The quantum key to unbreakable cryptography

The solution to secure communications may lie in the phenomenon derided by Einstein as 'spooky action at a distance'. Cathal O'Connell reports.

A forest of optical elements to guide light to each diamond. – Hanson lab@TUDelft

Quantum teleportation has arrived but it has nothing to do with beaming up Star Fleet officers. Instead, a team of Dutch physicists has exploited the tricks of quantum physics to reliably teleport data from one side of their lab to the other. Extending the range should be a simple next step, bringing closer the day of un-crackable quantum hotlines encrypting online data transfers for everything from finances to intelligence gathering.

“This is ultimately about the secure and reliable transfer of quantum information,” says Wolfgang Pfaff, the lead student author of the study, which was published in Science this month.

Quantum information, such as the orientation of an electron’s “spin”, is the quantum version of the 1s and 0s of modern computing. Some scientists are trying to use the strange properties of quantum systems to make powerful computers (See Quantum computing? Yes, no and maybe?). Others, including Pfaff and his colleagues, are developing advanced communication systems. One of the strange outcomes of quantum physics is that quantum information can be transmitted from place to place, without travelling through the space in between.

“You make the information disappear on the source side and reappear on the receiver’s side,” says Pfaff. “At no point in time is this information somewhere in between the sender and the receiver.”

The Dutch team’s teleportation system is based on a pair of trapped electrons, codenamed “Alice” and “Bob”, each housed inside its own diamond crystal at opposite ends of the lab. Using a beam of light travelling through a maze of mirrors, the team “entangles” the two electrons, forming a quantum link between them.

If any third party tries to intercept the signal, the message is instantly destroyed.

Entanglement is the idea Einstein famously derided as “spooky action at a distance”. His problem, not surprisingly, was that it appeared to violate basic physics.

But Einstein’s objections were put to bed more than 30 years ago when entanglement was experimentally proven, and it is now an accepted phenomenon with effects measured to be at least 10,000 times faster than the speed of light.

After enabling the quantum link, the Dutch team was able to encrypt a message using the spin of the Alice electron as the encryption key. The message was then transmitted via a conventional communications channel such as a telephone line.

The clever thing is that the encryption can only ever be decoded by Alice’s partner electron, Bob. The quantum link between them is a unique security key. If any third party tries to intercept the signal, the message is instantly destroyed.

Quantum teleportation has been achieved before using photons – particles of light. The major advance in this experiment is to achieve it with electrons – particles of matter. As electrons can be trapped at specific locations within their diamond crystals, they can be manipulated and read with relative ease. Particles of light are far harder to work with because they are in constant, very fast motion.

It’s a significant step forward, says Andrea Morello, who researches quantum computation and communication at the University of New South Wales. “It shows that you can teleport quantum information from one piece of solid material to another three metres away. There's no real limit to that, it could be further without much difficulty, so it's really impressive.”

The technology provides the great new hope for encrypting private data. Google, for instance, is already investing heavily in quantum communications, as are some of the world’s major intelligence agencies.

The obvious next step for this research, says Pfaff, is to extend the range of the teleportation to kilometres. That should be achievable simply by replacing the maze of mirrors with a fibre optic cable to transmit the light beams used to set up the entanglement of Bob and Alice. The group already has plans to test their “spooky” quantum hotline over 1,300 metres.

“It’s not like the applications are far in the future,” says Morello.

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