物理学家首次制造出时间晶体，经典对称性最后一块面纱被揭开
Original
2017-02-04
梁柱
DeepTech深科技
说到晶体，大家最容易想到的便是钻石，这些闪烁夺目的石头是由不计其数的碳原子有序重复排列而成的。而对于物理学家而言，晶体与宇宙中的一项基本性质对称性有关。
宇宙中所有自然形成的和人造出来的晶体都不同程度地打破了空间对称性，而现在又有一种新型晶体被人类造了出来，它除了满足一般晶体的特征外，还打破了时间维度上的对称性，这就是时间晶体（time crystals ）。在这种全新的晶体里，晶格将会在时间这个维度上进行有序重复。
至此，经典对称性最后一块面纱——时间对称——终于被人类揭开，这是物理学上的一项重大成果。
物理学家曾希望通过“离子阱”来创造“时间晶体”，这是一种在不消耗能量的前提下，以固定模式运动的物质。
尽管如同石墨烯最初一般，时间晶体到底有些什么应用现在还并不明确。但至少有一点是肯定的，时间晶体的超级稳定性可以作为对抗退相干效应的有力武器，来保证一个量子态的寿命。如果量子态可以被强化，它还可用作制造更稳定的量子计算机内存，和进行更精确的测量。
时间晶体
2016 年 8 月，arXiv上面放出了一篇文章声称时间晶体是可行的！该文刚刚在上周的物理评论快报上发表，作者是加州大学伯克利分校物理系的助理教授Norman Yao。在这篇论文中，Norman Yao详细描述了受到外部驱动的时间晶体所具有的性质：基态呈现周期性，且为驱动周期的整数倍，周期稳定，有清晰的相变边界。
他还提出了制备时间晶体的具体方案。在他的方案中，由加州大学伯克利领导的团队会制造一个只有头发丝十分之一细的微小“陷阱”，然后将100钙离子注入到这个陷阱中，库仑作用会让这100个离子均匀地散开到陷阱的边上。此时经过激光冷却后的离子都处于基态，然后研究人员会开启一个静磁场，根据Yao的理论，钙离子环会在磁场中开始转动（而且会永久地转下去）。如果一切顺利，这些离子每隔一段时间就会回到最初的位置上，从而形成一个个在时间上重复排列的“晶格”，时间对称性由此被打破。
Norman Yao
很快，两个研究团队就依照Norman Yao所描绘的蓝图分别制造出了他们的第一个时间晶体，使用的是完全不同的设备。这两个来自马里兰大学和哈佛大学的团队分别在网上公布了他们的成果并提交发表，Norman Yao是这两篇论文的共同作者。
时间晶体之所以在时间上不断重复是因为它们受到了一种周期性的冲力，就像不断地弹一块果冻让它周期性晃动一样。Norman Yao解释说，这里面最大的突破，与其说是这些晶体做到了在时间上周期性“重复”，倒不如说是我们第一次大规模制造出了非平衡态的新材料。它们的基态永不停歇，无法像钻石或者红宝石那样回到相对静止的平衡态。
“这是一种新的物质状态，仅此而已。但它同样酷的地方在于这是第一个非平衡态的物质状态”，Norman Yao说，“在过去的半个世纪里，我们一直在研究平衡态的物质，比如金属和绝缘体，而今天我们将开始探索非平衡态物质的奇妙世界。”
加州大学伯克利分校的时间晶体实验说明：通过电场将钙离子围绕在一个100微米宽的“陷阱”中， 使它们形成晶体圆环。科学家们相信，静磁场会使圆环转动。
不被看好的发现
那么为什么时间对称性如此难以打破呢？这还要从晶体与对称性破缺来说起。
众所周知，自然法则本身都是满足对称性的。在平地上做力学实验，和在一列高速行驶的列车上做的实验得到的结果是相同的，昨天和今天做相同的实验得到的结果也是相同的，因为力学原理满足空间平移对称性和时间平移对称性。此外，还有旋转对称性和反转对称性等。
对称性体现了自然的美与和谐，但是物理体系的稳定状态（基态）却不是对称的。举个简单的例子，在桌子上竖起一支铅笔，理论上铅笔受到的重力垂直向下，在水平面上是对称，也就是说铅笔向任意一个方向倒下的概率相等。这时候铅笔的状态体满足对称性。
然而，竖立并不是铅笔能量最低的状态，当一个微小扰动出现，它只能选择一个方向倒下。铅笔倒下的状态能量最低（即基态），也没有了对称性。更一般的来讲，晶体便是一个打破对称性的极佳例子，在不同的方向上看晶体，看到的是不同的样子。这种自然法则对称而物质状态不对称的现象叫做自发对称性破缺。
既然空间对称性可以被打破，那么时间对称性为什么不能呢？
Frank Wilczek
2012年，诺贝尔奖获得者Frank Wilczek 提出了这个问题。同年他在Physics ReviewLetters上面发表了两篇论文，分别阐述了经典力学和量子力学下的时间对称性破缺的可能性。
简单来说，就是通过一种特殊方式，可以让物质的基态处于一种周期中。对应于晶体结构在空间上的周期排列，这种材料在时间上也是“周期排列”的，他给这种材料取了个很形象的名字——时间晶体。值得注意的是，一般晶体在时间这个维度上是连续分布的，在任何一个时刻观察它们都会看到同样的晶体，而时间晶体在不同的时间却有着不同的基态。
