Quantum ‘Jamming’ Could Help Unlock the Mysteries of Causality

量子“干扰”或有助于揭开因果关系的奥秘

For the past few decades, researchers have understood that quantum computers should eventually be able to crack the widely used codes that secure much of the digital world. To protect against this fate, they’ve spent years developing new codes that appear to be safe from future safecrackers armed with quantum computers. 在过去的几十年里，研究人员已经意识到，量子计算机终将能够破解目前保护数字世界大部分安全的主流加密算法。为了防患于未然，他们花费多年时间开发了新的代码，旨在抵御未来配备量子计算机的“保险箱破解者”。

At the same time, they’ve also devised ingenious ways to use the rules of quantum mechanics to keep communications secure. But quantum mechanics, just like the “classical” mechanics that preceded it, is just a theory of nature. What if it eventually gets superseded by a fuller theory, just as quantum mechanics supplanted Newtonian physics a century ago? Will these quantum communication techniques still be secure in a world where there’s an even more fundamental set of rules? 与此同时，他们还设计了巧妙的方法，利用量子力学的规则来保障通信安全。然而，量子力学就像它之前的“经典”力学一样，仅仅是一种自然理论。如果它最终被一个更完备的理论所取代，就像一个世纪前量子力学取代牛顿物理学那样，又会怎样呢？在一个存在更基本规则的世界里，这些量子通信技术还能保持安全吗？

“In terms of these cryptographic protocols, it’s good to be paranoid,” said Ravishankar Ramanathan, a quantum information theorist at the University of Hong Kong who works on quantum cryptography. “Let’s try to minimize the assumptions behind the protocol. Let’s suppose that at some future date people realize that quantum mechanics is not the ultimate theory of nature.” “就这些加密协议而言，保持偏执是件好事，”香港大学研究量子密码学的量子信息理论家 Ravishankar Ramanathan 表示，“让我们尽量减少协议背后的假设。假设在未来的某一天，人们意识到量子力学并非自然的终极理论。”

It’s a possibility worth considering. The difficulty of outstanding problems—like reconciling quantum mechanics and gravity—suggests that a post-quantum theory of nature might involve something quite unexpected. 这是一种值得考虑的可能性。一些悬而未决的难题（例如调和量子力学与引力）的难度表明，后量子时代的自然理论可能涉及一些完全出乎意料的内容。

To guard against the possibility that their protocols are based on faulty assumptions, some quantum cryptographers search for even more basic principles to build upon. Instead of starting from quantum mechanics, they dig deeper, down to the very concept of causality. 为了防范协议可能基于错误假设的风险，一些量子密码学家开始寻找更基础的原则作为构建基石。他们不再从量子力学出发，而是深入挖掘，直抵因果关系这一核心概念。

A Subtle Sabotage

微妙的破坏

One way to understand developments in this area is to consider quantum key distribution, which involves taking advantage of the rules of quantum mechanics to pass along a key—something that can be used to decode a secret message—in a way that cannot be covertly tampered with. Quantum key distribution makes use of quantum entanglement, which locks two particles together through one of their properties, like spin. Quantum entanglement contains something of a trip wire. If anyone tries to mess with the entanglement—as they would if they tried to steal the key—the intrusion will destroy the entanglement, revealing the sabotage. This is because of a fundamental quantum mechanical principle called the “monogamy of entanglement.” 理解这一领域进展的一种方法是考虑量子密钥分发（QKD）。它利用量子力学规则来传递密钥（用于解码秘密信息的工具），并确保其无法被秘密篡改。量子密钥分发利用了量子纠缠，通过自旋等属性将两个粒子锁定在一起。量子纠缠包含一种类似“绊线”的机制：如果有人试图干扰纠缠（例如试图窃取密钥），这种入侵会破坏纠缠状态，从而暴露破坏行为。这是基于量子力学中一个被称为“纠缠单配性”（monogamy of entanglement）的基本原理。

But what if this principle no longer held? In such a case, if the people passing the message did not have complete control of their devices, an outsider could potentially subtly change the particles’ entanglement, disrupting the communication without leaving a trace. 但如果这一原则不再成立呢？在这种情况下，如果传递信息的人无法完全控制他们的设备，外部人员就有可能微妙地改变粒子的纠缠状态，从而在不留痕迹的情况下干扰通信。

This process is called quantum jamming, and efforts to understand it have surged in recent years. 这一过程被称为量子干扰（quantum jamming），近年来，对其进行研究的努力激增。

For many scientists, jamming is appealing because it can help them better understand both quantum mechanics and the nature of cause and effect. They wonder: Are there deep principles that forbid jamming, that make it impossible? Or, if no principle forbids it, could jamming occur in the real world? 对于许多科学家来说，干扰现象之所以具有吸引力，是因为它能帮助他们更好地理解量子力学以及因果关系的本质。他们不禁思考：是否存在某种深层原则禁止干扰，使其成为不可能？或者，如果没有原则禁止它，干扰是否会在现实世界中发生？

