April 2019. The LENS team has found experimentally a regime in which a quantum gas of magnetic atoms has supersolid properties, i.e. it behaves simultaneously as a solid and a superfluid [1,2]. This discovery ends a quest lasting more than 15 years, motivated by the seminal theory work by outstanding theorists, among which the QUIC researcher M. Lewenstein (ICFO) , and opens an exciting new direction to study exotic properties of quantum materials.
As is well known, solids are composed by particles arranged in a fixed structure, often periodic as in crystals. Particles in liquids are instead free to move, although with friction. During the last century, special types of liquids have been discovered – superfluids and superconductors – where the particles can move without friction. It had been predicted that, according to the laws of quantum mechanics, even more special systems could exist, where the particles exhibit the periodic structure of a solid and at the same time have superfluid properties. However, the search for such exotic types of material has proven so far unsuccessful in the primary candidate system, ultracold helium. Interest in such phenomena has recently re-emerged, with various laboratories worldwide realizing systems of ultracold atoms with special interaction properties that are appropriate to create such supersolids. Notable examples are the supersolid behaviors of Bose-Eintein condensates in coupled optical cavities studied by the QUIC team at ETHZ , and of the spin-orbit coupled condensates realized at MIT , a variation of those studied in the QUIC project by the University of Trento.
A system with supersolid properties determined solely by the interactions between the atoms has now been spotted in a gas of ultracold dysprosium atoms, by a collaboration between QUIC researchers in Pisa and Florence, led by G. Modugno, and a theoretical team in Hannover, led by L. Santos . These atoms behave like strong magnets, interacting between themselves in a way to form a periodic structure; the atoms are however not locked in position and can move freely across the system, as in a superfluid. Interestingly, the supersolid can be described as an array of quantum droplets, another new phase of matter that the QUIC project contributed to discover . Most of the properties of the supersolid are still to be tested, including the peculiar excitation spectrum that is predicted to derive from the simultaneous breaking of two symmetries. There is therefore a lot of excitement in the scientific community for the discovery, since it is opening a new direction to study and understand the properties of such an exotic, yet fundamental, quantum material. Other two groups in Stuttgart and in Innsbruck have indeed immediately confirmed the initial observations, with further experiments and theoretical analyses [7,8].
Figure: Distribution of the atoms in the velocity space. The coherent stripe regime with supersolid properties (middle row) shows a characteristic periodic structure, differently from the standard Bose-Einstein condensate (top row) or the incoherent droplets (bottom row) that appear at different strengths of the interaction between the atoms.
This type of discovery seems to confirm that the special quantum gases explored in the QUIC project can have an invaluable role in extending our knowledge of the quantum world. We are confident that this type of discovery will have also a long-term impact on our lives in terms of the discovery of new materials with special properties.
 L. Tanzi, E. Lucioni, F. Famà, J. Catani, A. Fioretti, C. Gabbanini, R. N. Bisset, L. Santos, and G. Modugno, Observation of a dipolar quantum gas with metastable supersolid properties, Phys. Rev. Lett. 122, 130405 (2019), Editors’ Suggestion.
 T. Donner, Viewpoint: Dipolar Quantum Gases go Supersolid, Physics 12, 38 (April 3, 2019).
 L. Santos, G. V. Shlyapnikov, and M. Lewenstein, Roton-maxon spectrum and stability of trapped dipolar Bose-Einstein condensates, Phys. Rev. Lett. 90, 250403 (2003)
 J. Léonard, A. Morales, P. Zupancic, T. Esslinger, and T. Donner, Supersolid formation in a quantum gas breaking a continuous translational symmetry, Nature 543, 87 (2017).
 J.-R. Li, J. Lee, W. Huang, S. Burchesky, B. Shteynas, F. Ç. Top, A. O. Jamison, and W. Ketterle, A stripe phase with supersolid properties in spin–orbit-coupled Bose–Einstein condensates, Nature 543, 91 (2017).
 See the previous QUIC posts on the liquid quantum droplets discovered at ICFO and at LENS.
 F. Böttcher, J.-N. Schmidt, M. Wenzel, J. Hertkorn, M. Guo, T. Langen, and T. Pfau, Transient supersolid properties in an array of dipolar quantum droplets, Phys. Rev. X 9, 011051 (2019).
 L. Chomaz et al., “Long-lived and transient supersolid behaviors in dipolar quantum gases“, arXiv:1903.04375
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