Ultracold atomic physics offers myriad possibilities to study strongly correlated many-body systems in lower dimensions. Typically, only ground state phases are accessible. Using a tunable quantum gas of bosonic cesium atoms, we realize and control in one dimensional geometry a highly excited quantum phase that is stabilized in the presence of attractive interactions by maintaining and strengthening quantum correlations across a confinement-induced resonance. We diagnose the crossover from repulsive to attractive interactions in terms of the stiffness and the energy of the system. Our results open up the experimental study of metastable excited manybody phases with strong correlations and their dynamical properties.In many-body quantum physics the interplay between strong interactions and confinement to a low-dimensional geometry amplifies the effects of quantum fluctuations and correlations. A remarkable example in one dimension is the Tonks-Girardeau (TG) gas, where bosons with strong repulsive interactions minimize their interaction energy by avoiding spatial overlap and acquire fermionic properties [1,2]. Evidence for this ground state phase was found using BoseEinstein condensates (BEC) loaded into optical lattices [3,4]. While many-body quantum systems are usually found in their ground state phases, long-lived excited state phases are responsible for some of the most striking physical effects, examples ranging from vortex lattices in superfluids to subtle topological excitations in spin liquids [5]. However, the experimental realization of excited phases is difficult, as these usually quickly decay by intrinsic effects or by coupling to the environment. In this context, cold atoms [3, 4, 6-12] may provide unique opportunities for the realization of long-lived, strongly interacting, excited many-body phases due to the excellent decoupling from the environment and the tunability of interactions via, for example, Feshbach resonances.For an ultracold one-dimensional (1D) system of bosons, we prepare a highly-excited many-body phase known as the super-Tonks-Girardeau (sTG) gas [13]. In this highlycorrelated quantum phase, interactions are attractive, and rapid decay into a cluster-type ground state is in principle possible. However, a surprising property of this many-body phase is its metastability. Attractive interactions strengthen correlations between particle positions and ensure, similar to an effective long-range repulsive interaction, that particles rarely come together. To realize this exotic phase, we observe and exploit a 1D confinement-induced resonance (CIR) [14,15]. This resonance allows us to first enter deeply into the repulsive TG regime to establish strong particle correlations and then to switch interactions from strongly repulsive to strongly attractive. The frequency ratio of the two lowest-energy collective modes [16] provides accurate diagnostics for the crossover from the TG to the sTG regime. In particle loss and expansion measurements we study the time evolution of the system through th...

Molecular cooling techniques face the hurdle of dissipating translational as well as internal energy in the presence of a rich electronic, vibrational, and rotational energy spectrum. In our experiment, we create a translationally ultracold, dense quantum gas of molecules bound by more than 1000 wave numbers in the electronic ground state. Specifically, we stimulate with 80% efficiency, a two-photon transfer of molecules associated on a Feshbach resonance from a Bose-Einstein condensate of cesium atoms. In the process, the initial loose, long-range electrostatic bond of the Feshbach molecule is coherently transformed into a tight chemical bond. We demonstrate coherence of the transfer in a Ramsey-type experiment and show that the molecular sample is not heated during the transfer. Our results show that the preparation of a quantum gas of molecules in specific rovibrational states is possible and that the creation of a Bose-Einstein condensate of molecules in their rovibronic ground state is within reach.