  The diagrammatic Monte Carlo (Diag-MC) method is a numerical technique which
samples the entire diagrammatic series of the Green's function in quantum
many-body systems. In this work, we incorporate the flat histogram principle in
the diagrammatic Monte method and we term the improved version "Flat Histogram
Diagrammatic Monte Carlo" method. We demonstrate the superiority of the method
over the standard Diag-MC in extracting the long-imaginary-time behavior of the
Green's function, without incorporating any a priori knowledge about this
function, by applying the technique to the polaron problem
