Engineers at the Cambridge Research Laboratory of Toshiba Europe has demonstrated quantum communications on optical fibres over 600 km for the first time,
The Quantum Internet would be a global network of quantum computers connected by long distance quantum communication links. Several large government initiatives to build a quantum internet have been announced, for example in the US, EU and China.
However a key problem is transmitting quantum bits over long optical fibres. Small changes in the ambient conditions, such as temperature fluctuations, cause the fibres to expand and contract. This scrambles the qubits that are encoded as a phase delay of a weak optical pulse in the fibre.
The researchers at Toshiba in the UK developed a dual band stabilisation technique to overcome this issue. This sends two optical reference signals, at different wavelengths, for minimising the phase fluctuations on long fibres.
The first wavelength is used to cancel the rapidly varying fluctuations, while the second wavelength, at the same wavelength as the optical qubits, is used for fine adjustment of the phase.
Toshiba found it is possible to hold the optical phase of a quantum signal constant to within a fraction of a wavelength, with a precision of 10s of nanometers, even after propagation through hundreds of kilometres of fibre. Without cancelling these fluctuations in real time, the fibre would expand and contract with temperature changes, scrambling the quantum information.
The first application for dual band stabilization will be for long distance Quantum Key Distribution (QKD), where Toshiba has set up a commercial business with manufacturing in Cambridge.
Commercial QKD systems are currently limited to around 100 to 200 km of fibre. In 2018, Toshiba proposed the Twin Field QKD protocol as a way to extend the distance, and tested its resilience to optical loss using short fibres and attenuators. The dual band stabilization technique with Twin Field QKD enables the 600km links for the first time.
"This is a very exciting result," said Mirko Pittaluga, first author of an article in Nature Photonics describing the results. "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".
“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,” said Andrew Shields, Head of the Quantum Technology Division at Toshiba Europe
Taro Shimada, Corporate Senior Vice President and Chief Digital Officer of Toshiba Corporation said “With this success in Quantum Technology, Toshiba is willing to further expand its quantum business with rapid speed. Our vision is a platform for quantum information technology services, which will not only enable secure communication on a global scale, but also transformational technologies such as cloud-based quantum computing and distributed quantum sensing.”
The work was partially funded by the EU through the H2020 project OpenQKD and Corning collaborated in the work by providing the SMF-28® ULL fibre used in the study. The team is now engineering the proposed solutions to simplify their future adoption and deployment.
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