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Linear optical amplifiers are described quantum mechanically by a linear differential equation with Langevin operator noise sources. Even if the gain medium is composed of fermions, in the limit when the amplifier is a linear amplifier, the noise sources must have bosonlike commutation relations. I show in detail how the Tomonaga approximation produces the bosonic commutators of the noise sources.
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H. A. Haus (2001) studied this question.
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