January and March 2018: In two recent papers published during the first trimester of 2018, the ICFO team has observed and studied for the first time a novel type of ultra-dilute quantum liquid that goes beyond the standard van der Waals paradigm. This liquid forms droplets: macroscopic clusters of ultra-cold atoms that are eight orders of magnitude more dilute than liquid Helium, but have similar liquid-like properties. In particular, they remain self-trapped in the absence of external confinement due to the compensation of attractive mean-field forces and an effective repulsion stemming from quantum fluctuations.
In a first series of experiments, the ICFO team observed these self-bound droplets in a mixture of Bose-Einstein condensates of potassium with attractive inter-state and repulsive intra-state interactions. Exploiting phase-contrast in situ imaging, they directly measured the droplet’s ultra-low densities and micro-meter scaled sizes, and demonstrated the many-body origin of their stabilization mechanism. Furthermore, the team observed that for small atom numbers quantum pressure is sufficient to dissociate the droplets and drive a liquid-to-gas transition, which they mapped out as a function of atom number and interaction strength .
In a second series of experiments, the team studied the difference existing between these liquid droplets and more conventional bright solitons. In analogy to non-linear optics, the former can be seen as one-dimensional matter-wave solitons stabilized by dispersion, whereas the latter correspond to high-dimensional solitons stabilized by a higher order non-linearity due to quantum fluctuations. They found that in an optical waveguide, and depending on the system parameters, solitons and droplets can be smoothly connected or remain distinct states coexisting only in a bi-stable region .
 C. R. Cabrera, L. Tanzi, J. Sanz, B. Naylor, P. Thomas, P. Cheiney, and L. Tarruell, Quantum liquid droplets in a mixture of Bose-Einstein condensates, Science 359, 301 (2018).
 P. Cheiney, C. R. Cabrera, J. Sanz, B. Naylor, L. Tanzi, and L. Tarruell, Bright Soliton to Quantum Droplet Transition in a Mixture of Bose-Einstein Condensates, Phys. Rev. Lett., in press, arXiv:1710.11079.
See also the general audience article by S. K. Blau in Physics Today, the Perspective article by I. Ferrier-Barbut and T. Pfau in Science, the News and Views article by D. S. Petrov in Nature Physics and the website of L. Tarruell’s group at ICFO.