THE MOST ETHEREAL DROPLETS EVER:

THE BOSELETS, THE FERMILETS AND THE FERBOLETS
 

The atoms in Bose-Einstein condensates (BECs) created
up to now are only condensed in a metaphorical sense:
they are very rarified, having up to a million times
less atoms per unit volume than air in the atmosphere.
Also, they are not bound together, but only held in
place by outside forces, despite the fact that atoms
really attract each other and easily will form solids
or liquids at low temperature.

There may be a completely new kind of condensed state
with properties of both the Bose condensates and
ordinary liquids, according to theoretical arguments. In
theory, there is a very long range attractive interaction
between the atoms that permits them to hold together as
rarified droplets, without the external field required
to contain the atoms. The origin of the long-range
attraction is a very unusual and strange quantum
phenomenon, concerning the properties of some three
particle systems and known as the Efimov effect. If
the interaction between two particles is just at the
edge of allowing a weakly bound state, then three
particles will have a large number of bound states,
even if there are no two-particle bound states. There
are hardly any particles in nature that satisfy the
requirement that pairs are "almost" bound, but the
experimenters, Those magnificent men and their trapped
BECs, have learned how to change the interactions by
using external magnetic fields. It is argued that
conditions could be made to produces the Efimov states
in three-particles clusters, and surprisingly, these same
conditions would stabilize an ensemble of many such
atoms and form a liquid droplet, of a density about
ten thousand times smaller then todays BECs. Such a
droplet, a boselet (if made of bosons) or a fermilet
(if made of fermions) or a ferbolet (if made of both
fermions and bosons) has the incredible property of
being liquid, even though the atoms are significantly
farther apart then in todays BECs, which are gaseous.
A liquid has a well defined density, it is imcompressible
and unlike a gas there is no need for a container to keep
the particles together. All this goes counter to our
normal expectations, that by lowering the density one
can make a gas even more perfect, since the atoms will
interact even less when they are further apart. Quantum
mechanics however leads us to this inescapable conclusion
that a whole host of quantum liquids with remarkable
properties could be created instead. It will not be easy
to form and observe these boselets and fermilets
experimentally, but the same could have been said about
the BEC itself ten years ago.