Reddit reviews Quantum Optics For Experimentalists

>seems to confuse me more, but I’m very interested. Can you please elaborate or provide somewhere I can read a bit more about that?

That's a hard question, but one of my favorite experiment-friendly textbooks is Quantum Optics for Experimentalists by Ou).

To elaborate more, I would say, that using certain parallels between classical physics and quantum physics, people have worked backwards from the classical theory of electromagnetism to develop a quantum theory of electromagnetism (the complete theory is known as quantum electrodynamics, of which quantum optics is a part).

(Seriously abbreviating/oversimplifying here, but...)

For each value of momentum, polarization, and frequency, the quantum electromagnetic field has evenly spaced energy levels, not unlike the different orbitals an electron has in an atom. Each quantum of energy in this mode of the electromagnetic field counts as one photon. The full state of the field would be a superposition of different photon number states for every value of frequency and momentum of the electromagnetic field.

For example, a laser beam would be described as a superposition of many different photon number states over a narrow spread in frequency and momentum, while thermal light from blackbody radiation would instead be a statistical distribution of photon number states. You can also create nonclassical states of light like squeezed light, and beams of entangled photon pairs in spontaneous parametric down-conversion.

Edit: "One Photon" can also be a single-photon state that's a superposition over different values of frequency and momentum. This would be a single-photon wave packet.