量子纠缠现象的研究综述
原文作者 Admin
摘要：量子力学的影响是巨大的，它不仅仅改变了我们对物理世界的认识，更改变着现代科学技术的方方面面。诸多较为成熟的量子信息技术或是理论已经证实，量子信息技术由于本身基于量子力学的先天优势，将会给经典信息技术带来深刻甚至是颠覆式的变革。尽管如此，受限于对量子力学的认识，量子信息技术中诸多的理论进展并不顺利，如被视为量子信息技术的关键资源的量子纠缠理论的研究。量子纠缠自从二十世纪伴随量子力学的诞生和发展，一直倍受物理科学、信息科学等领域专家的青睐而受到广泛研究。然而，直到今天，我们对量子纠缠的认识仍然存在诸多挑战。如量子系统中的纠缠现象与宏观系统的关系，如何判定一个量子系统是否存在量子纠缠，如何对这一关键物理资源进行定量的描述等等。
关键词：量子纠缠；W态；GHZ态；量子力学；量子信息；量子叠加原理；不确定性关系。
外文翻译：
量子力学有一个更令人讨厌的结果，揭示了现实在很大程度上是一种持久的幻觉。量子力学不仅仅是一种微观理论.所有物质基本上都是量子的，所以奇怪的量子效应很难在比几个原子大的物体上观察到。就像柏拉图洞穴寓言中墙上闪烁的剪影一样，宏观的、所谓的古典物体的存在，只不过是它们真正的量子形式投下的一个影子。这对物理学家来说不是什么新鲜事，他们已经在量子世界里瞎混了一个多世纪，对摇摇晃晃的现实大厦基本不感兴趣。
周四发表在《科学》杂志上的两篇新论文，推动了物理学家在宏观尺度上可以实现的量子效应的边界。两项研究都在红细胞大小的铝桶中观察到了这种效应。在第一项研究中，美国和以色列的研究人员直接而可靠地测量了两个鼓之间的量子纠缠。第二项研究是由一个芬兰团队领导的，他们在测量纠缠鼓的同时，避免了后退动作，也就是测量物体位置和动量时不可避免的噪音。在经典世界中，这种测量的精度没有理论上的限制。但是，由德国物理学家维尔纳·海森堡在20世纪20年代提出的不确定性原理指出，对于像鼓这样的物体，其位置和动量的已知程度存在一个基本极限。芝加哥大学的凝聚态物理学家Aashish Clerk没有参与这两项研究，他说，这两篇论文中描述的技巧是为了避免你可能认为的海森堡测不准原理对测量的限制。先前在宏观系统中观察到过纠缠和反作用力逃避，但方式不同，可以说更有限。
2018年，另一组研究人员缠结了两条硅带。其他实验甚至使钻石中的振动纠缠在一起。然而，这两个团队在最近的论文中所展示的技巧， 让他们可以在更少的限制下观察量子效应。联邦理工学院的量子研究员朱毅文说，我们在这里没有发现任何关于量子力学的新东西。他没有参与这两项研究。但他说，要获得这些测量值，仍需要非常令人印象深刻的技术进步。克拉克说，这个神秘的研究领域有一个简单的首要目标:将一些大的东西带入量子状态。应用范围从量子计算机到需要亚原子精度的物理问题，如探测暗物质或引力波。
一些研究人员，比如芬兰阿尔托大学的物理学家、第二篇论文的合著者Mika Sillanpauml;auml;，希望测量敏感的量子效应，但由于他们的宏观测量工具的经典性质而受到限制。通过将量子效应带入宏观领域，或者换句话说，使经典物体回归到它们真正的量子自我Sillanpauml;auml;希望研究量子引力。量子技术的进步有时被吹捧为潜在的消费者利益。Sillanpauml;auml;冷淡地表示，这些新发展虽然令人兴奋，但并不适用于手机。
在解释量子纠缠时，人们用的类比比物理学中几乎任何其他现象都要多。Shlomi Kotler是美国国家标准与技术研究所的物理学家，也是第一篇论文的合著者，他给出了一个简单的定义:当物体的位置或动量比这些位置或动量的初始不确定性更精确地已知时，物体就被缠结了。缠结只是物体之间的一种关联，无论是电子还是微米大小的铝桶，它超越了传统关系的可能性。
为了实现纠缠，两个团队精心制作了微调好的铝鼓，把它们放在一个晶体芯片上，将装置过冷到接近绝对零度，然后用微波辐射脉冲撞击两个鼓。NIST的物理学家、第一篇论文的合著者约翰·特伊费尔说，这两个鼓根本不会机械地相互交流。微波作为媒介，使它们能够相互交谈。而困难的部分是要确保他们彼此之间能强有力地交流，而宇宙中没有其他人得到关于他们的信息。在微波的冲击下，每个鼓振动，上下波动的幅度约为一个质子的宽度。这种微小的运动可以检测到与鼓相连的电路电压的变化。Teufel说，纠缠两个原子的运动已经是一个困难的、英雄的实验。相比之下，每个鼓大约有一万亿个原子。此外，虽然单个粒子具有离散的量子态，如自旋向上或向下，鼓可以在它们摆动时振幅或振动距离的连续分布。但如果鼓足够敏感，可以与微波脉冲纠缠，且相对无噪声，它们的振幅将是强相关的。测量一个鼓的振幅就能知道另一个鼓的振幅是多少。例如，如果一个鼓被测量为高振幅，那么另一个必须是低振幅。你只需要一个非常非常好的信噪比来进行测量，克拉克说。这可能是这类系统的第一次实验，达到了这个效果。
事实上，这一比率非常低，我们可以通过简单地绘制两个鼓位置之间的空间关系来看到缠结的效果。在那里，在成千上万的数据点中，有一种不可思议的关联——证明了两个独立的鼓的经典现实是一个更深层次的真理的影子，在这个真理中，纠缠使它们成为一个单一的量子物体。
第二组的研究人员并没有重复击打鼓来多次缠绕它们，而是用一种更像鼓声的方法创造了一种持久的缠绕，而不是一次敲击。通过创造这种稳定的状态，研究人员能够对相同的纠缠进行多次测量，以避开海森堡测不准原理。这一原理经常被错误地描述为:任何测量，无论多么小，都会给物体带来冲击，从而引入不确定性。克拉克说，不确定原理认为，有些东西你不能同时完美地测量两者。还有其他一些事情，你完全可以同时且完美地进行测量。
例如，对于如何精确地知道一个物体的位置或动量是没有限制的。当你试图同时衡量两者时，问题就来了。反行动规避是一种绕开这一限制而不违反海森堡的命令的方法。不是测量每个鼓的位置和动量，Sillanpauml;auml;和他的同事们实际上是通过它对电路电压的影响来测量鼓动量的总和。没有什么违反海森堡测不准原理。你只是选择了一组特定的问题，而不是问那些被禁止的事情，Chu说。这两个实验所证明的精确度的可能性是有趣的。可以毫不夸张地想象，类似的鼓有朝一日可以用于探测桌面上量子引力的微小效应，或者用作量子网络中继的一部分。
