Quantum noise

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).

Quantum noise may be observed in any system where conventional sources of noise (industrial noise, vibrations, fluctuations of voltage in the electric power supply, thermal noise due to Brownian motion, etc.) are somehow suppressed. Generally, quantum noise can be considered as the error of the description of any physical system within classical (not quantum) theory. It is reasonable to include consideration of quanta appearing or disappearing spontaneously in spacetime due to the most basic laws of conservation, hence, no area in spacetime is devoid of potential addition or subtraction of a least common denominator quanta element, causing "noise" in a given experiment. This could manifest as quantum decoherence in an entangled system, normally attributed to thermal differences in the conditions surrounding each entangled particle considered to be part of an entangled set. Because entanglement is studied intensely in simple pairs of entangled photons, for example, decoherence observed in these experiments could well be synonymous with "quantum noise" as to the source of the decoherence. e.g. If it were possible for a quanta of energy to spontaneously appear in a given field, a region of spacetime, then thermal differences must be associated with this event, hence, it would cause decoherence in an entangled system in proximity of the event. In an electric circuit, the random fluctuations of a signal due to the discrete character of electrons can be called quantum noise. The random error of interferometric measurements of position, due to the discrete character of photons registered during measurement, can be attributed to quantum noise. Even the uncertainty of position of a probe in probe microscopy may be partly attributable to quantum noise, although this is not the dominant mechanism that determines the resolution of such a device. In most cases, quantum noise refers to the fluctuations of signal in extremely precise optical systems with stabilized lasers and efficient detectors.

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