【AI前沿】Quantum ‘Jamming’ Could Help Unlock the Mysteries of Causality

Matt von HippelScienceMay 23, 2026 7:00 AMQuantum ‘Jamming’ Could Help Unlock the Mysteries of CausalityTo keep communications secure in a post-quantum world, cryptographers are digging down into the concept of cause and effect.Courtesy of Samuel Velasco/Quanta MagazineCommentLoaderSave StorySave this storyCommentLoaderSave StorySave this storyThe original versionofthis storyappeared inQuanta Magazine.For the past few decades, researchers have understood that quantum computers should eventually be able tocrack the widely used codesthat secure much of the digital world. To protect against this fate, they’ve spent years developing new codes that appear to besafe from future safecrackersarmed with quantum computers.At the same time, they’ve also devisedingenious waysto 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 What if it eventually gets superseded by a fuller theory, just as quantum mechanics supplanted Newtonian physics a century ago?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?“In terms of these cryptographic protocols, it’s good to be paranoid,” saidRavishankar 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.”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.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.A Subtle SabotageOne 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.”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.This process is called quantum jamming, and efforts to understand it have surged in recent years.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?Jim the JammerMichał 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.“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.’”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.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.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.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.When Alice and