New Technique Delivers Secure Quantum Networks Over Long Distances

'Now you're thinking with quantum entanglement!' Griffith University

Researchers from Australia’s Griffith University have demonstrated a new approach to test whether a quantum communication is secure. This method could be very important in the construction of future quantum networks. 

The new technique is called quantum steering and allows us to test whether two particles are entangled, even in real-life conditions where such a task would be intractable using current technology. As reported in Science Advances, quantum steering could be used to verify entanglement for up to 80 kilometers (50 miles) of telecommunication fibers.

Entanglement is a curious quantum phenomenon and a key player in the world of quantum computing. We are used to imagining particles as distinct and independent objects, but that's not exactly true in quantum mechanics. It is possible for them to interact in such a way that they need to be considered together. Such particles are considered entangled and even if they are separated by a great distance, when something happens to one particle it will affect the other.

Einstein didn’t like this at all since it appeared to violate the property of locality. He called entanglement “spooky action at a distance”, but it is nothing of the sort. And it will soon have a very important real-life application. An entanglement state is very delicate and this is what makes it useful for secure communications. It can’t be hacked without being destroyed.

But real-life cases are more complex. A huge number of entangled photons are being sent through fiber optics because only a small fraction will survive. So it might be possible to fake entanglement and get access to the network.

“As the length of quantum channel grows, fewer and fewer photons successfully pass through the link, because no material is perfectly transparent and absorption and scattering take their toll,” senior author Geoff Pryde, said in a statement. "This is a problem for existing quantum nonlocality verification techniques with photons. Every photon lost makes it easier for the eavesdropper to break the security by mimicking entanglement.”

Quantum steering uses another fascinating phenomenon to check if communication is secure, quantum teleportation. It removes some of the entangled photons from fiber optics and moves them to a clean and efficient quantum channel where the photons are assessed.

"Our scheme records an additional signal that lets us know if the light particle has made it through the transmission channel," lead author Dr Morgan Weston added. "This means that the failed distribution events can be excluded up front, allowing the communication to be implemented securely even in the presence of very high loss." 

Quantum technologies promise to revolutionize computers and communications over the next few decades, but there are still many challenges to overcome.

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