Loop updates for quantum Monte Carlo simulations in the canonical ensemble

We present a new non-local updating scheme for quantum Monte Carlo simulations, which conserves particle number and other symmetries. It allows exact symmetry projection and direct evaluation of the equal-time Green's function and other observables in the canonical ensemble. The method is applied to bosonic atoms in optical lattices, neutron pairs in atomic nuclei and electron pairs in ultrasmall superconducting grains.