Berezin quantization of noncommutative projective varieties

Abstract

We use operator algebras and operator theory to obtain new result concerning Berezin quantization of compact K\"ahler manifolds. Our main tool is the notion of subproduct systems of finite-dimensional Hilbert spaces, which enables all involved objects, such as the Toeplitz operators, to be very conveniently expressed in terms of shift operators compressed to a subspace of full Fock space. This subspace is not required to be contained in the symmetric Fock space, so from finite-dimensional matrix algebras we can construct noncommutative manifolds with extra structure generalizing that of a projective variety endowed with a positive Hermitian line bundle and a canonical K\"ahler metric in the class of the line bundle. Even in the commutative setting these constructions are very fruitful. Firstly, we show that the algebra of smooth functions on any smooth projective variety can be quantized in a strong sense of inductive limits, as was previously only accomplished for homogeneous manifolds. In this way the K\"ahler manifold is recovered exactly from quantization and not just approximately. Secondly, we obtain a strict quantization also for singular varieties. Thirdly, we show that the Arveson conjecture is true in full generality for shift operators compressed to the subspace of symmetric Fock space associated with any homogeneous ideal. For noncommutative examples we consider homogeneous spaces for compact matrix quantum groups which generalize $q$-deformed projective spaces, and we show that these can be obtained as the cores of Cuntz--Pimsner algebras constructed solely from the representation theory of the quantum group. We also discuss interesting connections with noncommutative random walks.