Key Chemistry Question Answered, No Quantum Computer Required

What Garnet Chan cares most about is basic science.

Garnet Chan最关心的是基础科学。

He entered chemistry decades ago to understand some of the most consequential biochemical processes on Earth.

几十年前，他进入化学领域，以了解地球上一些最重要的生化过程。

But since then, he’s become a central figure in a different arena: the debate over whether quantum computers will have a decisive advantage over ordinary “classical” ones.

但从那时起，他成为了另一个领域的中心人物：关于量子计算机是否比普通“经典”计算机具有决定性优势的争论。

Over the past decade, many quantum computing researchers have identified the very reactions Chan studies as an area in which quantum computers should excel.

在过去的十年中，许多量子计算研究人员已经将陈研究的反应视为量子计算机应该擅长的领域。

Chan, however, has long doubted that powerful quantum computers — which are still years away — will be necessary.

然而，陈长期以来一直怀疑强大的量子计算机（距离实现还需要数年时间）是否有必要。

“My main interest is in solving chemical problems.

“我的主要兴趣是解决化学问题。

If classical computers are the right tool to do it, we should,” he said.

如果经典计算机是做到这一点的正确工具，我们就应该这样做，”他说。

While he believes quantum computers will eventually play an important role in the field, “I don’t see why we should wait for a fault-tolerant quantum computer to be built.”

虽然他相信量子计算机最终将在该领域发挥重要作用，但“我不明白为什么我们应该等待容错量子计算机的建成。”

Now he has a result that strengthens his case.

现在他的结果证实了他的观点。

In early January, Chan and five other quantum chemists based out of the California Institute of Technology reached a key milestone in understanding the enzyme nitrogenase , which converts atmospheric nitrogen into ammonia and makes life on our planet possible.

一月初，陈和加州理工学院的其他五位量子化学家在理解固氮酶方面达到了一个重要的里程碑，固氮酶将大气中的氮转化为氨，使我们星球上的生命成为可能。

It was a major triumph for theoretical chemists, the outcome of decades of effort.

这是理论化学家的重大胜利，是数十年努力的成果。

But for years, nitrogenase had also served as a proof-of-concept target in the realm of quantum computing.

但多年来，固氮酶也一直作为量子计算领域的概念验证目标。

To understand the enzyme, researchers must follow the behavior of many electrons that are all linked together via quantum entanglement.

为了了解这种酶，研究人员必须追踪通过量子纠缠连接在一起的许多电子的行为。

The number of possible configurations grows explosively large.

可能的配置数量呈爆炸式增长。

Researchers hypothesized that they would likely only be able to decipher the system via a machine that could manipulate quantum states.

研究人员假设他们可能只能通过能够操纵量子态的机器来破译该系统。

But Chan and his colleagues used purely classical methods.

但陈和他的同事使用的是纯粹的经典方法。

That makes their result a pivotal statement not only about the chemistry that supports life, but also about whether quantum computers are needed to understand it.

这使得他们的结果不仅成为关于支持生命的化学的关键陈述，而且也成为关于是否需要量子计算机来理解它的关键陈述。

“I think it’s important to clarify that this is not an impossible task where you have to first build a quantum computer to say anything about the problem,” Chan said.

陈说：“我认为重要的是要澄清，这不是一项不可能完成的任务，你必须首先建造一台量子计算机才能解决这个问题。”

While Garnet Chan is excited for the day when quantum computers will help solve important problems in chemistry, he sees no need to wait: Contrary to popular belief, he argues, quantum computers aren’t needed to answer some of the field’s biggest questions.

虽然 Garnet Chan 对量子计算机将帮助解决化学中的重要问题的那一天感到兴奋，但他认为没有必要等待：他认为，与普遍看法相反，量子计算机不需要回答该领域的一些最大问题。

Not everyone agrees.

并非所有人都同意。

Some researchers cite the many years it took to obtain the result classically.

一些研究人员引用了经典方法需要很多年才能获得结果的例子。

Even if one chemistry problem has ultimately proved tractable with classical methods, they say, quantum computers are still needed to make these kinds of discoveries at scale.

他们说，即使最终证明一个化学问题可以用经典方法解决，仍然需要量子计算机来大规模地进行此类发现。

“If we pick any optimization problem and you put 20 years into it, you can figure out that one system,” said James Whitfield , a quantum computing theorist at Dartmouth College.

达特茅斯学院的量子计算理论家詹姆斯·惠特菲尔德 (James Whitfield) 表示：“如果我们选择任何优化问题，并投入 20 年的时间，你就可以找出一个系统。”

“But whether that solution is transferable?

