Avalanches in a Bose-Einstein condensate

Direct Laser Cooling to Bose-Einstein Condensation in a Dipole Trap

We present a method for producing three-dimensional Bose-Einstein condensates using only laser cooling. The phase transition to condensation is crossed with 2.5×10^{4} ^{87}Rb atoms at a temperature of T_{c}=0.6  μK after 1.4 s of cooling. Atoms are trapped in a crossed optical dipole trap and cooled using Raman cooling with far-off-resonant optical pumping light to reduce atom loss and heating. The achieved temperatures are well below the effective recoil temperature. We find that during the final cooling stage at atomic densities above 10^{14}  cm^{-3}, careful tuning of trap depth and optical-pumping rate is necessary to evade heating and loss mechanisms. The method may enable the fast production of quantum degenerate gases in a variety of systems including fermions.

Probing the Interface of a Phase-Separated State in a Repulsive Bose-Fermi Mixture

We probe the interface between a phase-separated Bose-Fermi mixture consisting of a small Bose-Einstein condensate of ^{41}K residing in a large Fermi sea of ^{6}Li. We quantify the residual spatial overlap between the two components by measuring three-body recombination losses for variable strength of the interspecies repulsion. A comparison with a numerical mean-field model highlights the importance of the kinetic energy term for the condensed bosons in maintaining the thin interface far into the phase-separated regime. Our results demonstrate a corresponding smoothing of the phase transition in a system of finite size.

Self-Bound Quantum Droplets of Atomic Mixtures in Free Space

Self-bound quantum droplets are a newly discovered phase in the context of ultracold atoms. In this Letter, we report their experimental realization following the original proposal by Petrov [Phys. Rev. Lett. 115, 155302 (2015)PRLTAO0031-900710.1103/PhysRevLett.115.155302], using an attractive bosonic mixture. In this system, spherical droplets form due to the balance of competing attractive and repulsive forces, provided by the mean-field energy close to the collapse threshold and the first-order correction due to quantum fluctuations. Thanks to an optical levitating potential with negligible residual confinement, we observe self-bound droplets in free space, and we characterize the conditions for their formation as well as their size and composition. This work sets the stage for future studies on quantum droplets, from the measurement of their peculiar excitation spectrum to the exploration of their superfluid nature.

Dynamics of single neutral impurity atoms immersed in an ultracold gas

We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. Elastic two-body collisions lead to a rapid thermalization of the impurity inside the Rb gas, representing the first realization of an ultracold gas doped with a precisely known number of impurity atoms interacting via s-wave collisions. Inelastic interactions are restricted to a single three-body recombination channel in a highly controlled and pure setting, which allows us to determine the Rb-Rb-Cs three-body loss rate with unprecedented precision. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum many-body system.

Universal dimer in a collisionally opaque medium: experimental observables and Efimov resonances

A universal dimer is subject to secondary collisions with atoms when formed in a cloud of ultracold atoms via three-body recombination. We show that in a collisionally opaque medium, the value of the scattering length that results in the maximum number of secondary collisions may not correspond to the Efimov resonance at the atom-dimer threshold and thus cannot be automatically associated with it. This result explains a number of controversies in recent experimental results on universal three-body states and supports the emerging evidence for the significant finite range corrections to the first excited Efimov energy level.

Large atom number Bose-Einstein condensate of sodium

We describe the setup to create a large Bose-Einstein condensate containing more than 120 x 10(6) atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical dark-spot MOT in our experiments contains 2.0 x 10(10) atoms with a temperature of 320 microK and a density of about 1.0 x 10(11) atoms/cm(3). The sample is spin polarized in a high magnetic field before the atoms are loaded in the magnetic trap. Spin polarizing in a high magnetic field results in an increase in the transfer efficiency by a factor of 2 compared to experiments without spin polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by evaporative cooling. To suppress the three-body losses at the end of the evaporation, the magnetic trap is decompressed in the axial direction.

Ionization rates in a bose-einstein condensate of metastable helium

We have studied ionizing collisions in a BEC of metastable He. Measurements of the ion production rate combined with measurements of the density and number of atoms for the same sample allow us to estimate both the two- and three-body contributions to this rate. A comparison with the decay of the atom number indicates that ionizing collisions are largely or wholly responsible for the loss. Quantum depletion makes a substantial correction to the three-body rate constant.