STARTING AT THE BEGINNING. “In quantum field framework the elementary degrees of freedom are the quantum oscillators describing the different momentum (and spin/helicity) modes of the field.”
https://onlinelibrary.wiley.com/doi/full/10.1002/prop.201800007?casa_token=GdPRXs_tGLEAAAAA%3Ag-pxPMtkcleTYDUNK3mYGfpEzKRyc2LqQknTgFFB7dkYFDL8rDe2s0f4_0Q0oKjXD4_AEgFPGWolVTQ