Hi all,
tomorrow we'll hear from Christian Schilling, who did his PhD with
last week's visitor Matthias Christandl and is now a postdoc in
Oxford. Here's the title and abstract:
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.
Best,
-joe