Triple entanglement paves way for quantum encryption


European scientists put three photons in a 3-D 'twist' – a feat so far only predicted possible on paper. Viviane Richter reports.


Long-exposure photo of laser beams with a twisted wavefront. The beams have holes in the middle due to destructive interference at the center of the twists – FACULTY OF PHYSICS, UNIVERSITY OF VIENNA

Just when you thought they couldn’t be more cryptic, physicists have designed a method for further encrypting quantum particles.

A team of scientists from the University of Vienna and the Universitat Autonoma de Barcelona entangled three photons in a 3-D “twist” in the lab – a feat so far only predicted possible on paper. Their method could be applied to quantum computing, encryption – even a quantum internet.

Since the 1970s, scientists have been working to exploit the uncanny properties of quantum theory to transmit information along an uncrackable quantum hotline.

This potential comes from the ability of photons to be entangled – meaning the measurement of one photon instantaneously influences another, no matter how far apart it is.

Theoretically entanglement could be used to code sensitive information with a quantum key, since any attempt to tamper with the link would immediately destroy the entanglement.

Physicists have managed to entangle many particles, including more than 100,000 photons. But so far the link was only symmetric – meaning it would only be useful for simple, two-party communication.

But now, the Austrian and Spanish team have produced an “asymmetric” entanglement. This required entangling three photons in three dimensions, where all three photons are entangled, but two of them on an extra layer.

In regular “two-dimensional” entanglement, the photons are coded using their direction of polarisation (whether they wave “up and down” or “side to side”). But the team were able to add an extra layer of communication between two of the photons by adding a twist. This means the photons corkscrew through the air, rather than simply waving up and down.

Crucially, the asymmetry allows for different layers of information to be shared among multiple parties. For example, you could log in to internet banking on your mobile without the phone network getting a sniff at your quantum security key.

“This type of asymmetric quantum entanglement has been predicted before on paper, but we are the first to actually create it in the lab,” study author Mehul Malik said.

Using their system, the team has already developed a quantum cryptographic protocol, which could one day allow quantum communication to pass information along photons in different layers.

“The experiment opens the door for a future quantum internet with more than two partners and it allows them to communicate more than one bit per photon,” senior author Anton Zeilinger explained.

For now, we’re just waiting for the first quantum computer that can compose an email.

The method was published in Nature Photonics.

For a primer on quantum theory see Quantum physics for the treminally confused.

  1. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.109.150502
  2. http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2016.12.html
  3. http://cosmosmagazine.com/physical-sciences/quantum-physics-terminally-confused
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