Time crystals might exist after all (Update)

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Time crystals might exist after all (Update)

September 9, 2016 by Lisa Zyga

                               
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(Phys.org)—Are time crystals just a mathematical curiosity, or could they actually physically exist? Physicists have been debating this question since 2012, when Nobel laureate Frank Wilczek first proposed the idea of time crystals. He argued that these hypothetical objects can exhibit periodic motion, such as moving in a circular orbit, in their state of lowest energy, or their "ground state." Theoretically, objects in their ground states don't have enough energy to move at all.

In the years since, other physicists have proposed various arguments for why the physical existence of time crystals is impossible—and most physicists do seem to think that time crystals are physically impossible because of their odd properties. Even though time crystals couldn't be used to generate useful energy (since disturbing them makes them stop moving), and don't violate the second law of thermodynamics, they do violate a fundamental symmetry of the laws of physics.

However, now in a new paper published in Physical Review Letters, physicists from the University of California, Santa Barbara (UCSB) and Microsoft Station Q (a Microsoft research lab located on the UCSB campus) have demonstrated that it may be possible for time crystals to physically exist.

The physicists have focused on the implication of time crystals that seems most surprising, which is that time crystals are predicted to spontaneously break a fundamental symmetry called "time-translation symmetry." To understand what this means, the researchers explain what spontaneous symmetry breaking is.

"The crucial difference here is between explicit symmetry breaking and spontaneous symmetry breaking," coauthor Dominic Else, a physicist at UCSB, told Phys.org. "If a symmetry is broken explicitly, then the laws of nature do not have the symmetry anymore; spontaneous symmetry breaking means that the laws of nature have a symmetry, but nature chooses a state that doesn't."

If time crystals really do spontaneously break time-translation symmetry, then the laws of nature that govern time crystals wouldn't change with time, but the time crystals themselves would change over time due to their ground-state motion, spontaneously breaking the symmetry.

Although spontaneously broken time-translation symmetry has never been observed before, almost every other type of spontaneous symmetry breaking has been. One very common example of a spontaneously broken symmetry occurs in magnets. The laws of nature do not impose which side of a magnet will be the north pole and which will be the south pole. The distinguishing feature of any magnetic material, however, is that it spontaneously breaks this symmetry and chooses one side to be the north pole. Another example is ordinary crystals. Although the laws of nature are invariant under rotating or shifting (translating) space, crystals spontaneously break these spatial symmetries because they look different when viewed from different angles and when shifted a little bit in space.

In their new study, the physicists specifically define what it would take to spontaneously break time-translation symmetry, and then use simulations to predict that this broken symmetry should occur in a large class of quantum systems called "Floquet-many-body-localized driven systems." The scientists explain that the key aspect of these systems is that they remain far from thermal equilibrium at all times, so the system never heats up.

The new definition of broken time-translation symmetry is similar to the definitions of other broken symmetries. Basically, when the size of a system (such as a crystal) grows, the time taken for a symmetry-breaking state to decay into a symmetry-respecting state increases, and in an infinite system the symmetry-respecting state can never be reached. As a result, symmetry for the entire system is broken.

"The significance of our work is two-fold: on one hand, it demonstrates that time-translation symmetry is not immune to being spontaneously broken," said coauthor Bela Bauer, a researcher at Microsoft Station Q. "On the other hand, it deepens our understanding that non-equilibrium systems can host many interesting states of matter that cannot exist in equilibrium systems."

According to the physicists, it should be possible to perform an experiment to observe time-translation symmetry breaking by using a large system of trapped atoms, trapped ions, or superconducting qubits to fabricate a time crystal, and then measure how these systems evolve over time. The scientists predict that the systems will exhibit the periodic, oscillating motion that is characteristic of time crystals and indicative of spontaneously broken time-translation symmetry.

"In collaboration with experimental research groups, we are exploring the possibility of realizing Floquet time crystals in systems of cold atomic gases," said coauthor Chetan Nayak at Microsoft Station Q and UCSB.