但也许这项工作最吸引人的方面，超越了任何应用，是它只是让我们更接近世界的真正量子本质。“你每天看到的都是影子，”科特勒说。“但只要有正确的技术，你就能看到这种纠缠已经存在了，准备用于下一步。”
外文文献出处：科学美国人
附外文文献原文
One of the more irksome results of quantum mechanics is the revelation that reality is largely a persistent illusion. Quantum mechanics is not merely a theory of the microscopic: all matter is fundamentally quantum—it just so happens that weird quantum effects are hard to observe in anything bigger than a few atoms. Like the flickering silhouettes on the wall in Platorsquo;s allegory of the cave, the existence of macroscopic, so-called “classical” objects is merely a shadow cast by their true quantum forms. This much is not news to physicists, who have been mucking around in the quantum world for more than a century and are mostly unbothered by the crumbling edifice of reality.
Two new papers published on Thursday in Science push the boundaries of the quantum effects physicists can achieve at a macroscopic scale. Both studies observed such effects in thin aluminum “drums” about the size of a red blood cell. In the first study, U.S. and Israeli researchers directly and reliably measured quantum entanglement between the drums. And the second study, led by a Finnish team, measured entangled drums while avoiding “back action,” the inevitable noise associated with the very act of trying to measure an objectrsquo;s position and momentum.
In the classical world, there is no theoretical limit to the precision of such measurements. But the uncertainty principle, formulated by German physicist Werner Heisenberg in the 1920s, states that there is a fundamental limit to how well the position and momentum of an object such as a drum can be known. “The tricks described in these two papers are ways of evading what you might have thought is the limit on measuring forces coming from the Heisenberg uncertainty principle,” says Aashish Clerk, a condensed matter physicist at the University of Chicago, who was not involved with either study.
Both entanglement and back-action evasion have been previously observed in macroscopic systems but in different, and arguably more limited, ways. In 2018 another group of researchers entangled two strips of silicon. Other experiments have even entangled vibrations in diamonds. Yet the tricks demonstrated by both teams in the recent Science papers have allowed them to observe quantum effects with far fewer caveats.
“Wersquo;re not discovering anything new about quantum mechanics here,” says Yiwen Chu, a quantum researcher at the Swiss Federal Institute of Technology Zurich, who was not involved in either study. But getting these measurements still requires “very impressive technological advances,” she says.
