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.

A powerful set of universal relations, centered on a quantity called the contact, connects the strength of short-range two-body correlations to the thermodynamics of a many-body system with delta-function interactions. We report on measurements of the contact, using RF spectroscopy, for an 85 Rb atomic Bose-Einstein condensate (BEC). For bosons, the fact that contact spectroscopy can be used to probe the gas on short timescales is useful given the decreasing stability of BECs with increasing interactions. A complication is the added possibility, for bosons, of three-body interactions. In investigating this issue, we have located an Efimov resonance for 85 Rb atoms with loss measurements and thus determined the three-body interaction parameter. In our contact spectroscopy, in a region of observable beyond-mean-field effects, we find no measurable contribution from three-body physics.Systems with strong quantum correlations represent a frontier in our understanding of the complex quantum systems found in nature, and atomic Bose-Einstein condensates (BEC) provide a versatile system in which to explore beyond mean-field physics. Ultracold atoms experience two-body, short-range interactions that are well described theoretically by a delta-function pseudopotential characterized by an s-wave scattering length a. In the simplest BEC experiments the values of a and of the density n are such that interactions are too weak, compared to the kinetic energy cost of correlations, to take the gas out of the mean-field regime. The presence of a lattice potential can greatly suppress this kinetic energy cost, thus freeing the system to explore a much richer portion of many-body state space [1]. The application of an external lattice potential, however, imposes an artificial orderliness not found in bosons in the wild. To explore strong interactions in a more naturalistic bulk three-dimensional gas, one can increase a by means of a magnetic-field-tunable Feshbach scattering resonance [2]. Such efforts are motivated for instance by a desire to make better conceptual connections to the iconic strongly correlated fluid, liquid helium.In practice it has proven difficult to study atomic BEC with increasing a and only a few experiments have measured beyond-mean-field interaction effects in these systems [3][4][5]. The difficulty comes from the fact that an increase in a is accompanied by a dramatic increase in the rate of inelastic three-body processes [6,7]. This leads to large losses and significant heating of the trapped gas on a timescale similar to that for global equilibrium of the trapped cloud. Probes of the gas that require global equilibrium, such as measurements of the density distribution or the amplitude or frequency of collective density oscillations in a trap, are therefore limited to systems that are only modestly out of the mean-field regime. Our strategy for exploring BEC with larger interaction strengths is to start from an equilibrated weakly interacting gas, change the interaction strength relatively quickly, forsa...

We demonstrate a photon-counting technique for detecting Bragg excitation of an ultracold gas of atoms. By measuring the response of the light field to the atoms, we derive a signal independent of traditional time-of-flight atom-imaging techniques. With heterodyne detection we achieve photon shot-noise limited detection of the amplification or depletion of one of the Bragg laser beams. Photon counting for Bragg spectroscopy will be useful for strongly interacting gases where atom-imaging detection fails. In addition, this technique provides the ability to resolve the evolution of excitations as a function of pulse duration.