Vector of a posteriori state of an emitting two-level particle

An equation has been obtained for the non-normalized vector of an a posteriori state of a two-level particle at its resonant pumping by a coherent field at the detection of photons emitted by the particle. The equation is linear and belongs to the class of stochastic differential equations controlled by the classical Wiener and Poisson (reading) processes. The reading process describes photo detection and the Wiener process describes an additional dephasing of particles, which has not yet been taken into account.