Multipartite Entanglement in Atomic Systems

The key element for most of quantum technologies is entanglement, a kind of correlation which only exists in quantum mechanics. For instance, in quantum sensing entanglement improves the sensitivity of interferometric measurements: the relative phase of two interfering waves is measured more precisely when the waves are initially entangled. This method is applied for example for photonic waves on the LIGO gravitational wave interferometer. In the MENTA project we aim at generating, verifying and characterizing multipartite entanglement between a small number (>2) of spatially separated atomic clouds containing many atoms. A central goal of our project is to discover robust, experimentally accessible criteria to witness, characterize and explore the many facets of multipartite quantum correlations. Multipartite entanglement provides formidable challenges arising from the exponential increase of possibilities (=Hilbert space dimension) with the number of the quantum systems. The full classification of multipartite entanglement is missing in the literature, and the possibility to witness the classes of entangled states allowing quantum advantages in different quantum information applications is still largely unexplored. MENTA is a collaboration of four experimental groups from Germany, France, Italy and Austria and two theoretical groups from Spain and Italy. A crucial asset in the project is the dedicated collaboration of the two theoretical groups to guide the exploration of the many possible experimental strategies to make these characterizations. We will explore different experimental approaches based on the same principle: the interactions present in a dense ultracold atomic will induce entanglement between atoms. Depending on the experimental configuration, either the momenta or position of the atoms or their spins are entangled. In this last case, the entanglement needs to be transferred to position or momenta in order to obtain separated entangled clouds. We will look for the approach providing the most robust and useful entanglement. This experimentally challenging project also requires to extend our theoretical understanding of multipartite entanglement. Combining experimental investigations and theoretical efforts, we will develop new concepts to describe, experimentally measure, verify and apply this type of entanglement.