  We analyze fast interaction cycles in bipartite quantum systems, showing that
the statistics in one of the parties (the detector) can be used to determine
the two-point correlator of the observable which mediates the coupling in the
other (the target). We apply the results to the response of particle detectors
coupled to quantum fields and subject to this kind of interactions. We show
that, in principle, such a setup can be used to experimentally obtain a direct
evaluation of the Wightman function (both its real and imaginary part) for any
field state.
