  The entanglement harvesting protocol is an operational way to probe vacuum
entanglement. This protocol relies on two atoms, modelled by Unruh-DeWitt
detectors, that are initially unentangled. These atoms then interact locally
with the field and become entangled. If the atoms remain spacelike separated,
any entanglement between them is a result of entanglement that is `harvested'
from the field. Thus, quantifying this entanglement serves as a proxy for how
entangled the field is across the regions in which the atoms interacted. Using
this protocol, it is demonstrated that while the transition probability of an
individual inertial atom is unaffected by the presence of a gravitational wave,
the entanglement harvested by two atoms depends sensitively on the frequency of
the gravitational wave, exhibiting novel resonance effects when the energy gap
of the detectors is tuned to the frequency of the gravitational wave. This
suggests that the entanglement signature left by a gravitational wave may be
useful in characterizing its properties, and potentially useful in exploring
the gravitational-wave memory effect and gravitational-wave induced
decoherence.
