All real systems suffer from the effects of random noise .
This noise is an important factor in determining how difficult it will be to detect and measure a given signal in a given amount of time. We therefore need to know the noise level of a given system if we want to decide if it is good enough to use for a particular purpose. Shot Noise produced by the 'particle-like' behaviour of electrons and photons. In theory, thermal noise can be reduced to a very low level by cooling a system to a temperature near absolute zero. For the sake of simplicity we will therefore assume that the thermal noise is small enough to be ignored and concentrate our attention on the effects of shot noise.