QUROPE Quantum Information Processing and Communication in Europe

The strength of classical correlations is subject to certain constraints, commonly known as Bell inequalities. Violation of these inequalities is the manifestation of nonlocality—displayed, in particular, by quantum mechanics, meaning that quantum mechanics can outperform classical physics at tasks associated with such Bell inequalities. Interestingly, however, there exist situations in which this is not the case.

We study the counting statistics of ultracold bosonic atoms that are released from an optical lattice. We show that the counting probability distribution of the atoms collected at a detector located far away from the optical lattice can be used as a method to infer the properties of the initially trapped states. We consider initial superfluid and insulating states with different occupation patterns. We analyze how the correlations between the initially trapped modes that develop during the expansion in the gravitational field are reflected in the counting distribution.

One of the unsolved problems in the characterization of the optimal entanglement witnesses is the existence of optimal witnesses acting on bipartite Hilbert spaces H_{m,n}=C^{m}\otimes C^{n} such that the product vectors obeying <e,f|W|e,f>=0 span H_{m,n}. So far, the only known example of such witness was found among indecomposable witnesses and is the one corresponding to the Choi map. However, it remains an open question whether there exist decomposable witnesses without the above property of spanning.

Various quantum phenomena like high-Tc superconductivity or quark confinement are still awaiting universally accepted explanations, because of the computational complexity of solving simplified theoretical models designed to capture their relevant physics. Feynman suggested solving such models by "quantum simulation" with a device designed to obey the same quantum many-body dynamics. So far, the community has mostly focused on developing the \emph{controllability} of quantum simulators.

We investigate the thermodynamics of a combined Dicke and Ising model that exhibits a rich phenomenology arising from the second-order and quantum phase transitions from the respective models. The partition function is calculated using mean-field theory, and the free energy is analyzed in detail to determine the complete phase diagram of the system.

We consider three level atoms driven by two resonant light fields in a ladder scheme where the upper level is a highly excited Rydberg state. We show that the dipole--dipole interactions between Rydberg excited atoms prevents the formation of single particle dark states and leads to strongly correlated photon emission from atoms separated by distances large compared to the emission wavelength. For two atoms, correlated photon pairs are emitted with an angular distribution given by a coherent sum of the independent dipolar fields.

We present a filtered backprojection algorithm for reconstructing the Wigner function of a system of large angular momentum j from Stern–Gerlach-type measurements. Our method is advantageous over the full determination of the density matrix in that it is insensitive to experimental fluctuations in j, and allows for a natural elimination of high-frequency noise in the Wigner function by taking into account the experimental uncertainties in the determination of j, its projection m and the quantization axis orientation.

On the 4th of February 2011 the European Commission has sent us the official approval to our request to add Basel Universität (UNIBAS) among the AQUTE partners, following the move of Professor Philipp Treutlein from München (LMU, AQUTE partner P11) to Basel.

We present a proposal for a versatile cold-atom-based quantum simulator of relativistic fermionic theories and topological insulators in arbitrary dimensions. The setup consists of a spin-independent optical lattice that traps a collection of hyperfine states of the same alkaline atom, to which the different degrees of freedom of the field theory to be simulated are then mapped. We show that the combination of bi-chromatic optical lattices with Raman transitions can allow the engineering of a spin-dependent tunneling of the atoms between neighboring lattice sites.