We report the experimental observation of the full phase diagram across a transition where the spatial parity symmetry is broken. Our system consists of an ultracold gas of 39K with tunable interactions trapped in a doublewell potential. At a critical value of the interaction strength, we observe a continuous quantum phase transition where the gas localizes in one well or the other, thus breaking the underlying symmetry of the system. Furthermore, we show the robustness of the asymmetric state against controlled energy mismatch between the two wells. This is the result of hysteresis associated with an additional discontinuous quantum phase transition that we fully characterize. Our results pave the way to the production of a broad class of quantum entangled states including Schroedinger cat states with macroscopic atom number. A. Trenkwalder et al., 
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An open issue in quantum physics is to understand the interplay of disorder and interactions, which has been predicted to give rise to exotic states of matter such as quantum glasses or manybody localization. In a collaboration with theorists in Geneva and in Orsay, we have employed ultracold atoms with controllable disorder and interaction to study the paradigmatic problem of disordered bosons in the full disorderinteraction plane. Combining measurements of coherence, transport and excitation spectra, we have got evidence of an insulating regime extending from weak to strong interaction and surrounding a superfluidlike regime, in general agreement with the theory. For strong interaction, we have revealed the presence of a stronglycorrelated Bose glass coexisting with a Mott insulator. We have analyzed the finitetemperature effects on the phase diagram by comparing experimental results to exact diagonalization for smallsized systems and to densitymatrix renormalization group (DMRG) computations. At weak interactions, we have found short thermal correlation lengths, indicating a substantial impact of temperature on the system coherence. Conversely, at strong interactions, the obtained thermal correlation lengths are significantly larger than the localization length, and the quantum nature of the T = 0 Boseglass phase is preserved up to a crossover temperature that depends on the disorder strength. Furthermore, in the absence of disorder, by comparing experimental results to quasiexact finiteT DMRG computations, we can estimate the temperature in the experimental system. C. D’Errico et al. L. Gori et al. 
The Mott insulator is a well know quantum phase appearing in periodic potentials at integer particle fillings. In ordinary matter the potential strength cannot be changed, and it is therefore impossible to study the fate of the Mott insulator for vanishing potential strength. We have now employed an ultracold quantum gas to investigate the superfluidinsulator transition of onedimensional bosons in shallow periodic potentials. Experimentally, we have performed transport measurements and we have analyzed them with a phase slip based model to accurately determine the Mott transition. We have compared the experimental results with a theoretical analysis based on quantum Monte Carlo simulations in continuum space and Luttinger liquid approach. Experiments and theory are in excellent agreement. Our study provides a quantitative determination of the critical parameters for the Mott transition and defines the regimes of validity of widely used approximate models, namely, the BoseHubbard and sineGordon models. G. Boéris et al. 
We report on the experimental observation of a strongly interacting gas of ultracold twoelectron fermions with an orbital degree of freedom and magnetically tunable interactions. This realization has been enabled by the demonstration of a novel kind of Feshbach resonance occurring in the scattering of two ^{173}Yb atoms in different nuclear and electronic states. The strongly interacting regime at resonance is evidenced by the observation of anisotropic hydrodynamic expansion of the twoorbital Fermi gas. These results pave the way towards the realization of new quantum states of matter with strongly correlated fermions with an orbital degree of freedom. G. Pagano et al., See also the Physics Viewpoint by S. Cornish: S. Cornish 

In ultracold atoms settings, inelastic light scattering is a preeminent technique to reveal static and dynamic properties at nonzero momentum. In this work, we investigate an array of onedimensional trapped Bose gases, by measuring both the energy and the momentum imparted to the system via light scattering experiments. The measurements are performed in the weak perturbation regime, where these two quantities — the energy and momentum transferred — are expected to be related to the dynamic structure factor of the system. We discuss this relation, with special attention to the role of intrap dynamics on the transferred momentum. N. Fabbri et al., 