物质受法则支配的状态不对称，这就是自发对称性破缺。
在之后的几年里，很多物理学家对时间晶体的存在依旧持反对态度，因为他们认为，时间晶体必然会引发时间维度上的对称性破缺，而这是不可能的。虽然周期现象在我们的生活中无处不在（钟表，地球自转等等），但还无法证明任何一种物质的基态可以在光溜溜的时间轴上打上周期的烙印。
时间晶体的本质
根据UC Berkeley 物理学家Norman Yao的蓝图，马里兰大学的物理学家第一次用一维的镱离子链造出了时间晶体。 每一个离子都像电子那样拥有自旋，它们之间的长程作用力如图中箭头所指。
依照Norman Yao的蓝图，马里兰大学的Chris Monroe和他的同事在2016年9月用10个镱离子构建了一条链，这10个镱离子像电子一样具有自旋。也就是说，它们不再各向同性，而是像箭头一样会指向一个特定的方向，并且这些带有自旋的离子在磁场中会发生相互作用。
研究人员用两束精确调制的激光脉冲交替照射镱离子链。其中一个脉冲使镱离子的自旋翻转（比如从向上到向下，或者相反），然后紧接着第二个脉冲产生的磁场使其进入混乱的状态，随后又一个脉冲使它们再翻转方向，如此往复。
研究人员发现自旋的翻转周期稳定在了驱动周期（T）的2倍长度（2T）。神奇的是，虽然每一次的翻转脉冲完全一样，但镱离子们却自发地偏向了一个与上一个方向不同的方向，而不是选择回去；更神奇的是，即使中途稍微改变一下脉冲的频率，镱离子仍然严格地以2T为周期翻转。这正是Norman Yao的论文中时间晶体必须具备的性质。
镱离子在两个脉冲下实现周期性翻转。脉冲频率是T，自旋系统频率是2T。
“当你晃动一个果冻，然后发现它以一个不同的频率作为回应，这难道不奇妙吗？”Norman Yao说。“但这就是时间晶体的本质，你的驱动周期为T，但是当系统达到同步后你却观察到它在以一个比T大的周期振动。”
Norman Yao曾参与Monroe和其团队制造这种新材料的全部过程，在他帮助下，团队测量了材料的一些重要特性以保证其具有时间晶体的特性。Norman Yao还描述了在不同的磁场和激光频率下，时间晶体如何像冰块熔化一样改变其状态。而另一边，Mikhail lukin领导的哈佛团队通过在钻石中掺杂氮原子得到空穴，也制造出他们的时间晶体。而他们的时间晶体还有3T的周期，同样具有极佳的稳定性和清晰的状态边界。
印地安那大学的Phil Richerme在物理评论快报上对这篇论文评论说：“如此相似的结果竟出自于两个毫不相干的系统，这更加印证了时间晶体是广泛存在的一种新的物质状态，而不是用某个复杂系统通过精心设计而成的什么新奇玩意儿。时间晶体的发现印证了对称性的破缺在自然界中广泛存在，也为全新领域的研究开辟了道路。”
目前，Norman Yao正在对一些新的可能的非平衡态材料做理论研究，继续着他的时间晶体探索之路。
https://www.wired.com/2013/04/time-crystals/amp/
Physicists unveil new form of matter—time crystalsJanuary 26, 2017
Following a blueprint created by UC Berkeley physicist Norman Yao, physicists at the University of Maryland made the first time crystal using a one-dimensional chain of ytterbium ions. Each ion behaves like an electron spin and exhibits long-range interactions indicated by the arrows. Credit: Chris Monroe, University of Maryland
Normal crystals, likes diamond, are an atomic lattice that repeats in space, but physicists recently suggested making materials that repeat in time. Last year, UC Berkeley's Norman Yao sketched out the phases surrounding a time crystal and what to measure in order to confirm that this new material is actually a stable phase of matter. This stimulated two teams to build a time crystal, the first examples of a non-equilibrium form of matter.
