From neutron stars to high-temperature superconductors, strongly interacting many-body systems at or near quantum degeneracy are a rich source of intriguing phenomena. The microscopic structure of the first-discovered quantum fluid, superfluid liquid helium, is difficult to access due to limited experimental probes. While an ultracold atomic Bose gas with tunable interactions (characterized by its scattering length, a) had been proposed as an alternative strongly interacting Bose system [1][2][3][4][5][6][7][8] , experimental progress [9][10][11][12] has been limited by its short lifetime. Here we present time-resolved measurements of the momentum distribution of a Bose-condensed gas that is suddenly jumped to unitarity, i.e. to a = ∞. Contrary to expectation, we observe that the gas lives long enough to permit the momentum to evolve to a quasi-steady-state distribution, consistent with universality, while remaining degenerate. Investigations of the time evolution of this unitary Bose gas may lead to a deeper understanding of quantum many-body physics.A powerful feature of atom gas experiments that provides access to these new regimes is the ability to change the interaction strength using a magnetic-field Feshbach resonance [13]. In particular, at the resonance location, a is infinite. For atomic Fermi gases [14][15][16][17][18][19][20], accessing this regime by adiabatically changing a led to the achievement of superfluids of paired fermions and enabled investigation of the crossover from superfluidity of weakly bound pairs, analogous to the Bardeen-Cooper-Schrieffer (BCS) theory of superconductors, to Bose-Einstein condensation (BEC) of tightly bound molecules [16,17]. For bosonic atoms, however, this route to strong interactions is stymied by the fact that three-body inelastic collisions increase as a to the fourth power [21][22][23]. This circumstance has limited experimental investigation of Bose gases with increasing interaction strength to studying either non-quantum-degenerate gases [24,25] or BECs with modest interaction strengths (na 3 < 0.008, where n is the atom number density) [9][10][11][12].The problem is that the loss rate scales as n 2 a 4 while the equilibration rate scales as na 2 v, where v is the average velocity. Thus, it would seem that the losses will always dominate as a is increased to ∞. Even if we were to forsake thermal equilibrium and suddenly change a in order to project a weakly interacting BEC onto strong interactions [12,[26][27][28], one might expect that three-body losses would still dominate the ensuing dynamics for large a. In this work, however, we use this approach to take a BEC to the unitary gas regime, and we observe dynamics that in fact saturate on a timescale shorter than that set by three-body losses and that exhibit universal scaling with density.

We convert a strongly interacting ultracold Bose gas into a mixture of atoms and molecules by sweeping the interactions from resonant to weak. By analyzing the decay dynamics of the molecular gas, we show that in addition to Feshbach dimers it contains Efimov trimers. Typically around 8% of the total atomic population is bound into trimers, identified by their density-independent lifetime of about 100 µs. The lifetime of the Feshbach dimers shows a density dependence due to inelastic atom-dimer collisions, in agreement with theoretical calculations. We also vary the density of the gas across a factor of 250 and investigate the corresponding atom loss rate at the interaction resonance.Experiments with ultracold atomic gases provide access to a vast array of intriguing phenomena, in part because of magnetically tunable Feshbach resonances. In particular, recent experimental [1][2][3][4][5] and theoretical [6][7][8][9][10][11][12][13][14][15][16] advances have made resonantly interacting Bose gases an exciting new research topic [17]. Unlike their fermionic counterparts, strongly interacting Bose systems are profoundly influenced by three-body phenomena, and help us understand the progression from two-through few-to many-body physics.At the Feshbach resonance the s-wave scattering length a diverges, and in the case of zero density the Feshbach molecule state, also of size a, merges with the free-atom state. This diatomic resonant scenario is the prelude for a set of exotic few-body phenomena, namely the Efimov effect. Although the Feshbach molecular state is unbound at the resonance, there exists an infinite log-periodic series of Efimov three-body bound states [18,19]. At 1/a → 0 the size of the p th Efimov state (p = 0, 1, 2...) is larger than the previous by a factor by 22.7, and its binding energy ET smaller by a factor of 22.7 2 [20,21]. At finite density n many-body effects complicate the physics. The system has an additional length scale, the interparticle spacing n −1/3 , that may determine how few-and many-body interactions scale. Many questions arise, such as: what are the structure, strength, length scale and dynamics of the two-, few-and many-body correlations? What does it mean to have a two-or three-atom molecule when it is embedded in a gas with interparticle spacing comparable to the molecular size?Both the ambiguous constitution of two-and three-body states in a many-body environment and the short-lived quasiequilibrium of a resonantly interacting Bose gas [3] complicate experiments. For these reasons, many experiments simplify matters by reducing interactions to a well-understood regime before imaging [1][2][3][4]. This interaction sweep can preserve resonance fossils in the form of perceived loss [1, 2, 4], momentum generation [3], and molecule formation.In this letter, we create a mixture of 85 Rb (free atoms), 85 Rb * 2 (Feshbach dimers), and 85 Rb * 3 (Efimov trimers) by sweeping a resonantly interacting degenerate Bose gas onto the molecular states in the weakly-interacting regime (na 3 1). O...

We measured ratios of van der Waals potential coefficients (C3) for different atoms (Li, Na, K, and Rb) interacting with the same surface by studying atom diffraction from a nanograting. These measurements are a sensitive test of atomic structure calculations because C3 ratios are strongly influenced by core electrons and only weakly influenced by the permittivity and geometry of the surface. Our measurement uncertainty of 2% in the ratio C(3)(K)/C(3)(Na) is close to the uncertainty of the best theoretical predictions, and some of these predictions are inconsistent with our measurement.

No abstract