Using a two-level atom as a quantum bit,We have figured the equatio n of density matrix evolution of the disintegratin two-level atomic system in the cl assic electromagnetism field. The quantum coherenct properties in evolution of the st ate as follows time is obtained.
The author work out the differential equation of the density matrix of the two level atom system in accordance with the Raman process of the two-level atom interacting the two-mode fields,then make research on its decoherence properties.The result reflects that the decoherence of the quit has something to do with its original state,the frequency of the two-mode fields and the average quantity of photon in the two-mode fields.Decoherence phenomenon is especially obvious when the average quantity of photon is more than one.