Original version With this story appeared in Quanta Magazine.
Over the past few decades, researchers have realized that quantum computers should eventually be able to do this crack commonly used codes that secure much of the digital world. To protect themselves from this fate, they have spent years developing novel codes that appear to be like this safe from future burglars armed with quantum computers.
At the same time they also invented inventive ways utilize the principles of quantum mechanics to ensure communication security. But quantum mechanics, like the “classical” mechanics that preceded it, is only a theory of nature. What if it is eventually replaced by a more complete theory, just as quantum mechanics replaced Newtonian physics a century ago? Will these quantum communication techniques still be safe and sound in a world with an even more basic set of rules?
“When it comes to cryptographic protocols, it’s good to be paranoid,” he said Ravishankara Ramanathanquantum information theorist at the University of Hong Kong who works in quantum cryptography. “Let’s try to minimize the assumptions of the protocol. Suppose that in some future people realize that quantum mechanics is not the final theory of nature.”
This is 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 guard against the possibility that their protocols are based on faulty assumptions, some quantum cryptographers are looking for even more fundamental principles to build on. Instead of starting with quantum mechanics, they go deeper, to the very concept of causality.
Subtle sabotage
One way to understand developments in this field is to consider quantum key distribution, which involves using the principles of quantum mechanics to convey a key – something that can be used to decrypt a secret message – in a way that cannot be secretly tampered with. Quantum key distribution uses quantum entanglement, which binds two particles together through one of their properties, such as spin. Quantum entanglement involves something like a trigger wire. If someone tries to tamper with the entanglement – as they would if they were trying to steal a key – the intrusion will destroy the entanglement, revealing the sabotage. This is because of a fundamental principle of quantum mechanics called “entanglement monogamy.”
But what if this rule no longer applies? In this case, if the people transmitting the message did not have full control over their devices, an outsider could potentially subtly change the entanglement of the particles, disrupting the communication without leaving a trace.
This process is called quantum jamming, and efforts to understand it have increased in recent years.
For many scientists, jamming is attractive because it can lend a hand them better understand both quantum mechanics and the nature of cause and effect. They wonder: Are there deep rules against jamming that make it impossible? Or, if no rule prohibits it, could jamming occur in the real world?
Jim Jammer
Michael Ecksteintheoretical physicist from the Jagiellonian University in Krakow, likes to illustrate jamming with a story. Its heroes are the classic characters from explanations of quantum mechanics, Alice and Bob.
“Let’s say you have Alice and Bob and they meet the magician, Jim Jammer,” Eckstein said. “The magician says: ‘I have two balls, one is white and the other is black’.”
The balls replace a pair of entangled particles. If two particles are entangled, they have a property that is related in some way – if, for example, you measure the first particle and find its spin is up, the second particle’s spin will inevitably be down, and vice versa. This happens even when the second particle is halfway across the universe. Here the balls are connected in such a way that if one is white, the other will always be black.