This arcane area of research has a simple overarching goal: “get something big into a quantum state,” Clerk says. Applications range
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Scientists super quantum effect and entangled drum duet
One of the more irksome results of quantum mechanics is the revelation that reality is largely a persistent illusion. Quantum mechanics is not merely a theory of the microscopic: all matter is fundamentally quantum—it just so happens that weird quantum effects are hard to observe in anything bigger than a few atoms. Like the flickering silhouettes on the wall in Platorsquo;s allegory of the cave, the existence of macroscopic, so-called “classical” objects is merely a shadow cast by their true quantum forms. This much is not news to physicists, who have been mucking around in the quantum world for more than a century and are mostly unbothered by the crumbling edifice of reality.
Two new papers published on Thursday in Science push the boundaries of the quantum effects physicists can achieve at a macroscopic scale. Both studies observed such effects in thin aluminum “drums” about the size of a red blood cell. In the first study, U.S. and Israeli researchers directly and reliably measured quantum entanglement between the drums. And the second study, led by a Finnish team, measured entangled drums while avoiding “back action,” the inevitable noise associated with the very act of trying to measure an objectrsquo;s position and momentum.
In the classical world, there is no theoretical limit to the precision of such measurements. But the uncertainty principle, formulated by German physicist Werner Heisenberg in the 1920s, states that there is a fundamental limit to how well the position and momentum of an object such as a drum can be known. “The tricks described in these two papers are ways of evading what you might have thought is the limit on measuring forces coming from the Heisenberg uncertainty principle,” says Aashish Clerk, a condensed matter physicist at the University of Chicago, who was not involved with either study.
Both entanglement and back-action evasion have been previously observed in macroscopic systems but in different, and arguably more limited, ways. In 2018 another group of researchers entangled two strips of silicon. Other experiments have even entangled vibrations in diamonds. Yet the tricks demonstrated by both teams in the recent Science papers have allowed them to observe quantum effects with far fewer caveats.
“Wersquo;re not discovering anything new about quantum mechanics here,” says Yiwen Chu, a quantum researcher at the Swiss Federal Institute of Technology Zurich, who was not involved in either study. But getting these measurements still requires “very impressive technological advances,” she says.
This arcane area of research has a simple overarching goal: “get something big into a quantum state,” Clerk says. Applications range from quantum computers to problems in physics that require subatomic precision, such as the detection of dark matter or gravitational waves.
Some researchers, such as Mika Sillanpauml;auml;, a physicist at Aalto University in Finland and a co-author of the second paper, wish to measure sensitive quantum effects but have been limited by the classical nature of their macroscopic measuring tools. By bringing quantum effects into the macroscopic realm—or, put another way, returning classical objects to their true quantum selves—Sillanpauml;auml; hopes to investigate quantum gravity.
Advances in quantum technology are sometimes touted for their potential consumer benefit. The new developments, while exciting, are “not for mobile phones,” Sillanpauml;auml; says dryly.
More analogies have been conjured to explain quantum entanglement than nearly any other phenomenon in physics. Shlomi Kotler, a physicist at the National Institute of Standards and Technology and a co-author of the first paper, offers a simple definition: objects are entangled when their positions or momenta are known more precisely than the initial uncertainty of those positions or momenta. Entanglement is simply a correlation between objects—whether they are electrons or micron-sized aluminum drums—that exceeds what is possible with just a classical relationship.
To achieve entanglement, the two teams crafted finely tuned aluminum drums, placed them on a crystal chip, supercooled the setup to near absolute zero and then hit both drums with a pulse of microwave radiation.
“These two drums donrsquo;t talk to each other at all, mechanically,” says John Teufel, a physicist at NIST and a co-author of the first paper. “The microwaves serve as the intermediary that lets them talk to each other. And the hard part is to make sure they talk to each other strongly without anybody else in the universe getting information about them.”
Struck by the microwaves, each drum vibrates, rising up and down by about the width of a proton. This minuscule motion is detectable as a change in the voltage of a circuit connected to the drums.
“Entangling the motion of two atoms is already a hard, heroic experiment,” Teufel says. In comparison, each drum has roughly one trillion atoms. Moreover, whereas single particles have discrete quantum states such as spin up or down, the drums can be in a continuous distribution of amplitudes, or distances of vibration, as they wobble.
But if the drums are sensitive enough to be entangled from the microwave pulse and relatively noise-free, their amplitudes will be strongly correlated. Measuring the amplitude of one drum tells you what the amplitude of the other is. For example, if one drum is measured to have a high amplitude, the other must have a low amplitude.
“You just need a really, really good signal-to-noise ratio for you
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