Etymology of "quantum leap"

What is the etymology of the term "Quantum Leap / Quantum Jump"? Doesn't 'Quantum' mean small and discrete?

Dictionary.com: says:

• A dramatic advance, especially in knowledge or method, as in Establishing a central bank represents a quantum leap in this small country's development. This term originated as quantum jump in the mid-1900s in physics, where it denotes a sudden change from one energy state to another within an atom. Within a decade it was transferred to other advances, not necessarily sudden but very important ones.

But how came Quantum Leap to mean sudden and conspicuous change?

Solution 1:

You ask:

how did quantum leap come to mean a sudden and conspicuous change?

In physics, a quantum jump is an instantaneous¹ change of a quantum system from one state to an entirely different² state. So the meaning of "sudden and conspicuous" is not very much of a leap at all from the scientific definition.

The meaning of "large" really isn't justified by the scientific meaning, but it's easy to see how "sudden and conspicuous" could acquire the implication of "large".

¹ If you're a philosopher of quantum mechanics, you can argue over whether a quantum jump is really instantaneous, or whether it just appears to be so.
² Here, the technical term in quantum mechanics for entirely different is orthogonal.

Solution 2:

As suggested by the following source, it refers to the connotation of quantum meaning a "sudden large increase".

Quantum leap:

• This is a transferred or figurative use of the concept of the same name in particle physics or quantum mechanics. From the OED: "quantum increase, a sudden large increase; cf. quantum jump; quantum jump, an abrupt transition between one stationary state of a quantized system and another, with the absorption or emission of a quantum; also transf., a sudden large increase or advance; quantum leap, a sudden large advance.

(The Phrase Finder)

The following source illustrates the evolution of "quantum" meaning "something big":

• The first use of “quantum leap” to mean “really big” was in 1956, the OED says, in a discussion of the US-Soviet balance of power in a nuclear postwar world, where a writer described “The enormous multiplication of power, the ‘quantum leap’ to a new order of magnitude of destruction.”

But note that:

• Though using “quantum leap” to mean “big jump” is fully idiomatic, it’s best to avoid using just plain “quantum” to mean “huge” (especially if addressing a physicist). Most dictionaries still define it as simply “an amount.” It’s only the hyperbole that gives it mass quantity.

(archives.cjr.org)