Cold atoms probe the magnetic field near a wire

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 ΔBz 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 K1(2πy/λ), as one would expect for the far field of a transversely oscillating current within the wire.