Update: A team of physicists at the Joint Quantum Institute at the University of Maryland have now experimentally confirmed the existence of time crystals for the first time. The team observed the time-crystal behavior predicted in a system of trapped ions. A pre-print is available at: arXiv:1609.08684 [quant-ph]

Explore further: Physicists propose new definition of time crystals—then prove such things don't exist

More information: Dominic V. Else, Bela Bauer, and Chetan Nayak. "Floquet Time Crystals." Physical Review Letters. DOI: 10.1103/PhysRevLett.117.090402 Also at arXiv:1603.08001 [cond-mat.dis-nn]

Journal reference: Physical Review Letters



Time crystals might exist after all
（Phys.org） - 时间晶体只是一个数学好奇心，或者他们实际上可以存在吗？物理学家自2012年以来一直在辩论这个问题，当时诺贝尔奖得主Frank Wilczek首次提出了时间晶体的想法。他认为这些假想物体可以呈现周期性运动，例如在其最低能量状态或其“基态”中在圆形轨道中运动。理论上，处于其基态的物体没有足够的能量来移动。

自从这些年来，其他物理学家提出为什么物理存在各种争论时间晶体是不可能的，而大多数物理学家似乎认为，时间晶体，因为它们的奇特性实际上是不可能的。尽管时间晶体不能被用来产生有用的能量（因为打扰他们，使他们停止移动），并没有违反热力学第二定律，他们不违反基本对称的物理定律。

然而，现在出版了新的纸张物理评论快报，来自加利福尼亚州圣巴巴拉分校（UCSB）和微软站Q（微软研究实验室位于加州大学圣巴巴拉分校校园）的大学的物理学家已经证明，它可能时间晶体是可能的到物理上存在。

物理学家已经集中在似乎最令人惊讶的时间晶体的含义，这是时间晶体被预测自发地打破称为“时间 - 翻译对称性”的基本对称。为了理解这是什么意思，研究人员解释了什么是自发对称破裂。

“这里的关键区别是明确的对称性破缺和自发对称性破之间，”合着者多米尼克否则，在加州大学圣巴巴拉分校的物理学家告诉Phys.org。“如果对称性被明确地打破了，那么自然规律就不再具有对称性;自然的对称性破坏意味着自然规律具有对称性，而自然选择不是对称性。

如果时间晶体真的自发地破坏时间 - 翻译对称性，则控制时间晶体的自然规律不会随时间改变，但是晶体本身将随着时间的推移而改变，这是由于它们的基态运动，自发地破坏对称性。

虽然自发破碎的时间 - 翻译对称从来没有被观察到，几乎所有其他类型的自发对称破裂已经。在磁体中发生自发断裂对称的一个非常常见的例子。自然规律不强加磁铁的哪一侧将是北极，哪一个将是南极。然而，任何磁性材料的区别特征在于它自发地断开这种对称性并且选择一侧作为北极。另一个例子是普通晶体。虽然自然规律在旋转或移动（平移）空间下是不变的，但晶体自发地破坏这些空间对称性，因为当从不同角度观察时和当在空间中移动一点时，它们看起来不同。

在他们的新研究中，物理学家具体地定义了自发地打破时间 - 翻译对称性所需要的东西，然后使用模拟来预测这种断裂的对称应该发生在一个称为“Floquet-many-body-localized driven系统“。科学家解释说，这些系统的关键方面是它们在任何时候都远离热平衡，因此系统不会升温。

断裂时间 - 翻译对称性的新定义类似于其他断裂对称性的定义。基本上，当系统（诸如晶体）的尺寸增大时，对称破裂状态衰减到对称关系状态所花费的时间增加，并且在无限系统中，永远不能达到对称关系的状态。结果，整个系统的对称性被破坏。

Microsoft Station Q的研究员Bela Bauer说，“我们的工作的重要性有两方面：一方面，它表明时间 - 翻译对称性不能免受自发性破坏，”另一方面，加深了我们的理解，即非平衡系统可以承载许多在平衡系统中不能存在的物质的有趣状态。

根据物理学家，应该可以进行一项实验，以观察时间平移对称性破坏，通过使用截留原子，被俘获离子，或超导量子位的一个大系统以制造时间晶体，然后再测量这些系统如何随时间变化。科学家预测，系统将呈现周期性的振荡运动，其是时间晶体的特征并且指示自发地断裂的时间 - 平移对称性。

Microsoft Station Q和UCSB的共同作者Chetan Nayak说：“与实验研究小组合作，我们正在探索在冷原子气体系统中实现Floquet时间晶体的可能性。

更新：马里兰大学联合量子研究所的一组物理学家现在已经实验性地首次证实了时间晶体的存在。团队观察到在被捕获离子系统中预测的时间 - 晶体行为。预印本可在以下网站获得：arXiv：1609.08684 [quant-ph]