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Researchers send “unhackable” quantum data over 370-mile optical fiber

Breakthrough could pave the way for highly secure worldwide communications

Fiber optic cable under yellow-colored light

Researchers have successfully transmitted quantum information over a 370-mile (600km) optical cable, breaking a record and heralding the beginning of long-distance quantum-secured information transfer between cities.

The scientists at Toshiba, working at the company's R&D lab in Cambridge in the UK, demonstrated they could send quantum bits (or qubits) over hundreds of kilometers of optical fiber without losing data, using a technology called 'dual band' stabilization.

One of the biggest problems in creating a quantum link is transferring quantum bits over long optical fibers. Slight changes, such as temperature fluctuations, cause fibers to expand and contract, destroying the fragile qubits, which are encoded as a phase delay of a weak optical pulse in the fiber.

The new technique sends two optical reference signals at different wavelengths, minimizing phase fluctuations on long fibers. The first wavelength is used to cancel the rapidly varying fluctuations, while the second wavelength, which is at the same wavelength as the optical qubits, fine-tunes the phase.

Researchers found it was possible to hold the optical phase of a quantum signal constant to within a fraction of a wavelength, with a precision of tens of nanometers, even after propagation through hundreds of km of fiber. Without compensating for these fluctuations in real-time, the fiber would expand and contract with temperature changes, scrambling the quantum information.

One of the first real-world applications of the technique will be for long-distance Quantum Key Distribution (QKD). These are currently limited to 130 miles — about half the length of New York — in length. 

Scientists said their "Twin Field QKD protocol" would extend this distance. They tested its resilience to optical loss using short fibers and attenuators. By using the dual-band stabilization technique, Toshiba has now implemented Twin Field QKD on long fibers and demonstrated QKD over 600 km for the first time.

Andrew Shields, head of the Quantum Technology Division at Toshiba Europe, said in a statement that QKD had been used to secure metropolitan area networks in recent years. 

"This latest advance extends the maximum span of a quantum link so that it is possible to connect cities across countries and continents, without using trusted intermediate nodes. Implemented along with Satellite QKD, it will allow us to build a global network for quantum secured communications," he added.

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Roger Grimes, a data-driven defense evangelist at KnowBe4, told ITPro that a more secure network won't necessarily mean more secure organizations.

"Ransomware isn't the huge problem it is today because they are breaking insecure network links. I haven't heard a single person complain to me in decades that if only they had more secure network transmission, then the way they were exploited would not have happened," he said.

"Network-layer interception exploits do happen...but it's very rare. So, this whole idea that if we get to a quantum network or quantum Internet and the whole world will celebrate that accomplishment, and hackers and malware will just be defeated, is more than a bit fanciful."

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