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 What if it eventually gets superseded by a fuller theory, just as quantum mechanics supplanted Newtonian physics a century ago?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

A Subtle Sabotage

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?

Jim the Jammer

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.