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 $\Delta B_z$ parallel to the wire. This field is proportional to the current in the wire and is approximately periodic along the wire with period $\lambda=230 \mu$m. We find that the decrease of this field with distance from the centre of the wire is well described by the Bessel function $K_1(2\pi y/\lambda)$, as one would expect for the far field of a transversely oscillating current within the wire.