“但是这个解决方案是否可以转移？

Questions like that won’t be answered by solving one instance of one molecular system.”

像这样的问题并不能通过解决一个分子系统的一个实例来回答。”

Solving this particular problem about nitrogenase may not settle the debate over quantum computers just yet, but each step toward understanding the enzyme’s full chemistry makes the debate less hypothetical.

解决固氮酶的这个特殊问题可能还无法解决有关量子计算机的争论，但了解酶的完整化学过程的每一步都使争论变得不那么假设。

Alongside photosynthesis, nitrogen fixation is one of the most essential chemical processes for life on Earth.

除了光合作用之外，固氮也是地球上生命最重要的化学过程之一。

Nitrogenase is what makes it possible.

固氮酶使这一切成为可能。

Before nitrogenase evolved, living things were limited by the amount of nitrogen available to be incorporated into organic matter.

在固氮酶进化之前，生物受到可掺入有机物中的氮量的限制。

It was an ironic obstacle, given that the planet was in fact suffused with nitrogen: The element accounts for about 80% of the atmosphere.

这是一个具有讽刺意味的障碍，因为这个星球实际上充满了氮：这种元素约占大气层的 80%。

But atmospheric nitrogen exists as the diatomic molecule N 2 , which is inert and therefore unusable in biological processes.

但大气中的氮以双原子分子 N 2 的形式存在，它是惰性的，因此无法用于生物过程。

Only rare high-energy events could break the molecule into nitrates that life could use.

只有罕见的高能事件才能将分子分解成生命可以使用的硝酸盐。

“Organisms were literally waiting for lightning to strike.

“生物体实际上是在等待闪电袭击。

That’s how you’d get nitrogen to be available for biomass,” said Daniel Suess , a chemist at the Massachusetts Institute of Technology who studies nitrogenase.

这就是如何获得可用于生物质的氮，”麻省理工学院研究固氮酶的化学家 Daniel Suess 说。

But 3 billion years ago, the nitrogen floodgates opened when nitrogenase evolved in early prokaryotes.

但30亿年前，当早期原核生物进化出固氮酶时，氮闸门打开了。

The enzyme accomplished what no other biological process could do: It broke the triple bond holding N 2 together and converted the inert gas into biologically useful ammonia.

这种酶完成了其他生物过程无法做到的事情：它打破了将 N 2 结合在一起的三键，并将惰性气体转化为生物上有用的氨。

The enzyme was effective but extraordinarily complicated.

这种酶很有效，但极其复杂。

That was beside the point to the early microbes that benefited from it, but it would come to matter enormously to the humans who, billions of years later, wanted to replicate its trick in order to make fertilizer.

对于从中受益的早期微生物来说，这不是重点，但对于数十亿年后想要复制其技巧来制造肥料的人类来说，它却意义重大。

Part of what makes nitrogenase so chemically difficult is its “active site” — a cluster of iron and molybdenum atoms called FeMo-co. Each iron atom carries four or five unpaired electrons whose behavior depends on that of the others.

固氮酶在化学上如此困难的部分原因在于它的“活性位点”——一簇铁和钼原子，称为 FeMo-co。每个铁原子携带四到五个不成对的电子，其行为取决于其他电子的行为。

In fact, FeMo-co is one of the most correlated systems in all of biology, and a prime example of what’s known as the electron correlation problem: Because its electrons can’t be treated independently, it’s extremely hard to determine properties of the overall system, such as its true electronic structure and energy.

事实上，FeMo-co 是所有生物学中相关性最高的系统之一，也是所谓电子相关问题的一个典型例子：因为它的电子无法独立处理，所以很难确定整个系统的属性，例如其真实的电子结构和能量。

For most of human history, the pressing question wasn’t how nitrogenase worked — it was how to get enough of what it produced.

在人类历史的大部分时间里，紧迫的问题不是固氮酶如何发挥作用，而是如何获得足够的固氮酶产生的物质。

As late as the 19th century, the most reliable source of usable nitrogen was guano harvested from islands off the coast of Peru, a resource so valuable and rare that nations went to war over it.

直到 19 世纪，最可靠的可用氮源是从秘鲁海岸附近的岛屿上收获的鸟粪，这种资源非常宝贵和稀有，以至于各国为争夺它而发动了战争。

Then the German chemists Fritz Haber and Carl Bosch cracked industrial nitrogen fixation in 1909, and the practical significance of the problem receded.

随后德国化学家弗里茨·哈伯（Fritz Haber）和卡尔·博世（Carl Bosch）于1909年破解了工业固氮，该问题的实际意义逐渐减弱。

The scientific one — understanding how nitrogenase, tucked inside an ordinary soil bacterium, accomplishes what the Haber-Bosch process requires an industrial furnace to do — remained open.

