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Bob and Alice get jiggy with Charlie while Randy plays voyeur

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University of Waterloo boffins have demonstrated a three-way quantum entanglement that shows quantum non-locality can work with more than two particles.*

The regulars of quantum physics, Bob and Alice, and the interloper Charlie, had an appropriately trailer-trash setting, but alas saw precious little one-on-one photon action since they were separated by hundreds of metres.

The details of the experiment are straightforward enough. To test the three-way “Greenberger-Horne-Zeilinger” (GHZ) state, the researchers started by generating photon triplets in a lab. The triplets were then split up: the first was sent through a 580m optical fibre to delay it; the other two were sent to two trailers (Bob and Charlie) that are 700m distant from both the source and from each other.

Randy, the fourth party in the experiment and located in yet another trailer, randomly selected the measurements Alice (the originating lab) would perform on her photons.

The university's release describes the setup in each location: “Each trailer contained detectors, time-tagging devices developed by IQC spin off company Universal Quantum Devices (UQD), and quantum random number generators. To ensure the locality loophole was closed, the random number generators determined how the photon at each trailer would be measured independently.”

By keeping the time window small – less than three nanoseconds – the university says the experiment can rule out any information transfer between the three locations, “a critical condition to prove the non-locality of entanglement.”

Chris Everen, lead author of the University of Waterloo group's paper in Nature Photonics (abstract here) and now a research assistant at the University of Bristol, says the result means groups of people could create quantum-secured networks rather than simple one-on-one pairings:

“QKD, so far, has been a pairwise system – meaning that it works best and with less assumptions when you’re only talking with one other person. This is the first experiment where you can now imagine a network of people connected in different ways using the correlations between three or more photons,” he said. ®

Bootnote: The author has corrected the first paragraph, which originally referred to faster-than-light entanglement. This was drawn from early wire releases such as this. However, the Waterloo University release focuses on nonlocality.

The Register notes that researchers in China last year claimed that entanglement appears to operate at speeds faster than light. ®

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