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These atoms interact over long ranges, much as do the atoms in liquid helium9, allowing for a roton — a kind of excitation that has a particular momentum.

What sets dipolar BECs apart from other cold-atom experiments is that no external influence is needed to generate the roton.

In experiments on dipolar BECs, the energy of this roton can be tuned by using an external magnetic field to adjust repulsive short-range interactions.

The existence of a roton allows a BEC to more easily occupy a superposition of two different momenta, effectively moving left and right at the same time.

When the energy of the roton hits zero, the superfluid becomes unstable and forms a structure that has a periodic density variation.