  Quantizing any model in which a Lagrange multiplier (LM) field is used to
restrict field configurations to those that satisfy the classical equations of
motion, leads to at most one-loop radiative corrections. This approach can be
used with both the Yang-Mills (YM) and Einstein-Hilbert (EH) action; the
resulting theory is both renormalizable and unitary, has a positive energy
spectrum and has no negative norm states contributing to physical processes.
Although this approach cannot be consistently used with scalar fields alone,
scalar fields can be coupled to gauge fields so that loop effects in the gauge
sector are restricted to one-loop order in a way that satisfies the usual
criterion for a consistent quantum field theory. The tree-level diagrams are
those of the classical theory in which the metric couples to the
energy-momentum tensor.
