Multi-party Quantum Communication Possible

quantum communication

Scientists have measured quantum communication between three entangled photons at faster than the speed of light. Previously, communication has been possible between nonlocal entangled photons but now, with the addition of a third photon, multi-party quantum communication has been proven possible. Physicists at the University of Waterloo’s Institute for Quantum Computing (IQC) have now managed three entangled photons at three separate locations designated as Alice, Bob, and Charlie. While both Alice and Bob have been able to communicate for years, the addition of Charlie has proven quantum non-locality for more than two entangled photons.

The different locations are several hundreds of meters apart and this possibility was once described by Albert Einstein as distant “spooky action.” The triple photon entanglement may lead to future possibilities for quantum communications. The term non-locality indicates how the particles, even removed by large distances, can instantly determine the state of the other. This could lead to the ability to transfer information not only faster than the speed of light but effectively creating instantaneous communication. Referring back to Einstein, this ability actually contravenes his local action principal. The principal means that objects can only be influenced by their immediate surroundings and cannot directly influence each other from a distance. However, the current experiment shows that multi-party communication is possible by means of the photons having knowledge of the others’ states even when separated by several hundreds of meters.

In order to support their theory, physicists needed to close the locality loophole and ascertain that local variables were not influencing the correlations between the photons. What they did was to beam the entangled photons several hundred meters from the laboratory into trailers parked in the fields. One of the co-authors, the Canadian Research Chair in Optical Quantum Technologies, Professor Kevin Resch, said that a lot about the fundamental level of nature can be told from the correlations seen from quantum systems. Resch is a recent winner of the Natural Sciences and Engineering Research Council of Canada’s E.W.R. Steacie Fellowship. He is ready to exploit the documented behaviors to utilize them as resources for a possible future multi-party quantum communication or other quantum technologies.

The correlations between the three photons were studied while they were in what is known as a Greenberger-Home-Zellinger (GHZ) state which indicates a particular kind of entanglement in the quantum state for a minimum of three particles. In this case, there were three photons generated for the study. After being created in Resch’s laboratory, the photon triplets travelled through optical fiber, one of them remaining in the lab after a delay and the other two being sent through telescopes to trailers approximately 700 meters from both the original lab and from their separate locations.

The experiment used Universal Quantum Devices (UQD) which are time tagging devices at each trailer to measure each particle independently. Additionally, the devices guaranteed that a three nanosecond time measurement was not exceeded which confirmed that there was no possibility of information transmission. This was a major condition in the quest to prove the non-locality. The experiment showed that three entangled particles may open the door for future multi-party quantum communication and make possible such things as quantum secret sharing and third man cryptography. The paper’s lead author Chris Erven states that until now, Quantum Key Distribution (QKD) has always been a pair system. He further states that this is the first experiment which uses at least three photons and it opens the imagination to consider different correlations and different networks of possible connections.

By Dee Mueller
on twitter @TuesdayDG

Sources
Nature
Phys.org
Science Daily
MarketWatch

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