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Comment on the paper "Electromagnetic Wave Dynamics in Matter-Wave Superradiant Scattering" by L. Deng, M.G. Payne, and E.W. Hagley

The paper by Deng et al. (Phys. Rev. Lett. 104, 050402 (2010)) presents an analytic theoretical description of matter-wave superradiance (Phys. Rev. Lett. 85, 4225-4228 (2000)) which claims to go beyond previous theoretical frameworks. I show here that the theory presented in this paper is not a description of superradiance per se, but rather an elegant perturbative description of a Raman amplifier far away from the superradiant threshold. As such, it merely is a limiting case of previously known treatments of superradiance. Two additional new findings of the paper are incorrect: (1) The claim that adiabatic elimination of the excited state of the atoms is only possible when the probe pulse propagates slowly. (2) The prediction that superradiance has a dependence on the sign of the detuning of the pump laser due to a phase-matching condition.