Engineers at the world-renowned Cambridge Research Laboratory of Toshiba Europe have demonstrated quantum key distribution (QKD) of optical fibers over a distance of 600 km which they claim is a major breakthrough in long-distance quantum communications.

The results hold out the prospect of quantum–secured information being sent between metropolitan areas, and the researchers say the work represents a significant advance towards building a future quantum internet.

They acknowledge that many technical barriers still remain before such a network of quantum computers connected via long-distance quantum communications becomes a reality, but maintain that their demonstration of how quantum bits can be transmitted over long optical fiber links is a significant step towards such a quantum internet.

The breakthrough being reported this week in Nature Physics now involves a ‘dual band’ stabilization technique. This is said to allow transmission of two reference signals (using different wavelengths) to minimize phase fluctuations on long fibers.

The first signal is used to cancel out fluctuations while the second, which is at the same wavelength as the qubits, is used for fine adjustment of the phase.

The set-up made it possible for the researchers to maintain the optical phase of a signal within a fraction of the wavelength, even through hundreds of kilometers of optical fiber.

Work showed that it was possible to hold the optical phase of a quantum signal constant to within a fraction of a wavelength, and with a precision of tens of nanometers, even after propagation through hundreds of kilometers of fiber.

The researchers note that, without cancelling these fluctuations in real time, the fiber would expand and contract with changes in temperature, thereby scrambling the quantum information.

The initial application for such dual band stabilization is expected to be long-distance Quantum Key Distribution (QKD), and Toshiba has established a commercial business in manufacturing in Cambridge. T

o date, QKD systems have been limited to around 100 to 200 km of connected fiber.

“This is a very exciting result. With the new techniques we have developed, further extensions of the communication distance for QKD are still possible and our solutions can also be applied to other quantum communications protocols and applications,” commented Mirko Pittaluga, lead author of the article.

Three years ago, the Toshiba researchers proposed the Twin Field QKD protocol as a way to extend the distance, and tested its resilience to optical losses using short fibers and actuators. It has now deployed the technology.

And the latest advance follows the announcement last year that BT and Toshiba had installed the UK’s first industrial quantum-secure network.

Andrew Shields, head of the Quantum Technology Division at Toshiba Europe, noted that “this latest advance extends the maximum span of quantum links, 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”.

The research was partially funded by the EU trough the H2020 Project OpenQKD, and Toshiba says the team is now engineering the proposed solutions to simplify their adoption and deployment.

Corning is said to have collaborated in the work by providing its SMF-28 ULL fiber.

By: DocMemory
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