谷歌宣称实现“量子霸权”，这意味着什么？

谷歌的研究人员近日宣布，他们已经实现了所谓的“量子霸权”，这标志着计算机科学迎来了一个重要的里程碑。

谷歌的科学家在一篇研究论文中详解了这一突破。该论文本周早些时候曾短暂刊登在美国国家航天局（NASA）的网站上，不过后来又被撤下了。《财富》杂志拿到了该论文的一个副本。

NASA一直在与谷歌就量子计算的某个方面开展合作研究。上周五，《金融时报》率先报道了这篇论文的存在。

谷歌拒绝就该报道发表论评。量子计算指的是用量子物理学的强大特性执行计算机的运算。如果谷歌真的达到了这一里程碑，这标志着科学界朝着量子计算机的实际应用迈出了重要的一步——终有一天，量子计算机将能以极其强大的性能，解决当今最先进的超级计算机也无法解决的各种复杂问题。

量子计算机最让人期待的用途之一，是它可以用来创造新的化学物质，比如生产氮肥或者高功率电池所需的各种催化剂。量子计算也可以破解一些常见的数字加密方法。或许有一天，它还会被用于简化物流和配送操作，以及加速机器学习的应用。

不过，所谓的“量子霸权”并不意味着量子计算机时代已经到来，它们在短期内也无法马上取代我们目前使用的传统计算机。

什么是“量子霸权”？

量子霸权，只的是研究人员利用量子计算机执行某种单一运算时，在合理时间内，其运算效率没有任何一台传统计算机能与之比拟，哪怕是最大的超级计算机也不行。

在谷歌的案例中，该运算主要是检查一个用于生成随机数据的算法，生成的是否是真正随机的数据。

据该研究论文称，量子计算机只需要3分20秒就能完成这一复杂的数学计算。如果换成目前世界上最强大的传统的商用计算机——IBM的Summit 3计算机，则需要花费大约1万年的时间才能完成相同的任务。

量子计算机的工作原理

量子计算机利用了量子力学的特性。经典计算机是以二进制处理信息的，二进制的一个位又称“比特”，可以表现为0或者1。而量子计算机使用的是被称为“量子比特”的逻辑单元，简称为“量子位”，它可以被置于一种量子态，也就是同时代表0和1。

经典计算机中的比特是独立运行的，互不影响。而在量子计算机中，一个量子比特，会影响系统中所有其他量子比特的状态。所以它们可以一起工作来实现一个解决方案。

这两个特性，也使得量子计算机的潜力远远超过了传统计算机。

针对一个问题，传统计算机每次运算时给出的答案都是相同的，而量子计算机给出的结果却是概率性的。也就是说，量子计算机对一个问题并非总会给出相同的答案。因此，要使用量子计算机，你必须在整个系统中进行几十万甚至几百万次的运算，输出结果会围绕最有可能正确的答案集中分布。

在谷歌的研究中，谷歌研究人员使用了一台新的量子处理器，它被命名为“Sycamore”，拥有54个量子比特（不过有一个量子比特没有正常工作，所以只有53个量子比特真正用于实验）。处理器会对生成的数字进行随机抽样，这次实验进行了大约100万次。

Sycamore有什么特殊之处？

Sycamore并非世界上最大的量子处理器。谷歌去年就造出了一个72量子位的处理器。美国加州一家从事量子计算机研究的创业公司Rigetti也表示，它计划很快推出一台128量子位的处理器。不过谷歌的研究人员表示，谷歌在如何保持量子位处于量子状态的时间，以及在如何让量子位与相邻的量子位互动等问题上，已经取得了重大进展。

这一点是非常重要的，因为当量子位脱离量子状态时，会给量子计算机执行的操作造成错误。这种错误必须使用额外的量子位才能纠正。正是因为这种出错概率比较高，所以在执行大多数的数学问题时，目前的量子计算机的效率还不如市面上一台普通的笔记本电脑。

“量子霸权”会使量子计算机超过传统计算机吗？

并不。谷歌的研究成果仅仅意味着在执行这一特定的复杂运算上，量子计算机的表现超过了经典的超级计算机。

不过，谷歌的研究人员在论文中指出，他们的量子计算机也可以用于问题优化、机器学习以及材料科学和化学等学科。

不过，谷歌最新的量子计算设备虽然已在特定问题上形成了“量子霸权”，但它如果用在其他应用场景上，能给效率带来多少提升，还是个未知数。

同时该研究论文也指出，在破解其他数学难题上（比如破解当前的加密系统，因为这个任务涉及分解很多非常大的素数），谷歌的这台量子计算机仍显得不够强大。

实际上，对于许多商业应用来说，今天的量子计算机的性能和计算的精确度，都无法与今天的传统笔记本电脑相比。

黑客们有了量子计算机，会不会偷走我的比特币？

到目前为止，比特币等数字加密货币所基于的公私密钥加密技术，尚无法被量子计算机破解。不过谷歌的研究人员在论文中预测称，量子计算能力将继续以“双指数速度”发展，因而比特币的安全性在长期仍将成为一个问题。

由于担心量子计算机有能力破解最常见的加密技术，美国国家安全局也呼吁发展新的加密技术，使用一些不容易被量子计算机破解的数学算法，以避免可能的“量子黑客”袭击。虽然美国还没有最终确定应该使用哪类新算法，但现在已有一些创业公司正在使用这种“后量子时代”的加密方法，帮助政府和金融机构提前应对“量子黑客”的威胁。

我的桌子上什么时候能有一台量子计算机？

短期内是不可能了。

理论上讲，任何能够进入量子态的材料，都可以被塑造成量子位，但是目前那些最先进的量子计算机使用的基本都是超导材料的量子位，有些还是用相当稀有的材料混合制成的。比如谷歌Sycamore处理器的量子位，使用的是铝环和铟的合成物。铟是一种与银一样稀有的元素。

为了将这些材料置于量子态，并保护量子位不受外部能源的干扰，量子处理器必须小心地悬浮在大型稀释冷藏箱中，其储存温度比太空深处的温度还低。

除了谷歌和Rigetti之外，还有一些公司正在竞相研发商用的量子计算机，比如IBM、微软、英特尔、D-Wave以及许多其他公司。他们都计划让客户通过云端调用量子计算机。所以很有可能的是，你的桌子上永远也不会出现一台量子计算机了。（财富中文网）

译者:朴成奎

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.