  Copper-oxide high TC superconductors possess a number of exotic orders
co-existing with or proximal to superconductivity, whose quantum fluctuations
may account for the unusual behaviors of the normal state, even affecting
superconductivity. Yet, spectroscopic evidence about such quantum fluctuations
remains elusive. Here, we reveal spectroscopic fingerprints for such
fluctuations associated with a charge order (CO) in nearly optimally-doped
Bi2Sr2CaCu2O8+d, using resonant inelastic x-ray scattering (RIXS). In the
superconducting state, while the quasi-elastic CO signal decreases with
temperature, the interplay between CO fluctuations and bond-stretching phonons
in the form of a Fano-like interference paradoxically increases, incompatible
with expectations for competing orders. Invoking general principles, we argue
that this behavior reflects the properties of a dissipative system near an
order-disorder quantum critical point, where the dissipation varies with the
opening of the pseudogap and superconducting gap at low temperatures, leading
to the proliferation of quantum critical fluctuations which melt CO.
