Shot noise
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Shot noise is a type of electronic noise that occurs when the finite number of particles that carry energy, such as electrons in an electronic circuit or photons in an optical device, is small enough to give rise to detectable statistical fluctuations in a measurement. It is important in electronics, telecommunications and fundamental physics.
The strength of this noise increases with the average magnitude of the current or intensity of the light. Often, however, as the signal increases more rapidly as the average signal becomes stronger, shot noise often is only a problem with small currents and light intensities.
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[edit] Explanation
[edit] In electronic devices
Shot noise in electronic devices consists of random fluctuations of the electric current in an electrical conductor, which are caused by the fact that the current is carried by discrete charges (electrons).
Shot noise is to be distinguished from current fluctuations in equilibrium, which happen without any applied voltage and without any average current flowing. These equilibrium current fluctuations are known as Johnson-Nyquist noise.
Shot noise is a Poisson process and the charge carriers which make up the current will follow a Poisson distribution. The noise is white noise with noise power given by
- <math>\Delta I^2=2e\langle I\rangle \Delta f
</math> where e is the elementary charge. The factor of 2 comes from the combination of negative and positive frequency components.
The noise power has SI units square amperes divided by hertz (A2/Hz).
Note: Shot noise and Johnson–Nyquist noise are both quantum fluctuations, and in some formulations are treated as a single unified concept.<ref>R. Sarpeshkar, T. Delbruck, and C. A. Mead, "White noise in MOS transistors and resistors", IEEE Circuits Devices Mag., pp. 23–29, Nov. 1993.</ref>
[edit] In quantum optics
In quantum optics, shot noise is caused by the fluctations of detected photons, again therefore a consequence of discretisation (of the energy in the electromagnetic field in this case). Shot noise is a main part of quantum noise.
Shot noise is measurable not only in measurements at the few-photons level using photomultipliers, but also at stronger light intensities measured by photodiodes when using high temporal resolution oscilloscopes. As the photocurrent is proportional to the light intensity (number of photons), the fluctuations of the electromagnetic field are usually contained in the electric current measured.
In the case of a coherent light source such as a laser, the shot noise scales as the square-root of the average intensity:
- <math>\Delta I^2 \ \stackrel{\mathrm{def}}{=}\ \langle\left(I-\langle I\rangle
\right)^2\rangle \propto I </math>
The only exception being if a squeezed coherent state can be formed through correlated photon generation.
[edit] Space charge
Low noise active electronic devices are designed such that shot noise is suppressed by the electrostatic repulsion of the charge carriers. Space charge limiting is not possible in photon devices.
[edit] References
<references/>
- This article contains material from the Federal Standard 1037C (in support of MIL-STD-188), which, as a work of the United States Government, is in the public domain.
