  We propose a new quantum Monte Carlo algorithm to compute fermion
ground-state properties. The ground state is projected from an initial
wavefunction by a branching random walk in an over-complete basis space of
Slater determinants. By constraining the determinants according to a trial
wavefunction $|\Psi_T \rangle$, we remove the exponential decay of
signal-to-noise ratio characteristic of the sign problem. The method is
variational and is exact if $|\Psi_T\rangle$ is exact. We report results on the
two-dimensional Hubbard model up to size $16\times 16$, for various electron
fillings and interaction strengths.
