Upper limits on the amplitude of ultra-high-frequency gravitational
  waves from graviton-photon mixing
  In this work, we present the first experimental upper limits on the presence
of stochastic ultra-high-frequency gravitational waves. We exclude
gravitational waves in the frequency bands from $(2.7 - 14)\times10^{14}~$Hz
and $(5 - 12)\times10^{18}~$Hz down to a characteristic amplitude of $h_c^{\rm
min}\approx6\times 10^{-26}$ and $h_c^{\rm min}\approx 5\times 10^{-28}$ at
$95~$% confidence level, respectively. To obtain these results, we used data
from existing facilities that have been constructed and operated with the aim
of detecting WISPs (Weakly Interacting Slim Particles), pointing out that these
facilities are also sensitive to gravitational waves by graviton to photon
conversion in the presence of a magnetic field. The principle applies to all
experiments of this kind, with prospects of constraining (or detecting), for
example, gravitational waves from light primordial black hole evaporation in
the early universe.
