Quantum noise

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Quantum noise is uncertainty of some physical quantity due to its quantum origin.

In the case of number of photons, the quantum noise is called also shot noise. Until year 2007, the most of optical communications use amplitude modulation. In this case, the quantum noise appears as shot noise only.

In the case of uncertainty of electric field in some laser, the quantum noise is not just shot noise; both incertainties of the amplitude and phase contribute to the quantum noise. This issue becomes important in the case of noise of a quantum amplifier, which preserves the phase. The phase noise becomes important at the frequency modulation or phase modulation of waves with energy of quantum comparable to the energy of a signal.