Von Neumann entropy penalization and low-rank matrix estimation

Author:
  • VKVladimir Koltchinskii
    Vladimir Koltchinskii

Abstract
We study a problem of estimation of a Hermitian nonnegatively definite matrix ρ of unit trace (e.g., a density matrix of a quantum system) based on n i.i.d. measurements (X1, Y1), …, (Xn, Yn), where Yj = tr(ρXj) + ξj, j = 1, …, n, {Xj} being random i.i.d. Hermitian matrices and {ξj} being i.i.d. random variables with . The estimator is considered, where is the set of all nonnegatively definite Hermitian m × m matrices of trace 1. The goal is to derive oracle inequalities showing how the estimation error depends on the accuracy of approximation of the unknown state ρ by low-rank matrices.

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