Comment on "Quantum Time Crystals from Hamiltonians with Long-Range Interactions"

• URL: http://arxiv.org/abs/2001.11037v1
• Date: Wed, 29 Jan 2020 19:00:06 GMT
• Title: Comment on "Quantum Time Crystals from Hamiltonians with Long-Range Interactions"
• Authors: Vedika Khemani, Roderich Moessner and S. L. Sondhi
• Abstract summary: Kozin and Kyriienko claim to realize "genuine" ground state time crystals. We point out that their models are unrealizable $it and$ they violate well established principles for defining phases of matter.
• Score: 0.0
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: In a recent paper (Phys. Rev. Lett. 123, 210602), Kozin and Kyriienko claim to realize "genuine" ground state time crystals by studying models with long-ranged and infinite-body interactions. Here we point out that their models are doubly problematic: they are unrealizable ${\it and}$ they violate well established principles for defining phases of matter. Indeed with infinite body operators allowed, almost all quantum systems are time crystals. In addition, one of their models is highly unstable and another amounts to isolating, via fine tuning, a single degree of freedom in a many body system--allowing for this elevates the pendulum of Galileo and Huygens to a genuine time crystal.

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  In this reply we answer one-by-one all questions raised in the discussion. As for the ideological dispute, it brightly highlights a bizarre nature of time crystalline order in closed quantum systems.
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