  We consider possible detection of nonclassicality of primordial gravitational
waves (PGWs) by applying Hanbury Brown - Twiss (HBT) interferometry to
cosmology. We characterize the nonclassicality of PGWs in terms of
sub-Poissonian statistics that can be measured by the HBT interferometry. We
show that the presence of classical sources during inflation makes us possible
to detect nonclassical PGWs with the HBT interferometry. We present two
examples that realize the classical sources during inflation. It turns out that
PGWs with frequencies higher than 10 kHz enable us to detect their
nonclassicality.
