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Monday, 2 December 2019

A new theoretical type of time crystal might not require energy input

A few years ago, physicists presented a new structure of matter with amazing properties: temporal crystals. In these crystals, atomic structures form patterns of arrangement that are repeated in both space and time. However, to induce such a structure, an initial energy supply to the ground state is necessary. But in a recent article, a team of physicists showed, using a model of string theory, that it would be possible to obtain such crystals without this energy input. An attractive theory, but whose experimental realization seems unlikely.

The method involves inciting entangled particles to influence the spin of others over a certain distance. Temporal crystals may seem like a science fiction concept, but they are a real phenomenon, theorized for the first time in 2012. From the outside, they look like normal crystals. But inside, the atoms - arranged in a repeating network structure otherwise normal - behave in a very special way.

They oscillate, turning in one direction then in the other. These oscillations, called "tics", are blocked on a very regular and particular frequency. Thus, where the structure of ordinary crystals repeats itself in space, in temporal crystals, it is repeated in space and in time.

Quantum intricacy: it would render unnecessary energy input into time crystals

Until now, experimentally produced time crystals have required an external stimulus (such as an electromagnetic radiation pulse) in the ground state, or in the lower energy state, to induce their ticking. This was done in 2016. According to a 2015 article, it seemed that a temporal crystal without adding energy to its ground state was simply physically impossible. In physics, we speak of "no-go theorem".

In the solution proposed by the researchers, each particle interacts with the others in the crystalline structure at long distances, thanks to the phenomenon of quantum entanglement (arrows). Credits: J. Zhang et al.

There is, however, a notable exception to this theorem with respect to time crystals. This is what Valerii Kozin of the University of Iceland in Reykjavík and Oleksandr Kyriienko of the University of Exeter in the United Kingdom, used to address the problem in their article published in the journal Physical Review Letters.

The 2015 article assumes that interactions between particles decrease with distance. But there is a practical exception. The entangled particles have a relationship that does not weaken with distance.

Measuring the spin of a particle will immediately determine the spin of its entangled partner, regardless of its distance. According to physicists, in time crystals, such a remote interaction could theoretically produce a fundamental state of temporal crystals requiring no energy injection.

A very theoretical solution brought by the string theoryb

In their new article, the researchers propose a system of particles in the temporal crystal, each having a rotation. They demonstrate that there is a way to describe entangled particle spins using a string theory model that corresponds to the definition of a time crystal given in the 2015 article.

Even if the particles were spinning out of sync, the interactions between the particles would produce the ticking of a time crystal, according to the authors. However, this system would be incredibly complicated, each particle can have a spin superposition. In fact, the whole thing might not be feasible in a laboratory. Intricating particles in this way is an idea that works well on paper, but is hardly practical in practice. We therefore look forward to any experiments that will ensue.


Quantum Time Crystals from Hamiltonians with Long-Range Interactions

Valerii K. Kozin and Oleksandr Kyriienko
Phys. Rev. Lett. 123, 210602 –

Published 20 November 2019


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