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

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 $and$ 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.

1.5 pages; Comment on Phys. Rev. Lett. 123, 210602

References in corpus (7)

• Universal High-Frequency Behavior of Periodically Driven Systems: from Dynamical Stabilization to Floquet Engineering
• Floquet Time Crystals
• On the phase structure of driven quantum systems
• Absence of Quantum Time Crystals
• Quantum Time Crystals from Hamiltonians with Long-Range Interactions
• A Brief History of Time Crystals
• Defining Time Crystals via Representation Theory

Cited by in corpus (4)

• Critical properties of the prethermal Floquet Time Crystal
• Critical properties of the Floquet time crystal within the Gaussian approximation
• Route to Extend the Lifetime of a Discrete Time Crystal in a Finite Spin Chain Without Disorder
• Non-stationarity and Dissipative Time Crystals: Spectral Properties and Finite-Size Effects