科学问题——了解隐藏在普通土壤细菌内部的固氮酶如何完成哈伯-博世过程需要工业炉才能完成的任务——仍然悬而未决。

It was an important question in its own right — and one that would achieve new prominence as people debated the best way to solve it.

这本身就是一个重要的问题——随着人们争论解决该问题的最佳方法，这个问题将获得新的关注。

A classical computer processes information as bits, which take one of two values: either 0 or 1.

经典计算机将信息处理为位，位取两个值之一：0 或 1。

A quantum computer instead uses qubits, which can exist in a superposition of 0 and 1 simultaneously and can become entangled with one another in ways that have no classical analogue.

相反，量子计算机使用量子位，量子位可以同时以 0 和 1 的叠加形式存在，并且可以以经典类似的方式相互纠缠。

That means that when (or if) a large-scale quantum computer exists, it will be able to explore many possible solutions to a problem at once, rather than grinding through them in sequence.

这意味着，当（或者如果）存在大型量子计算机时，它将能够同时探索问题的多种可能解决方案，而不是按顺序磨练它们。

The German chemist and engineer Carl Bosch developed the Haber-Bosch process, which converts atmospheric nitrogen to ammonia for use as a fertilizer.

德国化学家和工程师卡尔·博世开发了哈伯-博世工艺，将大气中的氮转化为氨用作肥料。

For certain kinds of problems with the right mathematical structure, this promises an exponential speedup over anything a classical machine could achieve.

对于具有正确数学结构的某些类型的问题，这有望比传统机器可以实现的任何目标实现指数级的加速。

The question, ever since quantum computing took off as a subject of theoretical study in the 1990s, has been which problems qualify.

自从量子计算在 20 世纪 90 年代成为理论研究的主题以来，问题就一直是哪些问题符合条件。

One of the most promising domains seems to be simulating chemical interactions: The electron interactions that govern how molecules behave are quantum mechanical at their core, which suggests that a quantum computer might be uniquely suited to modeling them.

最有前途的领域之一似乎是模拟化学相互作用：控制分子行为的电子相互作用的核心是量子力学，这表明量子计算机可能特别适合对它们进行建模。

The status of nitrogenase as an informal quantum computing benchmark traces back to a 2011 meeting Microsoft organized to explore applications for its nascent quantum computing group.

固氮酶作为非正式量子计算基准的地位可以追溯到 2011 年微软组织的一次会议，旨在探索其新生量子计算团队的应用程序。

Chan, who’d already been studying nitrogenase for more than a decade at the time, gave a talk on the enzyme.

陈当时已经研究固氮酶十多年了，他做了关于这种酶的演讲。

He doesn’t know to what extent that talk influenced later events, but in 2017, Microsoft researchers published a paper in the Proceedings of the National Academy of Sciences arguing that the entangled complexity of nitrogenase made it a compelling test for quantum computers .

他不知道这个言论在多大程度上影响了后来的事件，但 2017 年，微软研究人员在《美国国家科学院院刊》上发表了一篇论文，认为固氮酶的纠缠复杂性使其成为量子计算机引人注目的测试。

In Chan’s view, it was a strange fit from the start.

在陈看来，这从一开始就是一个奇怪的契合。

He disputed the claim, continuing to believe that it was possible to model nitrogenase using classical methods like the ones he’d spent his career developing.

他对这一说法提出异议，并继续相信可以使用经典方法（例如他在职业生涯中开发的方法）来模拟固氮酶。

Over the next decade, he would get to work proving it.

在接下来的十年里，他将开始努力证明这一点。

Chan and other researchers didn’t set out to explain how nitrogenase works end to end.

陈和其他研究人员并没有打算解释固氮酶是如何从头到尾地发挥作用的。

Rather, they turned to a widely used computational model of FeMo-co and asked a more preliminary question: What is its ground-state energy?

相反，他们转向广泛使用的 FeMo-co 计算模型，并提出了一个更初步的问题：它的基态能量是多少？

The ground state — FeMo-co’s lowest-energy electronic configuration — is the starting point for the whole reaction.

基态——FeMo-co 的最低能量电子构型——是整个反应的起点。

But FeMo-co contains a cluster of seven iron atoms, each with four or five unpaired electrons whose quantum “spins” can point up or down, whose orbitals can shift, and whose behavior depends on what the electrons around them are doing.

但 FeMo-co 包含七个铁原子簇，每个铁原子有四个或五个不成对的电子，其量子“自旋”可以向上或向下，其轨道可以移动，其行为取决于周围电子的行为。

This makes measuring FeMo-co’s ground-state energy extraordinarily complex.

