Higher Asymptotics of Unitarity in "Quantization Commutes with   Reduction"
  Let M be a compact Kaehler manifold equipped with a Hamiltonian action of a compact Lie group G. In [Invent. Math. 67 (1982), no.~3, 515--538], Guillemin and Sternberg showed that there is a geometrically natural isomorphism between the G-invariant quantum Hilbert space over M and the quantum Hilbert space over the symplectic quotient M//G. This map, though, is not in general unitary, even to leading order in h-bar.   In [Comm. Math. Phys. 275 (2007), no.~2, 401--422], Hall and the author showed that when the metaplectic correction is included, one does obtain a map which, while not in general unitary for any fixed h-bar, becomes unitary in the semiclassical limit that h-bar goes to zero. (cf. the work of Ma and Zhang in [C. R. Math. Acad. Sci. Paris 341 (2005), no.~5, 297--302], and [Ast\'erisque No. 318 (2008), viii+154 pp.]). The unitarity of the classical Guillemin--Sternberg map and the metaplectically corrected analogue is measured by certain functions on the symplectic quotient M//G. In this paper, we give precise expressions for these functions, and compute complete asymptotic expansions for them as h-bar goes to zero. 
William D. Kirwin