The Density Matrix
Chapter 1 presented the equations for the coherent evolution of the amplitudes of a two-level atom in a radiation field. However, the effects of spontaneous emission cannot be described in terms of such coherent evolution of the eigenstates of the system. Spontaneous emission is most readily handled by the density matrix, which is introduced in this chapter. Since this topic is covered by many textbooks in quantum mechanics, it is only briefly presented in the first section here (for instance, see Ref. 20). In the next section it is applied to the specific case of a two-level atom in a radiation field. The resulting equations are solved and discussed in terms of the effects of spontaneous emission on the interaction of atoms by radiation fields.
KeywordsExcited State Density Matrix Pure State Radiation Field Spontaneous Emission
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