谷歌的研究人員近日宣布，他們已經實現了所謂的“量子霸權”，這標志著計算機科學迎來了一個重要的里程碑。

谷歌的科學家在一篇研究論文中詳解了這一突破。該論文本周早些時候曾短暫刊登在美國國家航天局（NASA）的網站上，不過后來又被撤下了。《財富》雜志拿到了該論文的一個副本。

NASA一直在與谷歌就量子計算的某個方面開展合作研究。上周五，《金融時報》率先報道了這篇論文的存在。

谷歌拒絕就該報道發表論評。量子計算指的是用量子物理學的強大特性執行計算機的運算。如果谷歌真的達到了這一里程碑，這標志著科學界朝著量子計算機的實際應用邁出了重要的一步——終有一天，量子計算機將能以極其強大的性能，解決當今最先進的超級計算機也無法解決的各種復雜問題。

量子計算機最讓人期待的用途之一，是它可以用來創造新的化學物質，比如生產氮肥或者高功率電池所需的各種催化劑。量子計算也可以破解一些常見的數字加密方法。或許有一天，它還會被用于簡化物流和配送操作，以及加速機器學習的應用。

不過，所謂的“量子霸權”并不意味著量子計算機時代已經到來，它們在短期內也無法馬上取代我們目前使用的傳統計算機。

什么是“量子霸權”？

量子霸權，只的是研究人員利用量子計算機執行某種單一運算時，在合理時間內，其運算效率沒有任何一臺傳統計算機能與之比擬，哪怕是最大的超級計算機也不行。

在谷歌的案例中，該運算主要是檢查一個用于生成隨機數據的算法，生成的是否是真正隨機的數據。

據該研究論文稱，量子計算機只需要3分20秒就能完成這一復雜的數學計算。如果換成目前世界上最強大的傳統的商用計算機——IBM的Summit 3計算機，則需要花費大約1萬年的時間才能完成相同的任務。

量子計算機的工作原理

量子計算機利用了量子力學的特性。經典計算機是以二進制處理信息的，二進制的一個位又稱“比特”，可以表現為0或者1。而量子計算機使用的是被稱為“量子比特”的邏輯單元，簡稱為“量子位”，它可以被置于一種量子態，也就是同時代表0和1。

經典計算機中的比特是獨立運行的，互不影響。而在量子計算機中，一個量子比特，會影響系統中所有其他量子比特的狀態。所以它們可以一起工作來實現一個解決方案。

這兩個特性，也使得量子計算機的潛力遠遠超過了傳統計算機。

針對一個問題，傳統計算機每次運算時給出的答案都是相同的，而量子計算機給出的結果卻是概率性的。也就是說，量子計算機對一個問題并非總會給出相同的答案。因此，要使用量子計算機，你必須在整個系統中進行幾十萬甚至幾百萬次的運算，輸出結果會圍繞最有可能正確的答案集中分布。

在谷歌的研究中，谷歌研究人員使用了一臺新的量子處理器，它被命名為“Sycamore”，擁有54個量子比特（不過有一個量子比特沒有正常工作，所以只有53個量子比特真正用于實驗）。處理器會對生成的數字進行隨機抽樣，這次實驗進行了大約100萬次。

Sycamore有什么特殊之處？

Sycamore并非世界上最大的量子處理器。谷歌去年就造出了一個72量子位的處理器。美國加州一家從事量子計算機研究的創業公司Rigetti也表示，它計劃很快推出一臺128量子位的處理器。不過谷歌的研究人員表示，谷歌在如何保持量子位處于量子狀態的時間，以及在如何讓量子位與相鄰的量子位互動等問題上，已經取得了重大進展。

這一點是非常重要的，因為當量子位脫離量子狀態時，會給量子計算機執行的操作造成錯誤。這種錯誤必須使用額外的量子位才能糾正。正是因為這種出錯概率比較高，所以在執行大多數的數學問題時，目前的量子計算機的效率還不如市面上一臺普通的筆記本電腦。

