Quantum ‘Jamming’ Could Help Unlock the Mysteries of Causality

The original version of this story appeared in Quanta Magazine.

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.

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 What if it eventually gets superseded by a fuller theory, just as quantum mechanics supplanted Newtonian physics a century ago?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?

“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.”

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.

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.”

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?

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.

“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 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.

That’s the basic idea, though in reality the process of quantum jamming is a little more complicated.

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.

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, you could use a hypothetical jamming device to change the correlation between a pair of distant entangled particles. If this jamming procedure obeyed a few key rules, some physicists argue, it would secretly disrupt quantum entanglement without disrupting causality.

The idea of quantum jamming is so strange that initially physicists didn’t know quite what to do with it. “We wrote that paper and that was the end of it,” Popescu said.

Twenty years later, the time was right to explore it further.

Quantum cryptography had grown, as quantum computers went from theoretical ideas to experiments in the real world. In the first decade of the 2000s, several groups developed device-independent quantum key distribution, a quantum cryptography procedure that depends on the monogamy of entanglement.

In 2016, Ramanathan and Paweł Horodecki were thinking about these protocols when they found the paper by Grunhaus, Popescu, and Rohrlich. “We started to realize that this property of monogamy, upon which all of device-independent cryptography is based, completely fails once you start to allow these types of jamming correlations,” Ramanathan said.

Soon, jamming was the subject of vigorous discussion. Many researchers felt the thought experiment was missing something important: While jamming can’t be used to send signals faster than light, influencing the state of a distant quantum particle still feels like the kind of “spooky action at a distance” that long ago tormented Einstein.

But for some researchers, the discomfort that quantum jamming creates is inspiring new ideas. “I see it as a tool to try to help hone our intuitions of what the right definition of causation is,” said Roger Colbeck, who proposed one of the first protocols for device-independent cryptography in his 2006 doctoral thesis.

Now at King’s College London, Colbeck is working with V. Vilasini at Inria research center at the University of Grenoble Alpes to classify the way cause and effect work in different theories. For them, jamming serves as a useful edge case. They’re seeking another fundamental principle, like the no-signaling principle, that explains which rules jamming breaks.

The groups of Ramanathan and Horodecki responded to this work, as well as a recent paper by Weilenmann, in a preprint in December 2025 that they wrote with Eckstein, Tomasz Miller, and Paweł Horodecki’s father, Ryszard. Now, the researchers are in conversation, trying to clarify terms, fix misunderstandings, and look for the fundamental principles behind physical theories.

“That’s for me the most interesting question,” Eckstein said. “Is there any new physics behind it? Can physics include such phenomena?”

Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.