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 backing up qubits, the basic units of quantum computers, appears to allow a circumvention of this fundamental law of physics.

The no-cloning theorem was first discovered by scientists in the 1980s. It states that quantum states that describe all information about a system cannot be copied. Attempting to measure the information in order 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 that prevent information from being copied and hacked.

Achim Kempf at the University of Waterloo in Canada and Koji Yamaguchi at Kyushu University in Japan have now shown that a quantum system can indeed be cloned, as long as the information about it is encrypted and enclosed with a special, one-time decryption key.

“You can make many copies and generate redundancy this way, 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 only ever be a clear, obvious, readable, unencrypted copy of a qubit.”

Kempf and Yamaguchi came to this surprising conclusion after working on a seemingly unrelated problem—how a quantum Wi-Fi or radio station might work. This is something that is impossible under the traditional no-cloning theorem because multiple receivers will receive the same identical quantum information.

But when the pair looked at how random fluctuations, or noise, would affect the copies of information that receivers see, they realized their system could work. “We thought, what the hell? Why does quantum noise seem to mess with the no-cloning theorem?”

After analyzing the problem more closely, they realized that the noise acted as an effective encryption mechanism, distorting the original message, but in a way that could be reversed. If this was done on purpose, it could be exploited as a tool.

Once they had proven this result theoretically, the pair and other colleagues showed that this protocol could work on a real IBM Heron 156-qubit quantum computer processor.

Because the technique is fairly resistant to the noise and errors that are ubiquitous in today’s quantum computers, the team found that they could create hundreds of encrypted clones of single qubits by repeating the process over and over. “In fact, we ran out of real estate on the IBM processor. It only contains 156 qubits, but we estimated that we can do more than 1,000 encrypted clones before (the bugs) stop us.”

This modification of the no-cloning theorem could have applications for a quantum cloud storage or computing service, says Kempf. “If you send a file to Dropbox, it will save your data at least three times on three different computers that are geographically separated, so if one is hit by a fire, the other by a flood, there’s a good chance the third will survive,” says Kempf. “It used to be thought that you can’t do that with quantum information, because you can’t clone it. But what we showed is that you can.”

“It’s an interesting quantum cryptographic protocol,” says Aleks Kissinger of the University of Oxford, and may have applications in quantum communications where you need some redundancy in the information being transmitted. However, it does not affect the original non-cloning theorem because the method of Kempf and his team is clearly not cloning, he says. “It’s not so much cloning as sort of spreading the (quantum) state to many other parties, in such a way that one of those parties can later get it back,” Kissinger says. “It’s a clever trick, but I personally 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.”