> what we call a particle is usually a momentum eigenstate Not really. In non-relativistic QM, it's a wave packet. True momentum eigenstates are not physically

> what we call a particle is usually a momentum eigenstate Not really. In non-relativistic QM, it's a wave packet. True momentum eigenstates are not physically realizable. In quantum field theory, "particle" is just a shorthand name for certain quantum field states. Quantum fields aren't waves or particles, they're quantum fields.