attowatt

Attowatt (symbol aW) is the SI unit of power equal to 10^-18 watts. It is obtained by applying the SI prefix atto- to the watt, and since a watt is defined as one joule per second, 1 aW equals 1 × 10^-18 joules per second.

Because attowatt represents an extremely small amount of power, it appears mainly in specialized scientific and engineering contexts. Fields such as quantum optics, ultra-low-noise electronics, cryogenic detectors, and certain astronomical instrumentation discuss powers at the attowatt scale. In these areas, signals can be extremely weak or background noise levels can dominate, requiring carefully calibrated measurement setups, shielding from stray light and electromagnetic interference, and advanced detectors designed for single-photon or ultra-low-power operation.

Measurement and practical considerations: instruments capable of resolving attowatts typically operate at or near the limits of sensitivity for their domain. Calibrations often involve traceable references and methods to minimize loss and leakage. In many laboratories, attowatt-level signals are approached rather than directly measured, with techniques that infer power from related quantities or through carefully controlled attenuators and noise analysis.

Relation to other units: attowatt is three orders of magnitude smaller than a femtowatt (10^-15 W) and larger than zeptowatts (10^-21 W). It sits alongside other small prefixes such as femto-, pico-, nano-, micro-, and milli-, continuing a standardized scale for expressing tiny powers.

See also: watt, SI prefixes, photodetectors, quantum optics, low-noise instrumentation.