In
physics,
quantum noise refers to the uncertainty of a physical quantity that is due to its quantum origin. In certain situations, quantum noise appears as
shot noise; for example, most
optical communications use
amplitude modulation, and thus, the quantum noise appears as
shot noise only. For the case of uncertainty in the
electric field in some
lasers, the quantum noise is not just shot noise; uncertainties of both 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 when the energy of the
frequency modulation or
phase modulation of waves is comparable to the energy of the signal (which is believed to be more robust with respect to additive noise than an amplitude modulation).