QUROPE Quantum Information Processing and Communication in Europe

The robustness of fractional quantum Hall states is measured as the energy gap separating the Laughlin ground-state from excitations. Using thermodynamic approximations for the correlation functions of the Laughlin state and the quasihole state, we evaluate the gap in a two-dimensional system of dipolar atoms exposed to an artificial gauge field. For Abelian fields, our results agree well with the results of exact diagonalization for small systems, but indicate that the large value of the gap predicted in [Phys. Rev. Lett. 94, 070404 (2005)] was overestimated.

Using exact diagonalization for a small system of cold bosonic atoms, we analyze the emergence of strongly correlated states in the presence of an artificial magnetic field. This gauge field is generated by a laser beam that couples two internal atomic states, and it is related to Berry's geometrical phase that emerges when an atom follows adiabatically one of the two eigenstates of the atom-laser coupling. Our approach allows us to go beyond the adiabatic approximation, and to characterize the generalized Laughlin wave functions that appear in the strong magnetic field limit.

We propose a method to probe dynamical spin correlations of strongly interacting systems in optical lattices. The scheme uses a light-matter quantum non-demolition interface to map consecutively a given non trivial magnetic observable of the strongly correlated system to the light. The quantum memory is essential to coherently store the previously mapped observable during a time scale comparable to the many-body dynamics. A final readout of the memory yields direct access to dynamical correlations.

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.

We study the Mott phases and the superfluid-insulator transition of two-component ultracold bosons on a square optical lattice in the presence of a non-Abelian synthetic gauge field, which renders a SU(2) hopping matrix for the bosons. Using a resummed hopping expansion, we calculate the excitation spectra in the Mott insulating phases and demonstrate that the superfluid-insulator phase boundary displays a non-monotonic dependence on the gauge field strength.

We study quantum entanglement distribution on networks with full-rank bi-partite mixed states linking qubits on nodes. In particular, we use entanglement swapping and purification to partially entangle widely separated nodes. The simplest method consists of performing entanglement swappings along the shortest chain of links connecting the two nodes. However, we show that this method may be improved upon by choosing a protocol with a specific ordering of swappings and purifications. A priori, the design that produces optimal improvement is not clear.

We study the extended Bose--Hubbard model describing an ultra-cold gas of dipolar molecules in an optical lattice, taking into account all on-site and nearest-neighbor interactions, including occupation-dependent tunneling and pair tunneling terms. Using exact diagonalization and the multi-scale entanglement renormalization ansatz (MERA), we show that these terms can destroy insulating phases and lead to novel quantum phases. These considerable changes of the phase diagram have to be taken into account in upcoming experiments with dipolar molecules.

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.

S. Montangero (P1 UULM), seminar, Optimal control of Many-Body Quantum Systems

S. Montangero (P1 UULM), seminar