To most people, crystals mean diamond bling, semiprecious gems or perhaps the jagged amethyst or quartz crystals beloved by collectors.
To Norman Yao, these inert crystals are the tip of the iceberg.
If crystals have an atomic structure that repeats in space, like the carbon lattice of a diamond, why can't crystals also have a structure that repeats in time? That is, a time crystal?
In a paper published online last week in the journal Physical Review Letters, the University of California, Berkeley assistant professor of physics describes exactly how to make and measure the properties of such a crystal, and even predicts what the various phases surrounding the time crystal should be—akin to the liquid and gas phases of ice.
This is not mere speculation. Two groups followed Yao's blueprint and have already created the first-ever time crystals. The groups at the University of Maryland and Harvard University reported their successes, using two totally different setups, in papers posted online last year, and have submitted the results for publication. Yao is a co-author on both papers.
Time crystals repeat in time because they are kicked periodically, sort of like tapping Jell-O repeatedly to get it to jiggle, Yao said. The big breakthrough, he argues, is less that these particular crystals repeat in time than that they are the first of a large class of new materials that are intrinsically out of equilibrium, unable to settle down to the motionless equilibrium of, for example, a diamond or ruby.
"This is a new phase of matter, period, but it is also really cool because it is one of the first examples of non-equilibrium matter," Yao said. "For the last half-century, we have been exploring equilibrium matter, like metals and insulators. We are just now starting to explore a whole new landscape of non-equilibrium matter."
While Yao is hard put to imagine a use for a time crystal, other proposed phases of non-equilibrium matter theoretically hold promise as nearly perfect memories and may be useful in quantum computers.
This phase diagram shows how changing the experimental parameters can 'melt' a time crystal into a normal insulator or heat up a time crystal to a high temperature thermal state. Credit: Norman Yao, UC Berkeley
The time crystal created by Chris Monroe and his colleagues at the University of Maryland employs a conga line of 10 ytterbium ions whose electron spins interact, similar to the qubit systems being tested as quantum computers. To keep the ions out of equilibrium, the researchers alternately hit them with one laser to create an effective magnetic field and a second laser to partially flip the spins of the atoms, repeating the sequence many times. Because the spins interacted, the atoms settled into a stable, repetitive pattern of spin flipping that defines a crystal.
Time crystals were first proposed in 2012 by Nobel laureate Frank Wilczek, and last year theoretical physicists at Princeton University and UC Santa Barbara's Station Q independently proved that such a crystal could be made. According to Yao, the UC Berkeley group was "the bridge between the theoretical idea and the experimental implementation."
From the perspective of quantum mechanics, electrons can form crystals that do not match the underlying spatial translation symmetry of the orderly, three-dimensional array of atoms, Yao said. This breaks the symmetry of the material and leads to unique and stable properties we define as a crystal.
A time crystal breaks time symmetry. In this particular case, the magnetic field and laser periodically driving the ytterbium atoms produce a repetition in the system at twice the period of the drivers, something that would not occur in a normal system.
"Wouldn't it be super weird if you jiggled the Jell-O and found that somehow it responded at a different period?" Yao said. "But that is the essence of the time crystal. You have some periodic driver that has a period 'T', but the system somehow synchronizes so that you observe the system oscillating with a period that is larger than 'T'."
Yao worked closely with Monroe as his Maryland team made the new material, helping them focus on the important properties to measure to confirm that the material was in fact a stable or rigid time crystal. Yao also described how the time crystal would change phase, like an ice cube melting, under different magnetic fields and laser pulsing.
The Harvard team, led by Mikhail Lukin, set up its time crystal using densely packed nitrogen vacancy centers in diamonds.
"Such similar results achieved in two wildly disparate systems underscore that time crystals are a broad new phase of matter, not simply a curiosity relegated to small or narrowly specific systems," wrote Phil Richerme, of Indiana University, in a perspective piece accompanying the paper published in Physical Review Letters. "Observation of the discrete time crystal... confirms that symmetry breaking can occur in essentially all natural realms, and clears the way to several new avenues of research."
Yao is continuing his own work on time crystals as he explores the theory behind other novel but not-yet-realized non-equilibrium materials.
Explore further: Time crystals might exist after all (Update) Journal reference: Physical Review Letters Provided by: University of California - Berkeley
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发表于 2017-2-4 16:29
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