A loophole has been found that allows for quantum cloning

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 circumvent this fundamental law of physics.

The no-cloning theorem was first discovered by researchers in the 1980s. He says that the quantum states that describe all the information about the system cannot be copied. Trying to measure information in order to copy it would simply destroy the subtle quantum properties you want to measure. This fact has proven important for quantum technologies such as encryption, leading to simple protocols that prevent copying and hacking of information.

Achim Kempf at the University of Waterloo in Canada and his colleagues have now shown that a quantum system can actually be cloned if its information is encrypted and sealed with a special one-time 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 the no-cloning theorem because it says that there can always 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 quantum Wi-Fi or a radio station might work. This is something that is impossible according to the traditional no-cloning theorem, since multiple receivers would receive the same identical quantum information.

But when Kempf and his colleagues looked at how random fluctuations, or noise, would affect the copies of information the receivers see, they realized their system might work. “We thought, what the hell is this? Why does quantum noise seem to contradict the no-cloning theorem?”

After analyzing the problem more thoroughly, Kempf and his team realized that noise acts as an effective encryption mechanism that distorts the original message, but in a way that can be reversed. If done on purpose, it could be misused as a tool.

Once they proved this theoretical result, the team showed that the protocol could work on an actual 156-qubit IBM Heron quantum computing processor.

Because the technique is relatively robust 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 individual qubits by repeating the process over and over. “We actually ran out of real estate on the IBM processor. It only has 156 qubits, but we estimated that we could make over 1,000 encrypted clones before [errors] make us stop.”

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

“It’s an interesting quantum cryptographic protocol,” he says Alex Kissinger at the University of Oxford and could have applications in quantum communication where you need some redundancy in the information being transmitted. But it doesn’t affect the original no-cloning theorem because Kempf and his team’s method clearly doesn’t clone, he says. “It’s not so much cloning as it is a kind of propagation [quantum] to tell many other parties in such a way that any one of those parties can take it back later,” 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. “That’s just a refinement of the no-cloning theorem.

topics:

• quantum mechanics/
• quantum computing