Squeezing of primordial gravitational waves as quantum discord
  We investigate the squeezing of primordial gravitational waves (PGWs) in
terms of quantum discord. We construct a classical state of PGWs without
quantum discord and compare it with the Bunch-Davies vacuum. Then it is shown
that the oscillatory behavior of the angular-power spectrum of the cosmic
microwave background (CMB) fluctuations induced by PGWs can be the signature of
the quantum discord of PGWs. In addition, we discuss the effect of quantum
decoherence on the entanglement and the quantum discord of PGWs for
super-horizon modes. For the state of PGWs with decoherence effect, we examine
the decoherence condition and the correlation condition introduced by C. Kiefer
et al. (Class. Quantum Grav. 24 (2007) 1699). We show that the decoherence
condition is not sufficient for the separability of PGWs and the correlation
condition implies that the PGWs in the matter-dominated era have quantum
discord.
