10 October 2018 Congratulations to New ICFO PhD graduate

Dr. César Cabrera

Thesis committee

Dr. César Cabrera graduated with a thesis on “Quantum Liquid Droplets in a mixture of Bose-Einstein Condensates”

Dr. César Cabrera received his Master’s degree from the University of Bonn before joining the Ultracold Quantum Gases research group led by ICFO Prof. Dr. Leticia Tarruell. At ICFO, he centred his doctoral studies on Bose-Einstein condensates and a novel state of matter: quantum liquid droplets. Dr. Cabrera’s thesis, entitled “Quantum Liquid Droplets in a mixture of Bose-Einstein Condensates”, was supervised by Prof. Leticia Tarruell.


In this thesis, we report on the design and construction of a quantum simulator experiment using quantum gases in Spain. This experiment exploits mixtures of the three isotopes of potassium, which give access in an original approach to the study of Bose-Bose or Bose-Fermi mixtures using the same experimental setup. We validate our experimental setup with the observation of a Bose-Einstein condensate (BEC) of 41K and 39K. Moreover we observe the dual Bose-Einstein condensation of 39K–41K. These results represents the first observation of BECs in Spain and give access to a novel quantum degenerate mixture in the field. Since the control of interactions in our experiment are crucial, we characterize the scattering properties of the 39K–41K mixture, and spin mixtures of 39K and 41K. In addition, using a spin mixture of 39K BEC, we report on the observation of a novel state of matter: a composite quantum liquid droplet. This dilute quantum droplet is a liquid-like cluster of ultra-cold atoms self-trapped by attractive mean-field forces and stabilized against collapse by repulsive beyond mean-field many-body effects. This system follows the original proposal where D. Petrov predicted the formation of self-bound liquid droplets in mixtures of Bose-Einstein condensates. In the first series of experiments, we have observed the formation of quantum droplets in a regime where the Bose-Bose mixture should collapse from the mean-field perspective.We directly measure the droplet size and ultra-low density via high-resolution in situ imaging, and experimentally confirm their self-bound nature.We demonstrate that the existence of these droplets is a striking manifestation of quantum fluctuations. These droplets do not exist in single-component condensates with described with contact interactions. Finally, we observe that for small atom numbers, quantum pressure dissociates the droplets and drives a liquid-to-gas transition, which we map out as a function of interaction strength. These measurements open an intriguing point of investigation: the difference existing between droplets and bright solitons. In the second series of experiments, we address it by placing the mixture in an optical waveguide, realizing a system that contains both composite bright solitons and quantum liquid droplets. In analogy to non-linear optics, the former can be seen as one-dimensional matter-wave solitons stabilized by dispersion, whereas the latter corresponds to highdimensional solitons stabilized by a higher order non-linearity. We find that depending on atom number, interaction strength and confinement, solitons and droplets can be smoothly connected or remain distinct states coexisting only in a bi-stable region. We measure their spin composition, extract their density for a broad range of parameters, and map out the boundary of the region separating solitons from droplets. Our experiments demonstrate a novel type of ultra-dilute quantum liquid, stabilized only by contact interactions. They provide an ideal platform for benchmarking complex quantum many-body theories beyond the mean-field approximation in a quantum simulation approach. Furthermore, they constitute a novel playground to explore experimentally self-bound states stabilized by unconventional higher order nonlinearities, relevant in non-linear optics.

Thesis committe

Prof Dmitry Petrov, Laboratoire LPTMS, Université Paris Sud
Prof Dr Maciej Lewenstein, ICFO
Prof Robert Smith, University of Oxford