Triple entanglement feat adds new twist to quantum cryptography

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Scientists have added an extra dimension to the attributes of quantum entangled photons that enables the simultaneous and secure transmission of extra layers of data(Credit: Faculty of Physics, University of Vienna)

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In the world of quantum mechanics, entanglement is a weird realm where particles that were once joined exhibit mirror-opposite reactions when separated even when they are vast distances apart. Now researchers from the University of Vienna and the Universitat Autonoma de Barcelona have added a new twist to this phenomenon, by entangling three photons and adding a 3-D corkscrew motion to effectively allow multiple recipients to simultaneously receive information securely encoded in the one transmission.

Encoding entangled photons with particular spin states is a technique used in quantum cryptography to ensure that transmitted data arrives at its destination without being intercepted or changed. However, as each entangled pair is usually only capable of being encoded with one state – generally the direction of its polarization – the amount of data carried is limited to just one quantum bit (qubit) per photon.

In standard two-dimensional entanglement states, in line with quantum wave theory, a photon exists in all spin states at once. However, if a photon is passed through a polarizing filter that rejects given spin states, the photon can be made to exhibit just one of four possible states of spin – vertical, horizontal, left, or right – with particles in an entangled pair reflecting the polar opposite behavior of each other.

That is, if one entangled particle spins right, its entangled mate spins left. In this latest research, however, the scientists have managed to impart yet another state to two of the three photons in the mix, where they corkscrew in opposite directions as they travel. This is a feat never experimentally achieved before.

"This type of asymmetric quantum entanglement has been predicted before on paper, but we are the first to actually create it in the lab," said Dr Mehul Malik, from the University of Vienna, and lead author on the research.

In this research, two photons are imparted with attributes in a three-dimensional space, whereas the third only exists in two dimensions. This asymmetric entanglement structure allows for different layers of information to be encoded within the one entangled structure, allowing multiple recipients – both completely securely separated from each other – to receive information encoded in the one transmission simultaneously. This means, for example, that it may be possible to exchange data with a third-party, whilst using a public encryption key with a telecommunications carrier to secure it, all in the same transmission.

To prove this possibility, the researchers developed a new quantum cryptographic protocol using their added quantum state to allow sharing of separate layers of information asymmetrically amongst multiple parties with complete security between each layer.

"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," said Anton Zeilinger, a quantum physicist at the the University of Vienna, and co-author of the research.

Whilst other research has aimed at encoding entangled photons with more than a single state, such as using a frequency comb to produce hyper-entangled states to produce multi-frequency encoding, this method manages to add extra attributes to the entangled state by imparting spin. As such, this method of encoding may help move quantum information away from the standard qubit and into the realm of the qudit (a unit of quantum information encoded in any number of d states, where d is a variable), in which the quantum-encoded information can be increased by simultaneously encoding spin states and the motion of the particle through three dimensions.

The results of this research were published in the journal Nature Photonics.

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