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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.


EXPLANATION



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 ( Electron s).

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 strength of the current fluctuations can be expressed by giving the Variance

:\Delta I^2 \equiv \langle\left(I-\langle I angle
ight)^2 angle

of the current ''I'', where <''I''> is the average ("macroscopic") current. However, the value measured in this way depends on the frequency range of fluctuations which is measured (" squared divided by Hertz ). It may be defined as the zero-frequency Fourier Transform of the current-current correlation function:

:
S(t)=(\langle I(t)I(0) angle - \langle I(0) angle ^2)


''Note:'' This is the total noise power, which includes the equilibrium fluctuations ( Johnson-Nyquist Noise ). Some other commonly employed definitions may differ by a constant pre-factor.


In quantum optics

In Quantum Optics , shot noise is caused by the fluctations of detected Photon s, 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 Photo-multiplier s, but also at stronger light intensities measured by Photodiode s 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, e.g. a Laser , the shot noise scales as the square-root of the average intensity:
:\Delta I^2 \equiv \langle\left(I-\langle I angle
ight)^2 angle \propto I

The only exception being if a Squeezed Coherent State can be formed through correlated photon generation.


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.