A "Square-root" Method for the Density Matrix and its Applications to   Lindblad Operators
  The evolution of open systems, subject to both Hamiltonian and dissipative forces, is studied by writing the $nm$ element of the time ($t$) dependent density matrix in the form \ber \rho_{nm}(t)&=& \frac {1}{A} \sum_{\alpha=1}^A \gamma ^{\alpha}_n (t)\gamma^{\alpha *}_m (t) \enr The so called "square root factors", the $\gamma(t)$'s, are non-square matrices and are averaged over $A$ systems ($\alpha$) of the ensemble. This square-root description is exact. Evolution equations are then postulated for the $\gamma(t)$ factors, such as to reduce to the Lindblad-type evolution equations for the diagonal terms in the density matrix. For the off-diagonal terms they differ from the Lindblad-equations. The "square root factors" $\gamma(t)$ are not unique and the equations for the $\gamma(t)$'s depend on the specific representation chosen. Two criteria can be suggested for fixing the choice of $\gamma(t)$'s one is simplicity of the resulting equations and the other has to do with the reduction of the difference between the $\gamma(t)$ formalism and the Lindblad-equations. 
A. Yahalom and R. Englman