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. . . photon.
True, a photon is a quantum of energy, E=hf.
A photon
photon
photon
When a quantum leap occurs, a single photon is emitted. A "quantum leap" is generally used in the context of "business-speak" or "politico-speak" where the speaker is implying that some event, or process, or decision represents a large change of great significance.
A 'quantum' is a very very very small, but discrete amount of something. For example a quantum of light is called a photon, you cannot get a smaller 'piece' of light than one photon.
Max Planck came up with the idea of a photon of energy, suggesting that instead of waves, each photon had a discrete amount, or a quantum hence the foundation of quantum mechanics. I assume it was him who gave it the name.
Photon
The photon - definitely.
The ISBN of Three Roads to Quantum Gravity is 0465078354.
Photon