  We show in this letter that gravity coupled to a massless scalar field with
full cylindrical symmetry can be exactly quantized by an extension of the
techniques used in the quantization of Einstein-Rosen waves. This system
provides a useful testbed to discuss a number of issues in quantum general
relativity such as the emergence of the classical metric, microcausality, and
large quantum gravity effects. It may also provide an appropriate framework to
study gravitational critical phenomena from a quantum point of view, issues
related to black hole evaporation, and the consistent definition of test fields
and particles in quantum gravity.
