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Quantum communication demonstrated over 600 km of fiber

Toshiba Europe’s Cambridge Research Laboratory has announced a milestone in long-distance quantum communications, demonstrating the distribution of quantum keys on optical fibers over 600km in length.

The “breakthrough” enables long-distance communications secured by quantum cryptography, and marks a significant step towards the creation of a quantum internet – a network of quantum devices connected on an international scale.

The US, EU, and China all hope to lead the way in innovation in this area, quickly solving common optimization problems in the cloud; more precise global timing systems and highly secure communications.

There are still many technical barriers before the quantum internet can become a reality, including the problem of transmitting quantum bits in optical fibers over long distances.

Quantum bits, which can take many forms, can exist not only in binary states (usually denoted 0 and 1), but also in a superposition of these states.

The slightest changes in environmental conditions, such as changes in temperature, can cause optical fibers to expand and contract, distorting the vulnerable qubits, which are encoded as a phase delay of a weak optical pulse passing through the fiber.

Toshiba has demonstrated quantum communication over a record 600km distance by introducing a ‘dual band’ stabilization technique.

Simply put, this sends two reference signals (with different wavelengths) to minimize phase fluctuations on long fibers, hence ‘dual band’.

The first signal is used to cancel out fluctuations, while the second (which is on the same wavelength as the qubits) is used for fine-tuning the phase.

This allowed the Toshiba researchers to keep the optical phase of a signal within a fraction of a wavelength, even through hundreds of kilometers of fiber optics.

The result was described in the Nature Photonics paper as “an important step in promoting quantum communication as a trusted resource in today’s world”.

The first application for dual-band stabilization will be in long-range quantum key distribution (QKD). QKD enables secure communication that cannot be eavesdropped by distributing a key to encrypt the exchanged information;

the security of the key rests on fundamental physical properties. If a third party intercepts information en route, the intended recipient can always detect the eavesdropping.

Until now, commercial QKD systems have been limited to about 100-200 km of optical fiber. “This is a very exciting result,” said Mirko Pittaluga, lead author of the study.

“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 communication protocols and applications.

” Andrew Shields, head of Toshiba Europe’s Quantum Technology Division, commented: “QKD has been used in recent years to secure networks of metropolitan areas.

This latest development extends the maximum span of a quantum link, making it possible to connect cities across countries and continents.

without using trusted intermediate nodes. Implemented together with Satellite QKD, it allows us to build a global network for quantum secure communications.”