New states of matter with fine-tuned interactions: quantum droplets and dipolar supersolids.

Quantum fluctuations can stabilize Bose-Einstein condensates (BEC) against the mean-field collapse. Stabilization of the condensate has been observed in quantum degenerate Bose-Bose mixtures and dipolar BECs. The fine-tuning of the interatomic interactions can lead to the emergence of two new states of matter: liquid-like self-bound quantum droplets and supersolid crystals formed from these droplets.

The low-energy Goldstone mode in a trapped dipolar supersolid.

A supersolid is a counter-intuitive state of matter that combines the frictionless flow of a superfluid with the crystal-like periodic density modulation of a solid. Since the first prediction in the 1950s, experimental efforts to realize this state have focused mainly on helium, in which supersolidity remains unobserved. Recently, supersolidity has also been studied in ultracold quantum gases, and some of its defining properties have been induced in spin-orbit-coupled Bose-Einstein condensates (BECs) and BECs coupled to two crossed optical cavities.

Anisotropic Superfluid Behavior of a Dipolar Bose-Einstein Condensate.

We present transport measurements on a dipolar superfluid using a Bose-Einstein condensate of ^{162}Dy with strong magnetic dipole-dipole interactions. By moving an attractive laser beam through the condensate we observe an anisotropy in superfluid flow. This observation is compatible with an anisotropic critical velocity for the breakdown of dissipationless flow, which, in the spirit of the Landau criterion, can directly be connected to the anisotropy of the underlying dipolar excitation spectrum.