撰文 | Frank Wilczek (麻省理工學院教授、2004年諾貝爾獎得主）
翻譯 | 胡風、梁丁當
谷歌CEO桑達爾·皮查伊 (Sundar Pichai) 站在谷歌公司的量子計算機旁邊。
目前，絕大多數計算機都是處理由0、1組成的巨大數組。這兩個二進位數字經常被稱為比特。隨著時代進步，物理學家和工程師們能夠使用越來越小的器件來構建功能性比特。真空管和電磁繼電器逐漸被現代化的集成電路所代替。一個可以輕易置入手機或者手錶的小型晶片上聚集著數十億個電晶體。而且電晶體的尺寸仍然在繼續變小。
Currently, the vast majority of computers are machines that process huge arrays of 0s and 1s, also known as binary digits or simply bits. Over time, physicists and engineers have been able to create functional bits using smaller and smaller objects. Vacuum tubes and electromagnetic relays gave way to modern integrated circuits, which pack billions of transistors into chips that fit comfortably inside mobile phones or watches. And the transistors keep getting smaller.
然而，當微型化接近原子大小的時候——其實現在的電晶體不比原子大很多——我們便置身於一個嶄新的世界：量子力學奇境。比特的量子版，即量子比特，可以是0和1的任意組合（用專業術語講，即「疊加」）。相較於由比特構成的經典計算機，由量子比特構成的量子計算機的功能可能更加強大，但也更加複雜和脆弱。
But when miniaturization reaches the size of atoms-which are not much smaller than today's transistors-we find ourselves entering a new world: the wonderland of quantum mechanics. The quantum version of a bit, known as a qubit, can exist in a continuum of states that are mixtures (technically, "superpositions") of 0 and 1. Quantum computers built from qubits are potentially more powerful than classical computers built from bits, but they are also more complicated and delicate.
量子計算機已成為當今科學技術的前沿。量子比特的製造融合了低溫科學、超導和新型光電電路，是一項開創性的傑出技術成就。但它們其實比基於現代電晶體的經典比特更大，而非更小。我們目前能夠製造的量子計算機還沒有實際用途。事實上，針對它們的研究主要還是著眼於未來。
Today, quantum computers are a research frontier. Qubits are brilliant and pioneering feats of engineering, which make use of cryogenics, superconductivity and new kinds of optoelectronic circuitry. But they are actually larger, not smaller, than modern transistor-based bits. The quantum computers that we can presently construct are not ready for practical use. Rather, they point to the future.
在這條發展道路上，「量子霸權」標誌著一個里程碑：量子計算機能夠完成一項複雜的計算，而這樣的計算，通常的經典計算機無法在較短的時間內完成。谷歌的研究人員們最近在《自然》雜誌上發表了一篇讓人印象深刻的論文，宣稱用一台名為Sycamore的處理器實現了量子霸權。讓人驚訝的是，他們的這台量子計算機只由幾十個低質量（或「噪聲」較大）的量子比特構成，可它的性能卻能夠比肩由數百上千億高質量比特所構成的最先進的經典計算機。
Along that path, "quantum supremacy" is meant to mark a milestone. It is the successful performance, using a quantum computer, of a computation that could not be carried out by a classical computer of reasonable size in a reasonable time. An extremely impressive paper by Google researchers recently appeared in the journal Nature, announcing that they had achieved quantum supremacy with a processor called Sycamore. It is startling to see that their quantum computer, based on a few dozen low-quality (or "noisy") qubits, can compete successfully with top-of-the-line classical computers, which work with billions or trillions of high-quality bits.
儘管如此，我們最好謹慎、全面地看待它。首先，Sycamore實現的只是非常特殊的計算。即使對一名物理學家來說，該計算聽起來都很複雜，並沒有明顯的實用價值。而且，IBM的研究人員很快注意到，利用更好的算法的經典計算機也能在不太長的時間內完成。
Still, some perspective is in order. For one thing, the computation that Sycamore performed is very specialized. It is complicated to describe, even to physicists, and has no obvious practical use. Furthermore, IBM researchers quickly noted that better classical algorithms could perform almost as well.
而更加讓人深思的則是量子霸權的意義。其實解決重要的定量問題時，我們不用量子比特也可以比任何經典計算機都快。隨便一個碳原子，僅通過自己的行為，便能「計算」出一個非常重要的實際問題的答案--即碳會如何運動和變化？比如，通過檢測氣體碳在受熱或雷射輻射後的發射光譜，我們就可以「計算」出碳原子是如何輻射光子以及與光進行相互作用的。相比於任何超級計算機求解相關方程的速度，碳原子給出答案的速度顯然快得多。而且這個方案很容易擴展：只要多用幾個碳原子，你就可以解決化學中的一些重要 問題。
The most profound issue, however, concerns the meaning of quantum supremacy. After all, it doesn』t take qubits to solve important quantitative problems faster than any classical computer. Any carbon atom can "calculate" the solution of a very important practical problem—how does carbon behave? -simply by doing its thing. We can, for instance, "calculate" how carbon emits and interacts with light by examining the spectrum that gaseous carbon emits when heated or after exposure to laser light. Carbon atoms produce the answers much faster than any supercomputer can solve the relevant equations. And this strategy scales quite well: Using several carbon atoms, you can address important problems in chemistry.
毫無疑問，從長遠來看，利用物質量子特性的計算機將大大地增強我們解決有用問題的能力。但我們距離這個目標還很遙遠，甚至都不能確保一定成功。在可預見的將來，我們頂多只能在精心選擇的應用中擁有「量子優勢」，而並非在廣泛的領域擁有「量子霸權」。
There's little doubt that, in the long run, computers that exploit quantum features of matter will dramatically enhance our ability to address useful problems. But we're not there yet, nor is success guaranteed. For the foreseeable future we will have, at best, a "quantum advantage" in wellchosen applications, not "quantum supremacy" along a broad front.
作者簡介
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。因在夸克粒子理論（強作用）方面所取得的成就，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」。
特 別 提 示
1. 進入『返樸』微信公眾號底部菜單「精品專欄「，可查閱不同主題系列科普文章。
2. 『返樸』提供按月檢索文章功能。關注公眾號，回復四位數組成的年份+月份，如「1903」，可獲取2019年3月的文章索引，以此類推。
《返樸》，科學家領航的好科普。國際著名物理學家文小剛與生物學家顏寧共同出任總編輯，與數十位不同領域一流學者組成的編委會一起，與你共同求索。關注《返樸》（微信號：fanpu2019）參與更多討論。二次轉載或合作請聯繫返樸公眾號後台。