A mass attached to spring is an example of a "harmonic oscillator." For the quantum harmonic oscillator we can find creation and destruction operators which create and destroy one quantum of energy. This seems like a promising development in our quest to develop a rigorous quantum description of photons.
The theory of quantum mechanics we developed in the previous series has some loose ends. Notably: 1) We talk about photons being emitted (created) and absorbed (destroyed) but we haven't given a rigorous description of this process. 2) The Dirac equation implies the presence of a "Dirac sea" of invisible, negative-energy electrons. Exciting a negative-energy electron to a positive-energy state appears as the creation of an electron-positron pair. A "better" theory would predict this creation directly without the bizarre, invisible Dirac sea.
The theory developed to tie up these loose ends is called Quantum Electrodynamics. It's the first (of several) quantum field theories.