Abstract
Starting from an effective Hamiltonian describing the interaction of a multimode field with a two-photon absorber, an equation of motion for the quantised slowly varying field amplitude is derived which exhibits a memory effect due to the finite atomic linewidth. Averaging this equation, as a whole, over the memory time leads to a single-mode formalism, thus providing a basis for the physical interpretation of the conventional one-mode treatment of two-photon absorption, especially with respect to the photon antibunching phenomenon. In particular, a precise physical meaning can be attributed to the mode volume.

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