We present a non-perturbative method through which local probes can access the two-point function of a free quantum field within a region of spacetime. By considering a lattice of gapless particle detectors, we identified the probe observables that encode the field’s two-point function. We quantify the discrepancies introduced by physical finite-sized interaction regions by performing a spacetime multipole expansion of the smeared two-point function. Our protocol expresses the two-point function entirely in terms of measurable detectors correlations, providing an operational notion of states in QFT.