  Using the scaling theory of quantum critical phenomena we present a detailed
analysis of various superconducting properties of La$_{2-x}$Sr$_{x}$CuO$_{4}$,
Y$_{0.8}$Ca$_{0.2}$-123, Tl-1212 and Tl-2201. Our results include: the
experimental data are fully consistent with a 2D insulator to superconductor
(ISQ) transition in the underdoped limit. The quantum critical properties
appear to be equivalent to those of a dirty 2D bosonic system with long range
Coulomb interaction. Together with previous evidence, this transition appears
to be a generic feature of cuprate superconductors. The 3D superconductor to
normal state quantum transition (SNQ) turns out to be less unique. More
extended experimental data is needed to settle the nature of this transition
unambiguously.
