  We introduce a quantum Monte Carlo technique to calculate exactly at finite
temperatures the Green function of a fermionic quantum impurity coupled to a
bosonic field. While the algorithm is general, we focus on the single impurity
Anderson model coupled to a Holstein phonon as a schematic model for a
molecular transistor. We compute the density of states at the impurity in a
large range of parameters, to demonstrate the accuracy and efficiency of the
method. We also obtain the conductance of the impurity model and analyze
different regimes. The results show that even in the case when the effective
attractive phonon interaction is larger than the Coulomb repulsion, a
Kondo-like conductance behavior might be observed.
