  When a system undergoes a quantum phase transition, the ground-state
wave-function shows a change of nature, which can be monitored using the
fidelity concept. We introduce two Quantum Monte Carlo schemes that allow the
computation of fidelity and its susceptibility for large interacting many-body
systems. These methods are illustrated on a two-dimensional Heisenberg model,
where fidelity estimators show marked behaviours at two successive quantum
phase transitions. We also develop a scaling theory which relates the
divergence of the fidelity susceptibility to the critical exponent of the
correlation length. A good agreement is found with the numerical results.