这使得测量 FeMo-co 的基态能量变得异常复杂。

There are more than 78,000 plausible configurations the electrons might be in; the ground state is a superposition, or a sort of weighted combination, of all these configurations.

电子可能存在超过 78,000 种可能的构型；基态是所有这些配置的叠加或某种加权组合。

In principle, the Schrödinger equation tells you how all these different configurations contribute to the ground state and what its overall energy should be.

原则上，薛定谔方程告诉您所有这些不同的配置如何对基态做出贡献以及其总能量应该是多少。

But in practice, solving that equation directly and exactly for a system with as many interacting electrons as FeMo-co has is often impossible.

但在实践中，对于像 FeMo-co 这样具有如此多相互作用电子的系统，直接准确地求解该方程通常是不可能的。

This is true for both quantum and classical computers.

对于量子计算机和经典计算机来说都是如此。

In both cases, you have to start with a simpler approximation of the ground state’s basic structure — an educated guess, often reached only after years of research, about which configurations are contributing the most to the ground state.

在这两种情况下，你都必须从基态基本结构的更简单的近似开始——这是一种有根据的猜测，通常只有经过多年的研究才能得出，关于哪些配置对基态的贡献最大。

Fritz Haber (right) in his laboratory at the Kaiser Wilhelm Institute for Physical Chemistry in Berlin, alongside the chemical engineer Ladislaus Farkas.

弗里茨·哈伯（右）在柏林威廉皇帝物理化学研究所的实验室里，旁边是化学工程师拉迪斯劳斯·法卡斯。

Haber developed industrial processes for mass-producing both ammonia fertilizer and chemical weapons.

哈伯开发了大规模生产氨肥和化学武器的工业工艺。

Sueddeutsche Zeitung Photo/Alamy

南德意志报 照片/Alamy

Then, if you’re using a classical computer, you can try to progressively account for other configurations and show that you can safely ignore the huge number of remaining configurations because they don’t add much to the ground-state energy.

然后，如果您使用的是经典计算机，您可以尝试逐步考虑其他配置，并表明您可以安全地忽略大量剩余配置，因为它们不会给基态能量增加太多。

On the other hand, in theory, a quantum computer won’t require you to leave configurations out of your final estimate.

另一方面，理论上，量子计算机不会要求您将配置排除在最终估计之外。

Instead, the computer can represent your initial guess directly as a quantum state, and then evolve that state forward in time until it naturally reaches the right ground-state structure — allowing you to calculate the energy precisely.

相反，计算机可以将您最初的猜测直接表示为量子态，然后及时向前演化该状态，直到它自然达到正确的基态结构 - 让您能够精确计算能量。

Many researchers think quantum computers are at an advantage here, because the process of classically ruling out insignificant configurations can get prohibitively difficult.

许多研究人员认为量子计算机在这方面具有优势，因为传统上排除无关紧要的配置的过程可能会变得异常困难。

Chan and others, however, disagree.

然而，陈和其他人不同意。

For one thing, they argue , quantum computers still encounter the same bottleneck of needing that reasonable initial guess, and there’s no obvious reason why quantum methods should have any advantage at clearing that bottleneck.

一方面，他们认为，量子计算机仍然遇到同样的瓶颈，需要合理的初始猜测，并且没有明显的理由说明量子方法在消除该瓶颈方面应该具有任何优势。

Moreover, classical techniques have been rapidly maturing.

此外，经典技术已经迅速成熟。

But for Chan, asserting that quantum computers might not be needed after all was “like trying to resist the ocean tide,” he said.

但对于陈来说，断言可能根本不需要量子计算机“就像试图抵抗海潮一样”，他说。

Since receiving his doctorate from the University of Cambridge in 2000, Chan had been developing and refining ways to compress complicated quantum states by focusing only on their most important configurations.

自 2000 年获得剑桥大学博士学位以来，Chan 一直致力于开发和完善压缩复杂量子态的方法，只关注最重要的构型。

He and his team now hoped to apply these approaches to FeMo-co.

他和他的团队现在希望将这些方法应用于 FeMo-co。

They used two different techniques to winnow down the configurations they needed to look at.

他们使用两种不同的技术来筛选他们需要查看的配置。

Using one method, they started with their guess and incrementally adjusted the behavior of small numbers of electrons.

他们使用一种方法从猜测开始，逐步调整少量电子的行为。

They then showed that adjusting larger numbers of electrons didn’t lead to significant energy changes, giving them a clear recipe for which configurations they could ignore and which they couldn’t.

