  Since the relic gravitons are produced in entangled states of opposite
(comoving) three-momenta, their distributions and their averaged multiplicities
must determine the maximal frequency of the spectrum above which the created
pairs are exponentially suppressed. The absolute upper bound on the maximal
frequency derived in this manner coincides with the THz domain and does not
rely on the details of the cosmological scenario. The THz limit also translates
into a constraint of the order of $10^{-33}$ on the minimal chirp amplitudes
that should be effectively reached by all classes of hypothetical detectors
aiming at the direct scrutiny of a signal in the frequency domain that
encompasses the MHz and the GHz bands. The obtained high-frequency limit is
deeply rooted in the quantumness of the produced gravitons whose multiparticle
final sates are macroscopic but always non-classical. Since the unitary
evolution preserves their coherence, the quantumness of the gravitons can be
associated with an entanglement entropy that is associated with the loss of the
complete information on the underlying quantum field. It turns out that the
reduction of the density matrix in different bases leads to the same Von
Neumann entropy whose integral over all the modes of the spectrum is dominated
by the maximal frequency. Thanks to the THz bound the total integrated entropy
of the gravitons can be comparable with the cosmic microwave background entropy
but not larger. Besides the well known cosmological implications, we then
suggest that a potential detection of gravitons between the MHz and the THz may
therefore represent a direct evidence of macroscopic quantum states associated
with the gravitational field.
