Question: In page number 225 of Introduction to quantum field theory of peskin and schroeder, looking at the equation that follows to Eq. (7.41), I cannot understand how the general state $|\lambda_K>$ (one eigenstate of the full Hamiltonian Lorentz shifted so its momentum be K) is annihilated by two operators constrained to lie in distant wavepackets. This assumption allows to decompose this state as the product state of two excitations of the vacuum, and therefore is essencial to the proof of LSZ reduction formula. What is difficult for me to understand is that the fields that appear in the formula are the Heisenberg picture field for the interacting theory and as the vaccum of the interacting theory is also different to the free theory, I think that one cannot do any analogy with the free theory, so how must I think? Thanks!