Hi all,

Tomorrow Christian Schilling will visit us and give a talk at 3pm in E41.1. Christian did his PhD in our group “back in the day”, with Matthias, and now is at Oxford. See below for title and abstract.

Best,

-joe

Title: Reconstructing quantum states from single-party information

Abstract: The possible compatibility of density matrices for single-party subsystems is described by linear constraints on their respective spectra. Whenever some of those quantum marginal constraints are saturated, the total quantum state has a specific, simplified structure. We prove that these remarkable global implications of extremal local information are stable, i.e. they hold approximately for spectra close to the boundary of the allowed region. Application of this general result to fermionic quantum systems allows us to characterize natural extensions of the Hartree-Fock ansatz and to quantify their accuracy by resorting to one-particle information, only: The fraction of the correlation energy not recovered by such an ansatz can be estimated from above by a simple geometric quantity in the occupation number picture.