In physics, quantum spin is a mathematical construct applied to elementary particles like photons or electrons. It is called imaginary because if an electron is a particle with no size, it can’t actually spin, and if it had a size, its edges would have to move faster than light to explain the observed spin rate. Quantum spin is then imaginary because photons and electrons are points with no structure, but this model gives them a structure in quantum space. For example, a photon is a sine wave generated in a complex plane that exists outside our space, so it can actually spin on its movement axis.
QR3.7.1. The Curious Case of Quantum Spin
QR3.7.2. Quantum Directions
QR3.7.3. Polarization