 Quantum field theory, in theoretical physics. Quantum field theory QFT is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics and is used to construct physical models of subatomic particles in particle physics and quasi-particles in condensed matter physics. QFT treats particles as excited states, also called quanta, of their underlying fields, which are in a sense more fundamental. Interactions between particles are described by interaction terms in the Lagrangian involving their corresponding fields. Each interaction can be visually represented by Feynman diagrams, which are a formal computational tool in the process of relativistic perturbation theory.