# BEC and quantum liquid droplets

My first question is: Is atomic Bose-Einstein condensate a gas?

My second question is: What are the difference between BEC and quantum liquid droplets?

• 1) Yes, a Bose-Einstein condensate (BEC) is a gas. Hence, it is a metastable stable, as you know solids are the most stable state of matter. Three-body collisions happen at a rate $\propto n^2$, $n$ being the number density of atoms - these collisions make two atoms bond into a molecule, releasing the binding energy as kinetic energy of the third atom. The molecule is the start of the formation of the solid, while the third atom has enough energy to be ejected from whatever trap the BEC was sitting in, potentially colliding with other atoms in the process and heating them up.
This is why BECs only exist in dilute gases, i.e. low $n$, otherwise they would "die" very quickly.
In order to achieve this one usually starts off with a BEC (as it is the easiest macroscopic quantum wavefunction to experimentally manufacture) and then modifies something in order for it to increase in density without dying. Usually this comprises changing the scattering length a, which measures the strength and the sign (attractive or repulsive) of atomic interactions. As an example, $^{39}K$ has a negative background scattering length so it will have attractive interactions, meaning a cold cloud of $^{39}K$ will implode and eventually explode (when $n$ becomes large enough for the 3-body losses mentioned earlier to win) - this is called a Bose-nova.