Self-bound quantum droplets in Bose-Bose mixture

Self-bound quantum systems appear in different physical scenarios. They result from the balance between attractive and repulsive forces. Recently the existence of a new object belonging to this class has been discovered. Using a bosonic mixture of ultracold atoms, it is possible to generate a self-bound state resulting from the interplay between an attractive mean-field energy and the repulsive first-order perturbative correction, the so-called Lee- Huang-Yang term. This system is known as quantum droplet. During my PhD we have experimentally observed and characterized this novel quantum state. Thanks to an innovative technique, based on a time-averaged potential, we were able to levitate the mixture and study for the first time the self-bound nature of quantum droplets in 3D free space. We characterized their equilibrium properties, i.e. the size, the critical atom number for their formation and the spin imbalance, finding a very good agreement with the theoretical predictions. Despite being extremely dilute, for large atom numbers quantum droplets enter a liquidlike incompressible regime, highlighted by the formation of a bulk with uniform density. We investigate the occurrence of this incompressible regime by studying collisions between two droplets. This is indeed a powerful tool to gain information about the energy scales characterizing the system. To this aim, we implemented an experimental sequence able to create two separate quantum droplets and to imprint them a tunable relative velocity. By characterizing the outcomes of the collisions for different values of velocities and atom numbers in the droplets and comparing them with the results of energetic considerations and numerical simulations, we obtained the first evidence of a crossover between compressible and incompressible regimes.