Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a nonradiative coherence between the discrete Landau levels of an undoped quantum well and study its dynamics. We observe quantum beats that reflect the time evolution of the coherence between the two lowest Landau level magnetoexcitons. We interpret our observations using a many-body theory and find that the inter-Landau level coherence decays with a time constant, substantially longer than the corresponding interband magnetoexciton dephasing times. Our results indicate an intraband excitation dynamics that cannot be described in terms of uncorrelated interband excitations. © 2008 The American Physical Society.