“量子霸權”會使量子計算機超過傳統計算機嗎？

并不。谷歌的研究成果僅僅意味著在執行這一特定的復雜運算上，量子計算機的表現超過了經典的超級計算機。

不過，谷歌的研究人員在論文中指出，他們的量子計算機也可以用于問題優化、機器學習以及材料科學和化學等學科。

不過，谷歌最新的量子計算設備雖然已在特定問題上形成了“量子霸權”，但它如果用在其他應用場景上，能給效率帶來多少提升，還是個未知數。

同時該研究論文也指出，在破解其他數學難題上（比如破解當前的加密系統，因為這個任務涉及分解很多非常大的素數），谷歌的這臺量子計算機仍顯得不夠強大。

實際上，對于許多商業應用來說，今天的量子計算機的性能和計算的精確度，都無法與今天的傳統筆記本電腦相比。

黑客們有了量子計算機，會不會偷走我的比特幣？

到目前為止，比特幣等數字加密貨幣所基于的公私密鑰加密技術，尚無法被量子計算機破解。不過谷歌的研究人員在論文中預測稱，量子計算能力將繼續以“雙指數速度”發展，因而比特幣的安全性在長期仍將成為一個問題。

由于擔心量子計算機有能力破解最常見的加密技術，美國國家安全局也呼吁發展新的加密技術，使用一些不容易被量子計算機破解的數學算法，以避免可能的“量子黑客”襲擊。雖然美國還沒有最終確定應該使用哪類新算法，但現在已有一些創業公司正在使用這種“后量子時代”的加密方法，幫助政府和金融機構提前應對“量子黑客”的威脅。

我的桌子上什么時候能有一臺量子計算機？

短期內是不可能了。

理論上講，任何能夠進入量子態的材料，都可以被塑造成量子位，但是目前那些最先進的量子計算機使用的基本都是超導材料的量子位，有些還是用相當稀有的材料混合制成的。比如谷歌Sycamore處理器的量子位，使用的是鋁環和銦的合成物。銦是一種與銀一樣稀有的元素。

為了將這些材料置于量子態，并保護量子位不受外部能源的干擾，量子處理器必須小心地懸浮在大型稀釋冷藏箱中，其儲存溫度比太空深處的溫度還低。

除了谷歌和Rigetti之外，還有一些公司正在競相研發商用的量子計算機，比如IBM、微軟、英特爾、D-Wave以及許多其他公司。他們都計劃讓客戶通過云端調用量子計算機。所以很有可能的是，你的桌子上永遠也不會出現一臺量子計算機了。（財富中文網）

譯者:樸成奎

Google researchers claim to have achieved a major milestone in computer science known as "quantum supremacy."

Google scientists explain their breakthrough in a research paper, a copy of which was obtained by Fortune, that was briefly posted to a NASA website earlier this week before subsequently being taken down.

NASA has been working with Google on one aspect of their quantum computing research. News of the paper's existence was first reported by The Financial Times on Friday.

Google has declined to comment on the report. If the technology company has indeed achieved the milestone, it is a significant step towards the day when quantum computers, which use the powerful properties of quantum physics to perform their calculations, will be able to solve a vast array complex problems that lie beyond the abilities of today's most advanced supercomputers.

Among the most anticipated uses of quantum computers is the ability to create new chemicals, like catalysts for producing nitrogen-based fertilizers or for use in cells in higher-powered batteries. Quantum computing could also be used to crack most commonly used forms of digital encryption. It may one day also be used to streamline logistics and delivery operations, as well as speeding up machine learning applications.

But "quantum supremacy" does not mean quantum computers have yet arrived in the sense that they will soon replace the conventional computers that power our lives.

What is quantum supremacy?

Quantum supremacy means only that researchers have been able to use a quantum computer to perform a single calculation that no conventional computer, even the biggest supercomputer, can perform in a reasonable amount of time.

In the case of Google, this calculation involved checking whether the output of an algorithm for generating random numbers was truly random.

