Unwanted random fluctuations in an electrical signal.Electrical noise refers to unwanted, random fluctuations in voltage or current that ride on top of an intended signal. It is a fundamental limitation in all electronic systems, from audio amplifiers to quantum sensors. Noise can obscure weak signals, corrupt data, and set a floor on measurement precision. In physics experiments, distinguishing a true signal from noise is often the central challenge, especially when measuring tiny effects like single photons or atomic transitions.
The first quantitative description of thermal noise was given by John B. Johnson in 1926, and Harry Nyquist provided the theoretical explanation the same year. Shot noise was predicted by Walter Schottky in 1918.
Noise arises from several physical sources. Thermal noise (Johnson–Nyquist) comes from the random thermal motion of charge carriers in a conductor, increasing with temperature and resistance. Shot noise results from the discrete nature of electric charge, appearing as random fluctuations in current when electrons arrive at random times. Flicker noise (1/f noise) dominates at low frequencies and is linked to material imperfections. These fluctuations are inherently random, so they cannot be predicted exactly, only characterized statistically (e.g., by their power spectral density).
Engineers design low-noise amplifiers and use techniques like signal averaging and lock-in detection to extract signals buried in noise, essential in medical imaging (ECG, EEG), radio astronomy, and precision atomic clocks.
Even at absolute zero temperature, quantum mechanical zero-point fluctuations mean that electrical noise never completely disappears—a limit known as the quantum noise floor.
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