Measuring multipartite entanglement via dynamic susceptibilities

Entanglement plays a central role in our understanding of quantum many body physics, and is fundamental in characterising quantum phases and quantum phase transitions. Developing protocols to detect and quantify entanglement of many-particle quantum states is thus a key challenge for present experiments. Here, we show that the quantum Fisher information, representing a witness for genuinely multipartite entanglement, becomes measurable for thermal ensembles via the dynamic susceptibility, i.e., with resources readily available in present cold atomic gas and condensed-matter experiments. This moreover establishes a fundamental connection between multipartite entanglement and many-body correlations contained in response functions, with profound implications close to quantum phase transitions. There, the quantum Fisher information becomes universal, allowing us to identify strongly entangled phase transitions with a divergent multipartiteness of entanglement. We illustrate our framework using paradigmatic quantum Ising models, and point out potential signatures in optical-lattice experiments.Comment: 5+5 pages, 3+2 figure

Redetermination of the cation distribution of spinel (MgAl2O4) by means of neutron diffraction

The cation distribution of synthetic spinel powder of grain size ≤ 50 μ was redetermined by means of neutron diffraction experiments. The oxygen parameter found by Bacon (1952) has been confirmed, but, in contradiction to Bacon, the degree of inversion amounts to about 10 to 15 %, and depends upon the thermal history of the sample.La distribution des cations dans une poudre de spinelle synthétique d'une dimension de grains inférieure à 50 μ a été déterminée à nouveau au moyen d'expériences de diffraction neutronique. Tandis que le paramètre d'oxygène trouvé par Bacon (1952) a été confirmé, le degré d'inversion, par contre, est de 10 à 15 % et dépend de l'histoire thermique de l'échantillon

Light-induced dynamics of chlorophyll systems studied by neutron spectroscopy

Energy spectra of three chlorophyll-protein systems and of chlorophyll-a in solution were measured using the inelastic neutron scattering technique. The intensities of the observed energy spectra changed significantly when the sample was irradiated by light. This effect is interpreted in terms of electronic-vibrational relaxation mechanisms giving rise to radiationless transitions which enhance the population of the vibrational modes of all the constituents forming the sample either directly or by dissipation