A stripe phase with supersolid properties in spin–orbit-coupled Bose–Einstein condensates

Spin–orbit coupling in Bose–Einstein condensates creates a density modulation, which is a stripe phase with supersolid properties. Supersolid stripe phase (Li 21421, Physics Letter, Leonie Mueck) Supersolids exhibit long-range order, just like normal solids, while simultaneously displaying superfluid properties. This state of matter has been extremely difficult to generate and previous results that have suggested supersolidity in helium are yet to be unambiguously verified. Here, Jun-Ru Li and colleagues create a special stripe phase in a one-dimensional spin–orbit-coupled Bose–Einstein condensate and observe some of the predicted supersolid properties. They show that this stripe phase has long-range order in one direction, like a solid, while retaining a sharp momentum distribution, like a superfluid. The authors suggest that these results could be built upon to enable the demonstration of other exotic condensed matter effects, related to disorder and vortex creation. Elsewhere in this issue, Tilman Esslinger and colleagues couple a Bose–Einstein condensate of atoms to two optical cavities and observe the breaking of continuous translational symmetry along one direction. Supersolidity combines superfluid flow with long-range spatial periodicity of solids 1 , two properties that are often mutually exclusive. The original discussion of quantum crystals 2 and supersolidity focused on solid 4 He and triggered extensive experimental efforts 3 , 4 that, instead of supersolidity, revealed exotic phenomena including quantum plasticity and mass supertransport 4 . The concept of supersolidity was then generalized from quantum crystals to other superfluid systems that break continuous translational symmetry. Bose–Einstein condensates with spin–orbit coupling are predicted to possess a stripe phase 5 , 6 , 7 with supersolid properties 8 , 9 . Despite several recent studies of the miscibility of the spin components of such a condensate 10 , 11 , 12 , the presence of stripes has not been detected. Here we observe the predicted density modulation of this stripe phase using Bragg reflection (which provides evidence for spontaneous long-range order in one direction) while maintaining a sharp momentum distribution (the hallmark of superfluid Bose–Einstein condensates). Our work thus establishes a system with continuous symmetry-breaking properties, associated collective excitations and superfluid behaviour.