冷而酷的物理 | 諾獎得主Wilczek專欄_風聞

撰文 | Frank Wilczek

翻譯 | 胡風、梁丁當

來源：蔻享學術

孕育了眾多新物理的極低温，或許也是計算機技術取得新突破的關鍵。

Extremely low temperatures may be the key to new breakthroughs in computing.

現代製冷技術在很多方面改變了我們的生活。藉助冷藏技術，我們得以品嚐到來自全球各地的食物——無論是當季的還是反季節的；有了空調，家和汽車也變成了三伏天的避暑場所。

Modern cooling technology shapes our lives in many ways. Refrigeration gives us access to food from all over the world, in and out of season; airconditioning turns homes and cars into places of refuge from sweltering summers.

Illustration: Tomasz Walenta

相比於古老的冰塊製冷，這些技術無疑是重大的進步。在用冰製冷的年代，曾形成過工業規模的冰塊開採。我經常散步的地方——馬薩諸塞州劍橋的弗雷什塘（Fresh Pond），就曾經是一個冰礦。

These are big improvements over the time-honored method of using ice, which used to be mined on an industrial scale. Fresh Pond in Cambridge, Mass., where I often take walks, was once an ice mine.

將物體放到冰上冷卻，其物理原理是較簡單直接的。由於能量總是從高能態往低能態流動，熱量會從較熱的物體傳遞到冰上，結果就是前者變冷、後者融化。

Putting something on ice to cool it is straightforward physics. Since energy flows downhill from a higher-energy state to a lower-energy one, heat is transferred from a warm object to the cold ice, causing the former to become cooler and the latter to melt.

更先進的製冷方法則更難實現，因為這需要從物體中抽取能量並轉移到周圍環境中，從而保持物體比周圍環境温度更低。要實現這一點，就必須輸入能量。這一點看似矛盾，但藉助熱力學定律確實能實現。較為廉價的能源，再加上出色的工業設計，註定了冰工業的衰落。

More advanced methods of cooling are harder, because they require pumping energy out of a body to keep it cooler than its surroundings. Paradoxically, this can only be done by investing energy, as well as taking advantage of the laws of thermodynamics. Reasonably cheap energy, plus some brilliant engineering, doomed the ice industry.

極低温仍然是發現新物理的重要領域。物體在極低的温度下呈現出明顯的量子效應。量子力學的一個主要特徵是能量只能取離散的值，即某個能量單位的整數倍。要觀測到這種量子性， 這個倍數必須足夠小：1000002個能量單位和1000003個能量單位沒有本質不同，但如果是2個和3個能量單位，就會存在根本性的差別了。超冷材料的能量極低，在這種場景中量子力學的神奇定律能夠充分發揮它的魔力。

Extremely low temperature is still a major discovery zone in physics. It is where quantum mechanics comes into its own. The distinguishing feature of quantum mechanics is that energy comes in discrete units. To see the effects of this discreteness, you’ve got to keep the number of units small: There’s no qualitative difference between 1,000,002 and 1,000,003 units of energy, but there is between two and three units. Ultracold materials are energy-starved, allowing the strange laws of quantum mechanics to work their magic.

在接近絕對零度時，很多金屬和一些其他物質會突然變成超導態。超導材料中的電流沒有電阻，所以只需要很少的能耗——甚至零能耗——就能維持。這個美麗的性質賦予了超導材料很多用途，比如，它可以用於製造核磁共振成像（MRI）所需的強力電磁鐵。

Close to absolute zero, many metals and some other substances suddenly become superconductors. Electric currents flow through superconductors smoothly, without friction, so that it takes little or no energy to maintain them. Among many other applications, that beautiful fact makes it possible to create the powerful electromagnets used in magnetic resonance imaging (MRI).

類似地，液氦在低温下會轉變成超流體，從而在沒有摩擦阻力的理想情況下傳輸質量。因此，液氦是制 造低温冰箱的理想材料，而液氦超流已經成為了現代低温技術的主力軍。

Similarly, liquid helium becomes a superfluid at low temperature, able to transport heat without friction. This makes it ideal for low-temperature refrigerators, and superfluid helium is the workhorse of cryogenics.

信息處理是另一個非常活躍的低温物理前沿。現代計算機是通過電流工作的，而電流的熱效應是制約計算機發展的一個主要因素。目前研究人員正在研究如何用信息“超流” 在沒有熱損耗的情況下傳輸數據。

Information processing is another active frontier of low-temperature physics. The heat generated by modern computers, which are powered by electric currents, is a major limitation on computing technology. Researchers are working on ways to transfer data without generating heat, using “supercurrents” of information flow.

此外，計算中的許多重要問題，比如如何優化配電網絡或者航線規劃系統，都可以轉化為“製冷”的問題來思考。將一個問題數字化後，問題和（可能的）答案都成了一長串由0和1組成的編碼。我們可以把這些二進制碼翻譯成一個物理系統：0和1分別對應一個開關的“關”和“開”，或者是電子的“自旋向上”和“自旋向下”兩種狀態。

In addition, many important problems in computing, such as finding efficient distribution networks or airline routing systems, can be thought of in terms of making things cold. Whenever a problem is posed digitally, both the question and the potential answers are long strings of zeros and ones. These binary numbers can be translated into physical systems: Zero and one can become the “off” and “on” positions for a switch, or “spin up” and “spin down” for an electron.

通過這種“物理化”的編程，我們可以把問題和答案分別對應到一個物理系統和它的狀態上。其中，能量最低的那個態對應的就是最佳答案。

This kind of physical programming allows us to map questions and answers into states of physical systems, where the most efficient solution will be the one that contains the least heat.

製冷需要考慮多方面的因素，例如温度的高低、系統是處於大自然還是人工環境中。這些都不斷地挑戰着人們的聰明才智。發明新的冰箱和空調或許是計算機技術取得新突破的關鍵。這讓我在夏天游泳的時候有了新的思考目標。

The multifaceted problem of refrigeration, at high or low temperatures, in natural or engineered environments, continues to challenge human ingenuity. There are strong incentives to invent new kinds of ice and airconditioners—refreshing things to envision during my long summer swims.

Frank Wilczek：弗蘭克·維爾切克是麻省理工學院物理學教授、量子色動力學的奠基人之一。因發現了量子色動力學的漸近自由現象，他在2004年獲得了諾貝爾物理學獎。

本文經授權轉載自微信公眾號“蔻享學術”。