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arXiv:cond-mat/0308434 · Journal of Physics B Atomic Molecular and Optical Physics

Cold atoms probe the magnetic field near a wire

M. P. A. Jones, Chris J. Vale, Daniel Sahagún, B. V. Hall, Claudia Eberlein, B. E. Sauer, K Furusawa, David John Richardson, Edward A. Hinds

Published Dec 17, 2003• 49 citations
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Abstract

A microscopic Ioffe–Pritchard trap is formed using a straight, current-carrying wire, together with suitable auxiliary magnetic fields. By measuring the distribution of cold rubidium atoms held in this trap, we detect a weak magnetic field component Δ B z parallel to the wire. This is produced by the current in the wire and is approximately periodic along the wire with period λ = 230 µm. We have measured this field at distances in the range y = 250–350 µm from the centre of the wire. Over this range we find that the decrease of the field is well described by the Bessel function K 1 (2π y /λ), as one would expect for the far field of a transversely oscillating current within the wire.

DOI
10.1088/0953-4075/37/2/l01

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A microscopic Ioffe–Pritchard trap is formed using a straight, current-carrying wire, together with suitable auxiliary magnetic fields. By measuring the distribution of cold rubidium atoms held in this trap, we detect a weak magnetic field component Δ B z parallel to the wire. This is produced by the current in the wire and is approximately periodic along the wire with period λ = 230 µm. We have measured this field at distances in the range y = 250–350 µm from the centre of the wire. Over this range we find that the decrease of the field is well described by the Bessel function K 1 (2π y /λ), as one would expect for the far field of a transversely oscillating current within the wire.
Microscopic atom optical devices integrated on atom chips allow to precisely control and manipulate ultra-cold ( T < 1 µK) neutral atoms and Bose-Einstein condensates (BECs) close to surfaces. The relevant energy scale of a BEC is extremely small (down to < 10 −11 eV). Consequently, BECs can be utilized as a sensor for variations of the potential energy of the atoms close to the surface. Here we describe how to use trapped atoms as a measurement device and analyze the performance and flexibility of the field sensor. We demonstrate microscopic magnetic imaging with simultaneous high spatial resolution (3 µm) and high field sensitivity (4 nT). With one dimensional BECs, we probe the magnetic field variations close to the surface at distances down to a few microns. Measurements of the magnetic field of a 100 µm wide current carrying wire imply that the magnetic field variations stem from residual variations of the current flow direction, resulting from local properties of the wire. These disorder potentials found near lithographically fabricated wires are two orders of magnitude smaller than those measured close to electroplated conductors.
We report on the realization and characterization of a magnetic microtrap for ultracold atoms near a straight superconducting Nb wire with circular cross section. The trapped atoms are used to probe the magnetic field outside the superconducting wire. The Meissner effect shortens the distance between the trap and the wire, reduces the radial magnetic-field gradients, and lowers the trap depth. Measurements of the trap position reveal a complete exclusion of the magnetic field from the superconducting wire for temperatures lower than 6 K. As the temperature is further increased, the magnetic field partially penetrates the superconducting wire; hence the microtrap position is shifted towards the position expected for a normal-conducting wire.
This thesis presents the first demonstration of a guide for cold atoms based on a miniature structure of four current-carrying wires. The four wires are embedded within a hollow silica fibre. Atoms are guided along the centre of a fifth hole on the axis of the fibre by the Stern-Gerlach force. A vapour cell Magneto Optical Trap (MOT), formed 1 cm above the mouth of the waveguide is the source of cold "8"5Rb atoms. After cooling the atoms to 25 #mu#K in optical molasses they fall under the influence of gravity through a magnetic funnel into the waveguide. After propagating for 2 cm, the atoms are reflected by the field of a small pinch coil wound around the base of the guide. The atoms then travel back up the fibre and out into the funnel, where they can be imaged either in fluorescence or by recapturing in the MOT. A video sequence of atoms falling into the guide and re-emerging after reflection from the pinch coil graphically illustrates the operation of the guide. The coupling efficiency and transverse temperature of the atoms is measured experimentally and in a Monte-Carlo simulation. We find an optimum coupling efficiency of 12% and we measure the spatial extent of the cloud within the fibre to be of order 100 #mu#m. We find good agreement between experimental data and results from the numerical simulation. We have also been able to observe different thresholds for the reflection of different positive m_F levels. In another experiment we are able to trap the atoms in an elongated Ioffe trap for up to two seconds, increasing the distance over which the atoms are guided. We are able to guide the atoms over distances of 40 cm with a loss rate indistinguishable from the free space loss rate. (author)