Sometimes, there is an absolute limit on the number of terms an officeholder can serve, while, in other cases, the restrictions are merely on the number of consecutive terms.
For example, Maxwell's Equations would always be capitalized, while quantum mechanics is usually un-capitalized. I searched half a dozen sites for an answer to this but found no clue about a rule. So go either way that makes you happy, but (most of all) be consistent.
Quantum is a Latin word that means 'how much'. So a quantum of energy is a specific amount of energy. Light sources such as candles or lasers shoot out (or emit) light in bits called photons. Photons are like packets. Each one has a certain little bit of energy.