Spin Coupling between Cold Atoms and the Thermal Fluctuations of a Metal Surface

We describe an experiment in which Bose-Einstein condensates and cold atom clouds are held by a microscopic magnetic trap near a room-temperature metal wire $500\mu \mathrm{m}$ in diameter. The lifetime for atoms to remain in the microtrap is measured over a range of distances down to $27\mu \mathrm{m}$ from the surface of the metal. We observe the loss of atoms from the microtrap due to spin flips. These are induced by radio-frequency thermal fluctuations of the magnetic field near the surface, as predicted but not previously observed.

Figure 1

Construction of the atom chip (not to scale). Atoms are trapped near the surface of the guide wire.

Figure 2

Optical thickness of a $5\mu \mathrm{K}$ atom cloud measured by absorption imaging. As the cloud approaches the wire it begins to fragment along the $z$ direction. This effect has previously been seen above Cu wires.

Figure 3

Number of atoms remaining in the microtrap versus time at a distance of $29\mu \mathrm{m}$ from the conducting surface of the wire. The cloud is initially at a temperature of $1\mu \mathrm{K}$.

Figure 4

Lifetime of trapped atoms versus distance from the Al surface of the wire. Filled squares (open circles): measurements with spin-flip frequency $f_0=560\mathrm{k}\mathrm{H}\mathrm{z}$ ($f_0=1.8\mathrm{M}\mathrm{H}\mathrm{z}$). Solid (dashed) lines: calculated lifetimes above a thick slab of copper (aluminum) for these two spin-flip frequencies. Dotted line: expected scaling for technical noise.