Systems of three interacting particles are notorious for their complex physical behaviour. A landmark theoretical result in few-body quantum physics is Efimov's prediction of a universal set of bound trimer states appearing for three identical bosons with a resonant two-body interaction. Counterintuitively, these states even exist in the absence of a corresponding two-body bound state. Since the formulation of Efimov's problem in the context of nuclear physics 35 years ago, it has attracted great interest in many areas of physics. However, the observation of Efimov quantum states has remained an elusive goal. Here we report the observation of an Efimov resonance in an ultracold gas of caesium atoms. The resonance occurs in the range of large negative two-body scattering lengths, arising from the coupling of three free atoms to an Efimov trimer. Experimentally, we observe its signature as a giant three-body recombination loss when the strength of the two-body interaction is varied. We also detect a minimum in the recombination loss for positive scattering lengths, indicating destructive interference of decay pathways. Our results confirm central theoretical predictions of Efimov physics and represent a starting point with which to explore the universal properties of resonantly interacting few-body systems. While Feshbach resonances have provided the key to control quantum-mechanical interactions on the two-body level, Efimov resonances connect ultracold matter to the world of few-body quantum phenomena.

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...

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Bose-Einstein condensation of cesium atoms is achieved by evaporative cooling using optical trapping techniques. The ability to tune the interactions between the ultracold atoms by an external magnetic field is crucial to obtain the condensate and offers intriguing features for potential applications. We explore various regimes of condensate self-interaction (attractive, repulsive, and null interaction strength) and demonstrate properties of imploding, exploding, and non-interacting quantum matter.

An ultracold molecular quantum gas is created by application of a magnetic field sweep across a Feshbach resonance to a Bose-Einstein condensate of cesium atoms. The ability to separate the molecules from the atoms permits direct imaging of the pure molecular sample. Magnetic levitation enables study of the dynamics of the ensemble on extended time scales. We measured ultralow expansion energies in the range of a few nanokelvin for a sample of 3000 molecules. Our observations are consistent with the presence of a macroscopic molecular matter wave.