  Recent developments in quantum physics make heavy use of so-called "quantum
trajectories." Mathematically, this theory gives rise to "stochastic
Schr\"odinger equations", that is, perturbation of Schr\"odinger-type equations
under the form of stochastic differential equations. But such equations are in
general not of the usual type as considered in the literature. They pose a
serious problem in terms of justifying the existence and uniqueness of a
solution, justifying the physical pertinence of the equations. In this article
we concentrate on a particular case: the diffusive case, for a two-level
system. We prove existence and uniqueness of the associated stochastic
Schr\"odinger equation. We physically justify the equations by proving that
they are a continuous-time limit of a concrete physical procedure for obtaining
a quantum trajectory.
