Electrical noise arising from the discrete, random arrival of electrons at a contact or barrier.Shot noise is the random fluctuation in an electrical current that arises because charge is carried by discrete particles—electrons—rather than flowing as a continuous fluid. When electrons cross a barrier or junction, they arrive at random times, like raindrops hitting a roof. Even if the average current is perfectly steady, the instantaneous rate of arrival fluctuates, producing a small, broadband electrical noise that is fundamentally irreducible.
The phenomenon was first described by Walter Schottky in 1918, who called it the 'Schrot effect' (from the German word for small shot or pellets), while studying vacuum tubes.
The effect is most pronounced when electrons must overcome an energy barrier one by one, as in a vacuum tube diode or a semiconductor p-n junction. Each electron crossing the barrier contributes a discrete pulse of charge. The arrival times follow a Poisson process, so the variance of the current is proportional to the average current. This is distinct from thermal (Johnson) noise, which depends on temperature and resistance, and from flicker noise, which has a different frequency spectrum.
Shot noise sets a fundamental limit on the sensitivity of optical detectors and electronic amplifiers, but it is also exploited in quantum cryptography to generate truly random numbers.
In a typical electrical current of one ampere, about 6.24 × 10^18 electrons pass per second, yet the random timing of their arrival still produces a measurable noise signal.
