- This article is about the physical phenomenon. For the television program, see Quantum Leap.
In physics, a quantum leap or quantum jump is a change of an electron within an atom from one energy state to the next. This is a discontinuous change in which the electron goes from one energy level to another without passing through any intermediate levels. This phenomenon contradicted expectations set by theories older than quantum mechanics that energy should always change continuously.
More generally, a quantum leap is the smallest possible change, as when one's bank account balance goes from $500.00 (five hundred dollars) to $500.01 (five hundred dollars and one cent). There are no possible amounts intermediate between those.
Vernacular, slang, or metaphorical usage
In the vernacular, the term quantum leap has come to mean an abrupt change, especially an advance or augmentation. The term dates back to early-to-mid-20th century. The vernacular usage is not always in accord with the original meaning, in that a large abrupt change is often implied. A quantum leap in quantum mechanics is by definition the smallest change possible. The usages agree, however, in that both describe an advance that happens all at once, rather than gradually over time. A 'quantum leap in technology' is thus a revolutionary advance, rather than an evolutionary one.