Toshiba Quantum Cryptography experimental setup

Quantum communication demonstrated over 600km of fibre

Image credit: Toshiba Corporation

The Cambridge Research Laboratory of Toshiba Europe has announced a milestone in long-distance quantum communication, demonstrating quantum key distribution on optical fibres of over 600km in length.

The “breakthrough” will enable long-distance communication secured by quantum cryptography, and marks a major advance towards the establishment of a quantum internet - a network of quantum devices connected on an international scale. The US, EU and China all hope to lead in innovation in this area, which would allow common optimisation problems to be solved rapidly in the cloud; more precise global timing systems, and extremely secure communications.

Many technical barriers remain before the quantum internet can become a reality, including the problem of transmitting quantum bits over long distances in optical fibres. Quantum bits, which can take many forms, can exist in not just binary states (commonly labelled 0 and 1) but also in a superposition of these states. The smallest changes in ambient conditions, such as fluctuations in temperature, can cause optical fibres to expand and contract, scrambling the fragile qubits, which are encoded as a phase delay of a dim optical pulse passing through the fibre.

Toshiba has demonstrated quantum communication over a record distance of 600km through the introduction of a 'dual band' stabilisation technique. Speaking simply, this sends two reference signals (using different wavelengths) to minimise phase fluctuations on long fibres, hence 'dual band'. 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.

Toshiba quantum cryptography diagram

Toshiba quantum cryptography diagram/Toshiba Corporation

Image credit: toshiba corporation

This made it possible for the Toshiba researchers to maintain the optical phase of a signal within a fraction of a wavelength, even through hundreds of kilometres of optical fibre. The result was described in the Nature Photonics paper as “a major step in promoting quantum communications as a dependable resource in today’s world”.

The first application for dual-band stabilisation will be in long-distance quantum key distribution (QKD). QKD enables secure communication which cannot be eavesdropped on 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 in transit, the intended recipient will always be able to detect the eavesdropping.

So far, commercial QKD systems have been limited to around 100-200km of optical fibre.

“This is a very exciting result,” said Mirko Pittaluga, first 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 communications protocols and applications.”

Andrew Shields, head of Toshiba Europe’s Quantum Technology Division, commented: “QKD has 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.”

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