Résumé : We study the relationship between metric and algebraic structures on the section ring of a projective manifold and an ample line bundle over it. More precisely, we prove that once the kernel is factored out, the multiplication operator of the section ring becomes an approximate isometry (up to normalization) with respect to the $L^{2}$-norm. We then show that, in fact, those algebraic properties characterise $L^{2}$-norms and describe some applications of this classification. The semiclassical version of Ohsawa-Takegoshi theorem lies at the heart of our approach.

Mots-Clés : cohomology; plurisubharmonic metrics; ample line bundles; section ring; holomorphic extension theorem; norms on tensor products