One of the cornerstones of inflationary cosmology is that the large
scale structure of the Universe has a quantum mechanical origin. This
invites the question of whether compelling observational evidence for
the quantum nature of the origin can be found. Primordial
gravitational waves are generated directly from quantum fluctuations
during inflation. Since they interact with matter very weakly, travel
through the Universe virtually unimpeded, it is expected that they
keep their nonclassicality until today. In this talk, I will present
some methods to clarify whether the universe has a quantum mechanical
origin by detecting the nonclassical primordial gravitational waves.