Jim the Jammer

干扰者吉姆

Michał Eckstein, a theoretical physicist at the Jagiellonian University in Krakow, Poland, likes to illustrate jamming with a story. Its protagonists are the classic characters from explanations of quantum mechanics, Alice and Bob. 波兰克拉科夫雅盖隆大学的理论物理学家 Michał Eckstein 喜欢用一个故事来阐述干扰。故事的主角是量子力学解释中的经典人物：爱丽丝（Alice）和鲍勃（Bob）。

“Suppose you have Alice and Bob, and they meet a magician, Jim the Jammer,” Eckstein said. “The magician says, ‘I have two balls; one is white, and one is black.’” “假设爱丽丝和鲍勃遇到了一位魔术师，‘干扰者吉姆’，”Eckstein 说，“魔术师说：‘我有两个球，一个是白色的，一个是黑色的。’”

The balls stand in for a pair of entangled particles. If two particles are entangled, they have a property that is linked in some way—if you measure the first particle and find that its spin is up, for example, the other particle’s spin will inevitably be down, and vice versa. This holds true even if the other particle is halfway across the universe. Here the balls are linked such that if one is white, the other will always be black. 这些球代表了一对纠缠粒子。如果两个粒子处于纠缠状态，它们的某种属性就会以某种方式关联——例如，如果你测量第一个粒子并发现其自旋向上，那么另一个粒子的自旋必然向下，反之亦然。即使另一个粒子在宇宙的另一端，这一结论依然成立。在这里，球的关联方式是：如果一个是白色的，另一个必然是黑色的。

In the classic trope of stage magic, Jim lets members of the audience see the balls get placed into two boxes, mixed up, and given to Alice and Bob. No one, at this point, knows which ball is in which box. 在经典的舞台魔术桥段中，吉姆让观众看到球被放入两个盒子里，混淆后分别交给爱丽丝和鲍勃。此时，没有人知道哪个盒子里装的是哪个球。

Then Alice and Bob get into rocket ships that fly off in opposite directions at close to the speed of light. After a while, Alice opens her box, and Bob opens his. But in the meantime, Jim has performed a trick, and the balls have changed. 随后，爱丽丝和鲍勃乘坐火箭以接近光速的速度向相反方向飞去。过了一会儿，爱丽丝打开了她的盒子，鲍勃也打开了他的。但在那期间，吉姆施展了戏法，球已经发生了变化。

At first, neither Alice nor Bob notices Jim’s interference. Each expects to have a 50 percent chance of seeing a white or black ball, and when each opens up their box, the ball is either white or black. Nothing Jim does can change that. 起初，爱丽丝和鲍勃都没有察觉到吉姆的干扰。每个人都预期有 50% 的概率看到白球或黑球，当他们打开盒子时，球确实是白色或黑色的。吉姆所做的一切都无法改变这一点。

When Alice and Bob meet back on Earth, though, the magician’s trick is revealed. When Alice and Bob compare their measurements, they find that the balls are the same color. Jim has shifted the nature of the balls’ entanglement—from being opposite colors to being perfect matches. 然而，当爱丽丝和鲍勃回到地球重逢时，魔术师的戏法被揭穿了。当他们对比测量结果时，发现球的颜色竟然相同。吉姆改变了球的纠缠本质——从原本的颜色相反变成了完全匹配。

That’s the basic idea, though in reality the process of quantum jamming is a little more complicated. 这就是基本概念，尽管在现实中，量子干扰的过程要复杂得多。

In the mid-1990s, Jacob Grunhaus, Sandu Popescu, and Daniel Rohrlich were exploring just how far a theory could go beyond the rules of quantum mechanics while still respecting a core principle of Einstein’s: You can’t send information faster than the speed of light. Einstein’s mid-century thought experiments showed that without this “no-signaling” principle, the very notion of cause and effect would start to fray. Since then, the no-signaling principle has become a core assumption when physicists consider what might lie beyond quantum mechanics. “When we work in quantum foundations, what we take very seriously is the no-signaling principle,” said Mirjam Weilenmann of the French national research institute Inria. 20 世纪 90 年代中期，Jacob Grunhaus、Sandu Popescu 和 Daniel Rohrlich 探索了一个理论在超越量子力学规则的同时，还能在多大程度上遵循爱因斯坦的核心原则：即信息传递速度不能超过光速。爱因斯坦在 20 世纪中叶的思想实验表明，如果没有这个“无信号传输”（no-signaling）原则，因果关系的概念就会开始瓦解。从那时起，当物理学家思考量子力学之外可能存在什么时，“无信号传输”原则就成为了一个核心假设。法国国家信息与自动化研究所（Inria）的 Mirjam Weilenmann 表示：“当我们从事量子基础研究时，我们非常重视无信号传输原则。”

Grunhaus, Popescu, and Rohrlich imagined jamming as a kind of super-entanglement that could interfere with entangled particles. Just as you could use a measuring device to determine the fate of a distant entangled particle… Grunhaus、Popescu 和 Rohrlich 将干扰想象成一种可以干预纠缠粒子的“超纠缠”。正如你可以使用测量设备来决定遥远纠缠粒子的命运一样……