Supersolid formation in a quantum gas breaking continuous translational   symmetry
  The concept of a supersolid state is paradoxical. It combines the crystallization of a many-body system with dissipationless flow of the atoms it is built of. This quantum phase requires the breaking of two continuous symmetries, the phase invariance of a superfluid and the continuous translational invariance to form the crystal,. Proposed for helium almost 50 years ago, experimental verification of supersolidity remains elusive. A variant with only discrete translational symmetry breaking on a preimposed lattice structure, the `lattice supersolid', has been realized based on self-organization of a Bose-Einstein condensate (BEC). However, lattice supersolids do not feature the continuous ground state degeneracy that characterizes the supersolid state as originally proposed. Here we report the realization of a supersolid with continuous translational symmetry breaking. The continuous symmetry emerges from two discrete spatial ones by symmetrically coupling a BEC to the modes of two optical cavities. We establish the phase coherence of the supersolid and find a high ground-state degeneracy by measuring the crystal position over many realizations through the light fields leaking from the cavities. These light fields are also used to monitor the position fluctuations in real-time. Our concept provides a route to creating and studying glassy many-body systems with contrallably lifted ground-state degeneracies, such as supersolids in the presence of disorder. 
Julian L\'eonard, Andrea Morales, Philip Zupancic, Tilman Esslinger   and Tobias Donner
cond-mat.quant-gas cond-mat.other quant-ph