Loophole found that makes quantum cloning possible

In quantum mechanics, the idea that quantum information cannot be duplicated is ironclad – or at least, it was. A surprising approach to saving qubits, the basic units of quantum computers, seems to make it possible to circumvent this fundamental law of physics.

The no-cloning theorem was first discovered by researchers in the 1980s. It says that quantum states that describe all the information about a system cannot be copied. Attempting to measure information to copy it would simply destroy the delicate quantum properties you wish to measure. This fact has proven important for quantum technologies such as encryption, leading to simple protocols preventing information from being copied and hacked.

Achim Kempf of the University of Waterloo in Canada and his colleagues have shown that a quantum system can in fact be cloned, provided that the information about it is encrypted and surrounded by a special, unique decryption key.

“This way you can make a lot of copies and generate redundancy, but you have to encrypt the copies and the decryption key can only be used once,” says Kempf. “This makes it compatible with a no-cloning theorem, because it says that there can be at most one clear, obvious, readable, unencrypted copy of a qubit.”

Kempf and his team came to this surprising conclusion after working on a seemingly unrelated problem: how a Wi-Fi network or quantum radio station works. This is something that is impossible according to the traditional no-cloning theorem, because multiple receptors would obtain the same identical quantum information.

But when Kempf and his colleagues studied the impact of random fluctuations, or noise, on the copies of information seen by receivers, they realized their system could work. “We asked ourselves: What is it? Why does quantum noise seem to disrupt the no-cloning theorem?”

After analyzing the problem more carefully, Kempf and his team realized that the noise acted as an effective encryption mechanism, truncating the original message, but in a way that could be reversed. If done intentionally, it could be exploited as a tool.

Once this result was proven theoretically, the team then showed that this protocol could run on a real 156-qubit IBM Heron quantum computing processor.

Because the technique is quite resilient to the noise and errors that are ubiquitous in today’s quantum computers, Kempf and his team found that they could create hundreds of encrypted clones of unique qubits, repeating the process over and over again. “We actually ran out of space on the IBM processor. It only has 156 qubits but we estimated that we could make over 1,000 encrypted clones before the IBM processor launched.” [errors] make us stop.

This modification of the no-cloning theorem could be useful to a quantum cloud storage or computing service, Kempf says. “If you send a file to Dropbox, your data will be backed up at least three times on three different computers that are geographically separated, so that if one is hit by a fire, the other by a flood, there’s a good chance that the third will survive,” says Kempf. “We used to think you couldn’t do this with quantum information, because you couldn’t clone it. But what we’ve shown is that it’s possible.”

“It’s an interesting quantum cryptographic protocol,” says Aleks Kissinger of the University of Oxford, and it could be used in quantum communication where some redundancy is needed in the information transmitted. However, this doesn’t affect the original no-cloning theorem, because Kempf and his team’s method is obviously not cloning, he says. “It is not so much a question of cloning as of a sort of diffusion of [quantum] “It’s a clever trick, but personally I wouldn’t call it cloning.”

Kempf agrees. “It’s not cloning. It’s encrypted cloning,” he says. “It’s just a refinement of the no-cloning theorem.”