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Comment on "Quantum Quasicrystals of Spin-Orbit-Coupled Dipolar Bosons"

In a recent Letter, Gopalakrishnan, Martin, and Demler [Phys. Rev. Lett. 111 (2013) 185304] show that quasi-two-dimensional dipolar Bose gases, subject to a Rashba spin-orbit coupling, exhibit a variety of spatially ordered, or crystalline, ground states, including a pentagonal quasicrystal. Indeed, as the authors say, realizing quasicrystalline condensates would provide new ways to explore the physics of quasicrystals, and in particular to study the quantum dynamics of their unique collective phason modes. Yet, the authors conclude that "there are typically additional phasons in quantum-mechanical quasicrystals, when compared with their classical equivalents." In this Comment I review the notion of phason modes in quasicrystals, and explain why their number does not depend on whether they are classical or quantum.