Evolution of the Density Operator in a Dissipative System --A Non-master-Equation Approach

For a typical dissipative system, a driven harmonic oscillator put in a heat bath, we obtain a simple and exact solution for the reduced density operator of the system, which gives the result obtained by the master equation approach under the small damping approximation. This method gives a simple way to get exact solutions of many problems about dissipative systems in quantum optics.