The researchers were able to use a quantum computer to perform this complex mathematical calculation in three minutes and 20 seconds, according to the paper. They say it would have taken Summit 3—an IBM-built machine that is the world's most powerful commercially-available conventional computer—about 10,000 years to perform the same task.

How do quantum computers work?

Quantum computers work by harnessing the properties of quantum mechanics. Classical computers process information in a binary format, called bits, which can represent either a 0 or 1. Quantum computers, in contrast, use logical units called quantum bits, or qubits for short, that can be put into a quantum state where they can simultaneously represent both 0 and 1.

What's more, while the bits in a classical computer all operate independently from one another, in a quantum computer, the status of one qubit effects the status of all the other qubits in the system, so they can all work together to achieve a solution.

These two properties are what give quantum computers so much more potential power than conventional computers.

But while a conventional computer outputs the same answer to a problem every time you run a calculation, the outputs of a quantum computer are probabilistic. That means it does not always produce the same answer. So to use a quantum computer, you have to run a calculation through the system thousands or even millions of times, and the array of outputs converge around the answer that is most likely to be correct.

In the case of Google's research, the company used a new quantum processor, which it named Sycamore, that has 54 qubits (although one did not function properly, the researchers said, so only 53 were actually used in the experiment) which sampled the random number generating circuit it was testing some 1 million times.

What's so special about Sycamore?

Sycamore is not the world's largest quantum processor. Google itself had produced a 72 qubit system last year. And Rigetti, a California startup working on quantum computers, has said it plans to have a 128 qubit system ready soon. But Google's researchers said they made major advances in how long its qubits can remain in a quantum state and how each qubit interacts with the other qubits next to it.

That's important because when qubits fall out of a quantum state, they introduce errors into the calculations the quantum computer is performing. Those errors then have to be corrected by using additional qubits. These error rates are the reason that your laptop can beat today's quantum computers in getting a correct answer to most mathematical problems.

Does quantum supremacy make quantum computers better than conventional computers?

No. Google's achievement only means its quantum computer could outperform a classical supercomputer on this one complex calculation.

The Google researchers say in their paper that their quantum computer may also have uses in optimization problems, machine learning as well as materials science and chemistry.

But it is unclear how much of an advantage or increase in speed Google's new quantum computing hardware, which it used to achieve quantum supremacy, will have in these other applications.

And Google's machine is not yet powerful enough to tackle other difficult mathematical problems, such as breaking current encryption systems, a task which involves factoring very large prime numbers, according to the research paper.

For many business applications, in fact, today's quantum computers are no match for the power and accuracy of today's conventional laptops.

Could hackers armed with quantum computers steal my bitcoin?

For the moment, the public-private key encryption techniques on which bitcoin and other cryptocurrencies are based cannot be broken by a quantum computer. But Google's researchers, in their paper, predict that quantum computing power will continue to advance at a "double exponential rate," so those bitcoins may not be safe for all that much longer.

The fear of quantum computers being capable of breaking most common encryption techniques has lead the U.S. National Security Agency to call for the adoption of new techniques that use different kinds of math that are not susceptible to attack from a quantum computer. Although the U.S. has not yet settled on which class of new algorithms should be used, a number of startups are currently helping financial firms and governments prepare their systems to use such "post-quantum" encryption methods.

When can I have a quantum computer on my desk?

Not any time soon.

While almost any material that can be put into a quantum state can be used to form a qubit, the most advanced quantum systems today tend to use tiny bits of superconducting materials, often bonded together using fairly exotic materials. The qubits in Google's Sycamore processor used aluminum loops bonded with indium, an element that is about as rare as silver.

To put those materials into a quantum state, and to safeguard the qubits from interference from outside energy sources, the quantum processors have to be carefully suspended in large dilution freezers at temperatures colder than those found in deep space.

Ultimately, the companies racing to commercialize quantum computers— which besides Google and Rigetti, include IBM, Microsoft, Intel, D-Wave and a host of others—plan to offer customers the ability to run calculations on a quantum computer through the cloud. So it's more likely that one will never grace your desk, at all.