Long Atomic Coherence Times in an Optical Dipole Trap

Sodium atoms have been stored in a blue-detuned dipole trap based on sheets of argon ion laser light which support against gravity. In this trap, the atoms spend most of their time in free fall, resulting in a large reduction in the perturbation of the atomic levels due to the trapping potential. This reduction enabled us to probe the ground state hyperfine splitting with a measurement time of 4 s, yielding a linewidth of 0.125 Hz and a Ramsey fringe contrast of 43%. The coherence time was $\sim$300 times longer than achieved in a red-detuned Nd:YAG laser dipole trap with comparable depth.