additional corrugations appear with a characteristic period of about 170 ñm.…”

Collective dynamics and expansion of a Bose-Einstein condensate in a random potential

“…Recently, effects of disorder created by a laser speckle have been observed on the dynamics of a BEC, including uncorrelated shifts of the quadrupole and dipole modes [4] and localization phenomena during the expansion in a one-dimensional (1D) waveguide [5,6]. Effects of disorder have also been observed for BECs in microtraps as a consequence of intrinsic defects in the fabrication of the microchip [17,18].…”

Evaporative cooling of cold atoms at surfaces

“…Since the advent of microchip traps for cold atoms [1][2][3][4][5][6][7][8][9][10][11][12], interest in developing quantum hybrid systems, which exploit the long coherence times of Bose-Einstein condensates with the flexibility of modern micro-and nanoelectronics, continues to grow. There is potential to use such systems as quantum memory devices [13][14][15], precision measurement devices [16][17][18][19] and even rewritable electronic systems [20].…”