  The analytic structure of non-Gaussian correlators in inflationary
cosmologies has recently been proposed as a test of the quantum origin of
structure in the universe. To further understand this proposal, we explore the
analogous equal-time in-in correlators in flat space and show they exhibit the
same features as their cosmological counterparts. The quantum vacuum is
uniquely identified by in-in correlators with a total energy pole and no
additional poles at physical momenta. We tie this behavior directly to the
S-matrix and show that poles at physical momenta always arise from scattering
of particles present in the initial state. We relate these flat-space in-in
correlators to the probability amplitude for exciting multiple Unruh-de Witt
detectors. Localizing the detectors in spacetime, through the uncertainty
principle, provides the energy and momentum needed to excite the vacuum and
explains the connection to cosmological particle production. In addition, the
entanglement of these detectors provides a probe of the entangled state of the
underlying field and connects the properties of the correlators to the range of
entanglement of the detectors.
