attojoule
An SI unit of energy, work, and heat equal to 10⁻¹⁸ joules. Symbol: aJ
attojoule: one quintillionth of a joule
An attojoule is a unit of energy equal to 10−18 joules, or one quintillionth of a joule. The prefix atto comes from the Danish word atten, meaning eighteen, and denotes the 10−18 scale. In metrology and precision measurement, attojoules describe the energy scales of atomic and subatomic phenomena: the binding energy of electron pairs in superconductors, the energy dissipated in single molecular bonds, or thermal noise in ultra-sensitive instruments.
The attojoule occupies an extreme position in practical measurement. For context, a single photon of visible light carries roughly 3 attojoules of energy. A molecule at room temperature has thermal energy in the picojoule range, roughly a million times larger. The attojoule is therefore rarely encountered in routine industrial work; it appears primarily in fundamental physics laboratories, quantum metrology, and research into nanoscale systems. Most calibration work, inspection, and materials testing operates at scales of millijoules or higher.
Where the attojoule matters
Attojoules become relevant when measuring energy transfer in cryogenic or quantum systems. In scanning tunneling microscopy, the energy of tunneling electrons is expressed in attojoules. Precision calorimetry in fundamental research, studies of quantum dots, and detection of gravitational waves all operate at energy scales where the attojoule appears as a natural unit. Metrology laboratories establishing traceability chains for energy standards must account for noise and systematic effects at this scale when achieving the lowest uncertainties.
The term is rarely used in practice because 10−18 joules is often expressed instead as femtoelectronvolts (feV), which maps more naturally to quantum energy scales. A single attojoule is approximately 0.00624 feV. Most technical literature in quantum physics and nanoscience prefers the electronvolt family of units, leaving the attojoule as a formal SI unit of limited practical utility outside fundamental metrology standards and SI base unit definitions.