  Quantum droplets are small clusters of atoms self-bound by the balance of
attractive and repulsive forces. Here we report on the observation of a novel
type of droplets, solely stabilized by contact interactions in a mixture of two
Bose-Einstein condensates. We demonstrate that they are several orders of
magnitude more dilute than liquid helium by directly measuring their size and
density via in situ imaging. Moreover, by comparison to a single-component
condensate, we show that quantum many-body effects stabilize them against
collapse. We observe that droplets require a minimum atom number to be stable.
Below, quantum pressure drives a liquid-to-gas transition that we map out as a
function of interaction strength. These ultra-dilute isotropic liquids remain
weakly interacting and constitute an ideal platform to benchmark quantum
many-body theories.
