  The inflationary 1-loop tensor power spectrum from an excited spectator
scalar field is calculated. Recent studies on primordial black holes suggest
that the inflationary curvature perturbation may be huge on small scales. An
enhanced curvature perturbation may arise from a drastic enhancement of
spectator scalar field fluctuations. In this letter, using the in-in formalism,
we calculate 1-loop quantum corrections to primordial gravitational waves by
such an excited spectator field with a sharp peak in momentum space. We find
scale-invariant loop corrections in this full quantum setup, in contrast to the
sharply peaked corrections in the previously calculated scalar-induced tensor
modes. Especially, on super Hubble scales, the primordial gravitational waves
are also amplified, which can be understood as a Bogolyubov transformation of
the vacuum due to the excited scalar field. This mechanism allows us to probe
the scalar field properties on extremely short-distance scales with the current
and future cosmic microwave background and gravitational wave experiments,
opening a novel window for inflationary cosmology.
