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Ultracold atoms could help in understanding the physics of strongly interacting many-body systems, but the creation of degenerate Bose gases at unitarity has been hampered by the losses. An experiment overcomes these problems and investigates the time evolution of a unitary Bose gas. 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 owing to limited experimental probes. Although 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, where . 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.

Polar molecules are desirable systems for quantum simulations and cold chemistry. Molecular ions are easily trapped, but a bias electric field applied to polarize them tends to accelerate them out of the trap. We present a general solution to this issue by rotating the bias field slowly enough for the molecular polarization axis to follow but rapidly enough for the ions to stay trapped. We demonstrate Ramsey spectroscopy between Stark-Zeeman subleveis in ¹⁸⁰Hf¹⁹F⁺ with a coherence time of 100 milliseconds. Frequency shifts arising from well-controlled topological (Berry) phases are used to determine magnetic g factors. The rotating-bias-field technique may enable using trapped polar molecules for precision measurement and quantum information science, including the search for an electron electric dipole moment.

A Bose-Einstein condensate was produced in a vapor of rubidium-87 atoms that was confined by magnetic fields and evaporatively cooled. The condensate fraction first appeared near a temperature of 170 nanokelvin and a number density of 2.5 × 10$^{12}$ per cubic centimeter and could be preserved for more than 15 seconds. Three primary signatures of Bose-Einstein condensation were seen. (i) On top of a broad thermal velocity distribution, a narrow peak appeared that was centered at zero velocity. (ii) The fraction of the atoms that were in this low-velocity peak increased abruptly as the sample temperature was lowered. (iii) The peak exhibited a nonthermal, anisotropic velocity distribution expected of the minimum-energy quantum state of the magnetic trap in contrast to the isotropic, thermal velocity distribution observed in the broad uncondensed fraction.

We have measured magnetic trap lifetimes of ultra-cold(87) Rb atoms at distances of 5-1000 mum from surfaces of conducting metals with varying resistivity. Good agreement is found with a theoretical model for losses arising from near-field magnetic thermal noise, confirming the complications associated with holding trapped atoms close to conducting surfaces. A dielectric surface (silicon) was found in contrast to be so benign that we are able to evaporatively cool atoms to a Bose-Einstein condensate by using the surface to selectively adsorb higher energy atoms.

A cold-atom experiment confirms Boltzmann’s special case predicted more than a century ago: the ‘breathe’ mode of a gas in a perfectly isotropic three-dimensional harmonic potential is never damped by elastic collisions. Boltzmann noticed that his transport equation predicts special cases in which gases never reach thermal equilibrium. One example is the monopole breathe mode of atoms confined in a perfectly isotropic three-dimensional (3D) harmonic potential 1 . Such a complete absence of damping had not been observed in nature, and this anomaly weakened Boltzmann’s then-controversial claim to have established a microscopic, atomistic basis for thermodynamics. Only recently has non-damping of a monopole mode in lower-dimensional systems been reported in cold-atom experiments performed in highly elongated trap geometries 2 , 3 . The difficulty in generating a sufficiently spherical harmonic confinement for cold atoms has so far prevented the observation of Boltzmann’s fully 3D, isotropic case. Here, thanks to a new magnetic trap 4 capable of producing near-spherical harmonic confinement for cold atoms, we report a long-delayed vindication for Boltzmann: the observation of a 3D monopole mode for which the collisional contribution to damping vanishes.

We designed and constructed a simplified experimental system to create a Bose-Einstein condensate in (87)Rb. Our system has several novel features including a mechanical atom transfer mechanism and a hybrid Ioffe-Pritchard magnetic trap. The apparatus has been designed to consistently produce a stable condensate even when it is not well optimized.

When atoms in a gas are cooled to extremely low temperatures, they will-under the appropriate conditions-condense into a single quantum-mechanical state known as a Bose-Einstein condensate. In such systems, quantum-mechanical behaviour is evident on a macroscopic scale. Here we explore the dynamics of how a Bose-Einstein condensate collapses and subsequently explodes when the balance of forces governing its size and shape is suddenly altered. A condensate's equilibrium size and shape is strongly affected by the interatomic interactions. Our ability to induce a collapse by switching the interactions from repulsive to attractive by tuning an externally applied magnetic field yields detailed information on the violent collapse process. We observe anisotropic atom bursts that explode from the condensate, atoms leaving the condensate in undetected forms, spikes appearing in the condensate wavefunction and oscillating remnant condensates that survive the collapse. All these processes have curious dependences on time, on the strength of the interaction and on the number of condensate atoms. Although the system would seem to be simple and well characterized, our measurements reveal many phenomena that challenge theoretical models.

We study the dynamics of large vortex lattices in a dilute gas Bose–Einstein condensate. Rapidly rotating condensates are created that contain vortex lattices with up to 300 vortices. The condensates are held in a parabolic trapping potential, and rotation rates exceeding 99% of the radial trapping frequency are achieved. By locally suppressing the density while maintaining the phase singularities, we create vortex aggregates. To illustrate the underlying Coriolis force driven dynamics, oscillation frequencies of the vortex aggregate area are measured. A related technique also enables us to excite and directly image Tkachenko modes in a vortex lattice. These modes provide evidence for the shear strength that a vortex lattice in a superfluid can support.

Using lasers and ultracold atoms, physicists have found a way to stop and start a pulse of light. A pulse of laser light 3 microseconds in duration and about 1 kilometre in length was shot into a specially prepared sample of ultracold sodium gas. The gas sample was about 0.2 mm long and had the unusual property that the velocity of light within it was ten million times slower than in free space. Similar experiments have reduced the speed of light below 30 metres per second. This phenomenon may one day be used to store data in a quantum computer. (Original abstract - amended)

We have begun a series of experiments on mixed bosonic quantum fluids. Our system is mixed Bose-Einstein condensates in dilute Rb-87. By simultaneously trapping the atoms in two different hyperfine states, we are able to study the dynamics of component separation and of the relative quantum phase of two interpenetrating condensates. Population can be converted from one state to the other at a rate that is sensitive to the relative quantum phase.