  An open quantum system with multiple levels coupled to a bosonic environment
at zero temperature is investigated systematically using the non-Markovian
quantum-state-diffusion (QSD) method [W. T. Strunz, L. Di\'osi, and N. Gisin,
Phys. Rev. Lett. 82, 1801 (1999)]. We have established exact time-local QSD
equations for a set of interesting multilevel open systems, including high-spin
systems, multiple-transition atomic models, and multilevel atomic models driven
by time-dependent external fields. These exact QSD equations have paved the way
to evaluate the dynamics of open multilevel atomic systems in the general
non-Markovian regimes without any approximation.
