Hyperfine-State Coherence in the Presence of Spontaneous Photon Scattering*

Hyperfine-state superpositions are important to many atomic physics applications and experiments. In many of these experiments, off-resonance laser light is used either to manipulate or trap atoms. Spontaneous scattering of photons can therefore play an important role in hyperfine-state decoherence. We study experimentally the coherence of a hyperfine-state of a trapped $^9$Be$^+$ ion, in the presence of off-resonance light. It is shown that it is only Raman spontaneous scattering of photons which affects coherence. Raman scattering of photons is largely suppressed at a laser detuning which is much larger than the fine-splitting of the excited state. Coherence times that exceed the average scattering time of 19 photons are measured. This result implies that laser light can be used to manipulate hyperfine-state superpositions with very small decoherence.