然后他们证明，调整更多的电子不会导致显着的能量变化，这为他们提供了一个清晰的方法，告诉他们哪些配置可以忽略，哪些配置不能忽略。

Their second method was the one that Chan had spent his career working on.

他们的第二种方法是陈毕生致力于研究的方法。

It involved breaking their initial state into pieces and allowing only a limited amount of information to flow between those pieces.

它涉及将它们的初始状态分成几部分，并只允许有限数量的信息在这些部分之间流动。

They then showed that they only needed to consider changes in that information flow up to a particular limit.

然后他们表明，他们只需要考虑信息流在特定限度内的变化。

“Realizing that the description could be achieved by ‘simpler’ methods and pushing these methods extremely hard (as the problem is still computationally challenging) was the key,” Chan wrote in an email.

“认识到可以通过‘更简单’的方法来实现描述，并极其努力地推动这些方法（因为问题在计算上仍然具有挑战性）是关键，”陈在一封电子邮件中写道。

Both methods produced the same energy estimate for FeMo-co’s ground state (and matched what scientists had observed experimentally), giving the researchers confidence that they had found the true ground state.

这两种方法对 FeMo-co 的基态产生了相同的能量估计（并且与科学家在实验中观察到的结果相匹配），使研究人员相信他们已经找到了真正的基态。

Chan hopes that the technical breakthroughs his team made can now be extended to model the full nitrogenase enzyme and its reaction.

陈希望他的团队取得的技术突破现在可以扩展到模拟完整的固氮酶及其反应。

“My hope is that all these people advocating ‘We need to build a quantum computer to solve the nitrogenase problem’ will join this mission now that we have a route to doing it,” he said.

他说：“我希望所有这些主张‘我们需要建造一台量子计算机来解决固氮酶问题’的人都会加入到这一使命中，因为我们已经找到了实现这一目标的途径。”

But getting from the ground state to a full mathematical description of the reaction will be far more difficult, involving calculating energies for a whole sequence of intermediate chemical states.

但从基态到反应的完整数学描述将会困难得多，其中涉及计算整个中间化学态序列的能量。

“We’re not even close to achieving the holy grail of this,” Suess said.

“我们距离实现这一目标还差得很远，”苏斯说。

“We’ve still just described the resting state.

“我们刚刚描述了静息状态。

But the method is promising in that it suggests we can proceed with some confidence.”

但这种方法很有希望，因为它表明我们可以充满信心地继续进行。”

It’s also unclear what the result might mean for researchers’ hopes for quantum computing.

目前还不清楚这一结果对研究人员对量子计算的希望意味着什么。

Whitfield argues that calculating a single ground-state energy value was never where quantum computers were expected to best their classical counterparts.

惠特菲尔德认为，计算单一基态能量值从来都不是量子计算机有望超越经典计算机的地方。

Their likely advantage, he said, instead lies in that next question on the table: modeling how the system evolves over time.

他说，他们可能的优势在于摆在桌面上的下一个问题：对系统如何随时间演变进行建模。

That’s likely to showcase how inefficient classical methods can get — and how much more powerful quantum computers can be.

这可能会展示经典方法的效率有多低，以及量子计算机的功能有多强大。

After years of friendly sparring with the quantum computing community, Chan does not expect the new result to change many minds.

经过与量子计算界多年的友好争论，陈并不认为新结果会改变许多人的想法。

After all, he said, quantum chemistry simulation via quantum computers still holds great promise: If a quantum computer were to become available tomorrow, he would gladly use it.

毕竟，他说，通过量子计算机进行量子化学模拟仍然具有巨大的前景：如果明天就有一台量子计算机可用，他会很乐意使用它。

But he hopes his team’s new result will help correct the misconception that the hardest chemical problems are simply out of reach until quantum hardware arrives.

但他希望他的团队的新结果将有助于纠正这样一种误解：在量子硬件出现之前，最困难的化学问题根本无法解决。

“Science is self-correcting,” he wrote in an email, “but quite often, the corrections do not receive the same attention as the initial claim, because the field has moved on to other claims.”

“科学是自我修正的，”他在一封电子邮件中写道，“但很多时候，修正并没有像最初的主张那样受到同样的关注，因为该领域已经转向其他主张。”

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获取发送到您的电子邮件收件箱的最重要新闻的摘要

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《Quanta》杂志对评论进行审核，以促进知情、实质性、文明的对话。

Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected.

辱骂、亵渎、自我推销、误导、语无伦次或偏离主题的评论将被拒绝。

Moderators are staffed during regular business hours (New York time) and can only accept comments written in English.

版主在正常工作时间（纽约时间）有工作人员，并且只能接受英文评论。