  The effective quantum field theory description of gravity, despite its
non-renormalizability, allows for predictions beyond classical general
relativity. As we enter the age of gravitational wave astronomy, an important
and timely question is whether measurable quantum predictions that depart from
classical gravity, analogous to quantum optics effects which cannot be
explained by classical electrodynamics, can be found. In this work, we
investigate quantum signatures in gravitational waves using tools from quantum
optics. Squeezed-coherent gravitational waves, which can exhibit sub-Poissonian
graviton statistics, can enhance or suppress the signal measured by an
interferometer, a characteristic effect of quantum squeezing. Moreover, we show
that Gaussian gravitational wave quantum states can be reconstructed from
measurements over an ensemble of optical fields interacting with a single copy
of the gravitational wave, thus opening the possibility of detecting quantum
features of gravity beyond classical general relativity.
