諾獎得主Wilczek：出人意料的電子分裂_風聞
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撰文 | Frank Wilczek
翻譯 | 胡風、梁丁當
中文版
電子是最基本的一種粒子。在基礎物理學中，電子被視作沒有結構的點，具有質量、電荷和角動量（或“自旋”）。根據量子力學和相對論的嚴格規則，這個看上去有些簡陋的描述成為了構建化學和電子學的基礎元素。
在不久之前，把電子注入特定物質使其分裂還是一個近乎荒誕的想法。就像哥白尼時代的自然哲學家都認為日心説極其荒唐一樣，對於多數嚴謹的物理學家而言，電子會分裂成其他物質的想法也是非常離譜的。
但地球確實在繞着太陽轉動，而電子也確實能夠分裂。早在20世紀80年代，這個令人震驚的可能性就已初現端倪。當時，物理學家發現了一種被稱為分數量子霍爾效應的奇異物質態 ：如果把極其薄且純淨的特定半導體嵌入到特定的絕緣體中，在超強磁場和極低温度下，就會發生分數量子霍爾效應。
霍爾效應（Hall effect）最初是由19世紀的物理學家埃德温 · 霍爾（Edwin Hall）發現並以他的名字命名的。霍爾效應指的是在垂直於外磁場的方向對導體施加電流，在垂直於磁場和電流的方向會產生電勢差，也就是霍爾電壓。這種現象為電效應與磁效應之間的轉換提供了一種極為便捷的方式，是設計速度計和防抱死剎車系統等眾多常見儀器的核心機制。
在分數量子霍爾效應中，電流異常的小，卻也異常穩定。這些特徵意味着形成電流的粒子具有奇怪的屬性 ：它們的流動呈現出不同尋常的有序性，且每個粒子只攜帶很少的電荷。在最簡單的情況下，這種粒子攜帶的有效電荷只有電子電荷的三分之一，這表明薄層材料中的電子分裂成了三個相等的部分。
直到不久前，人們對分數電子的研究還只是純粹受好奇心驅動的學術性研究。分數電子成功地挑戰了科學家對物質的傳統認知，因此引發了高度關注。但要想實現這種效應需要極其苛刻的實驗條件，因此它的實際應用似乎只是空中樓閣。
然而，最近科學家對分數電子的興趣暴漲，因為他們發現分數電子具有一種特殊的集體記憶。更具體地講 ：如果你使一個分數電子圍繞着另一個分數電子移動，那麼根據繞轉的方式，兩個分數電子其後的行為也會有所不同。由於這種“記憶力”，分數電子——一種任意子——有望成為構建、存儲量子信息以及實現量子計算機的基本單元。
量子信息雖然具有豐富的潛力，但也極其脆弱。如果想要開發它的實際用途，我們需要能融合量子信息的複雜性與物理可操作性的方案。利用任意子，我們有望實現這個目標。目前，科學家正在致力於研發更容易實現的任意子，學習如何有效地纏繞它們、並測量它們的行為——也就是如何給它們賦予特定的記憶並使其呈現所需的結果。事實上，這項研究已經超越了純粹的學術範疇，微軟和谷歌等企業都深度參與其中。
任意子的故事是彰顯好奇心所驅動的基礎研究價值的一個典型例子。探索新奇的現象會給探索者帶來深刻的快樂。這本身就很有價值。但有的時候，它的價值會輻射更廣的領域。正如只有少部分敢於冒險的創業者可以獲得巨大的成功，也只有少數瘋狂的智力冒險最終會發展成突破性技術。無論哪種情況，成功都是罕見的，失敗才是大多數。儘管如此，基礎研究可能帶來的鉅額回報仍然使得對它的大量投資物有所值。
英文版
The Surprise of Splitting Electrons
The once-outrageous idea that the most elementary particles can break apart is spurring furious research into the new field of ‘anyonics’
Nobel and Templeton Prize-winning physicist Frank Wilczek explores the secrets of the cosmos. Read previous columns here.
Electrons are the most elementary of elementary particles. In fundamental physics they appear as structureless points where definite amounts of mass, electric charge, and angular momentum (or “spin”) reside. From that meager description, the stringent rules of quantum mechanics and relativity produce the splendid building block that dominates chemistry and-of course-electronics.
Not long ago, the outrageous idea that electrons, when injected into the right sort of material, would break into other objects seemed as far-fetched to most right-thinking physicists as the idea that the Earth moves seemed to sober natural philosophers in the time of Copernicus.
Yet the Earth moves-and electrons do break apart. That shocking possibility emerged in the 1980s, in studies of an exotic state of matter known as the fractional quantum Hall effect. This effect occurs when extremely pure, thin layers of the right semiconductors, embedded within the right insulators, are subjected to extremely high magnetic fields at extremely low temperatures.
The original Hall effect, named after the 19th-century physicist Edwin Hall, refers to the appearance of a sideways electric current in response to an applied voltage in this kind of setup. It provides a convenient way to translate between electrical effects and magnetic ones, and is at the heart of the operation of many useful devices including speedometers and anti-lock brakes.
In the fractional quantum Hall effect, the currents are both unusually small and unusually stable. Those features indicate that the particles that make the current have weird properties: their flow is unusually orderly, yet each one carries little charge. In the simplest case, the apparent charge is one-third that of an electron, which indicates that electrons injected into the material layer have fragmented into three equal pieces.
Until quite recently, electron fractionalization had the air of a scientific curiosity. Because it challenged traditional wisdom successfully, professional physicists paid close attention. But practical applications seemed remote, because the effect was visible only in difficult experiments.
Recently, however, interest in fractionated electrons has exploded, because it turns out that they have a kind of collective memory. To put this more concretely: After you move them around one another, their subsequent behavior reliably reflects how you treated them. Because of this “memory,” fractional electrons-known as anyons-are promising ingredients for building up and storing quantum information, and ultimately for making quantum computers.
Quantum information, while potentially very rich, is also very delicate. To use it for practical purposes, we need embodiments that combine complexity with physical toughness. Anyons could fit the bill. People are making progress by making them in more user-friendly forms, learning how to move them around efficiently, and probing their behavior-in essence, giving them things to remember and getting them to display the results. This work has expanded beyond the borders of academia; Microsoft and Google are heavily involved.
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The anyon story is a lovely example of the value of curiosity-driven research. Exploring surprising phenomena for their own sake gives profound joy to the people who do it. That is valuable in itself. But there’s sometimes (much) more. Just as only a small proportion of adventurous startups make it big, few wild intellectual adventures blossom into breakthrough technologies. In either case, lots of things can go wrong or fizzle out. But big payoffs from pure research, even though they are rare, make big investment in it profitable overall.
Frank Wilczek
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