Note on canonical quantization and unitary equivalence in field theory

The problem of defining and constructing representations of the canonical commutation relations can be systematically approached via the technique of algebraic quantization. In particular, when the phase space of the system is linear and finite dimensional, the 'vertical polarization' provides an unambiguous quantization. For infinite-dimensional field theory systems, where the Stone-von Neumann theorem fails to be valid, even the simplest representation, the Schroedinger functional picture has some non-trivial subtleties. In this letter we consider the quantization of a real free scalar field - where the Fock quantization is well understood - on an arbitrary background and show that the representation from the most natural application of the algebraic quantization approach is not, in general, unitarily equivalent to the corresponding Schroedinger-Fock quantization. We comment on the possible implications of this result for field quantization. (letter to the editor)