  Self-bound quantum droplets are a newly discovered phase in the context of
ultracold atoms. In this work we report their experimental realization
following the original proposal by Petrov [Phys. Rev. Lett. 115, 155302
(2015)], 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 equilibrium properties. 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.
