Quantum ‘noise’ could help unravel the secrets of causality

The original version This story appeared in Quanta Magazine.

Over the past few decades, researchers have realized that quantum computers should eventually be able to break the widely used codes that secure much of the digital world. To protect against this fate, they spent years developing new codes that appeared safe from future vault hackers armed with quantum computers.

At the same time, they have also devised ingenious ways to use the rules of quantum mechanics to keep communications secure. But quantum mechanics, like “classical” mechanics before it, is only a theory of nature. What if it is eventually replaced by a fuller theory, just as quantum mechanics replaced Newtonian physics a century ago? Would these quantum communication techniques still be safe in a world with a 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 reduce the assumptions behind the protocol. Suppose that at some point in the future people realize that quantum mechanics is not the final theory of nature.”

It’s a possibility worth considering. The difficulty of outstanding problems – such as reconciling quantum mechanics and gravity – suggests that a post-quantum theory of nature may involve something completely unexpected.

To protect against the possibility that their protocols are built on faulty assumptions, some quantum cryptographers are looking for more fundamental principles to build on. Instead of starting from quantum mechanics, they delved deeper, all the way to the concept of causality itself.

Hidden sabotage

One way to understand developments in this field 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 decrypt a secret message—in such a way that it cannot be secretly tampered with. The quantum key distribution uses quantum entanglement, which locks two particles together through one of their properties, such as spin. Quantum entanglement has something like a tripwire. If anyone tried to tamper with the interlock—as they would if they tried to steal the key—the break-in would destroy the interlock and reveal the sabotage. This is due to a fundamental principle in quantum mechanics called “monogamy.”

But what if this principle no longer applies? In such a case, if the people passing the message do not have complete control over their devices, it is possible that an outsider could subtly alter the entanglement of the particles, disrupting the communication without leaving a trace.

This process is called quantum distortion, and efforts to understand it have been increasing in recent years.

For many scientists, perturbation is attractive because it can help them better understand quantum mechanics and the nature of cause and effect. They wonder: Are there deep principles that prevent confusion and make it impossible? Or if there is no principle against it, can interference occur in the real world?

C. jamming

Michal Eckstein, a theoretical physicist at Jagiellonian University in Krakow, Poland, likes to clear up the confusion with a story. Its heroes are the classic figures from interpretations of quantum mechanics, Alice and Bob.

“Let’s say you have Alice and Bob, and they meet the magician, Jim the Jammer,” Eckstein said. “The magician said, ‘I have two balls, one white and the other black.’”

The balls represent a pair of entangled particles. If two particles are entangled, they have a property that is related in some way – if you measure the first particle and find that its spin is up, for example, then the other particle’s spin will inevitably be down, and vice versa. This is true even if the other particle is halfway across the universe. Here the balls are connected so that if one is white, the other will always be black.