  At quantum critical points (QCP)
\cite{Pfeuty:1971,Young:1975,Hertz:1976,Chakravarty:1989,Millis:1993,Chubukov:1
994,Coleman:2005} there are quantum fluctuations on all length scales, from
microscopic to macroscopic lengths, which, remarkably, can be observed at
finite temperatures, the regime to which all experiments are necessarily
confined. A fundamental question is how high in temperature can the effects of
quantum criticality persist? That is, can physical observables be described in
terms of universal scaling functions originating from the QCPs? Here we answer
these questions by examining exact solutions of models of correlated systems
and find that the temperature can be surprisingly high. As a powerful
illustration of quantum criticality, we predict that the zero temperature
superfluid density, $\rho_{s}(0)$, and the transition temperature, $T_{c}$, of
the cuprates are related by $T_{c}\propto\rho_{s}(0)^y$, where the exponent $y$
is different at the two edges of the superconducting dome, signifying the
respective QCPs. This relationship can be tested in high quality crystals.
