Toshiba Europe’s Cambridge Research Laboratory has achieved a milestone in long-distance quantum communication, demonstrated for the first time on optical fibres exceeding 600 km in length. Achieved with support from the EU-funded OPENQKD and QCALL projects, this accomplishment will allow the long-distance transfer of information secured through quantum cryptography. It’s also a big step forward towards the creation of a quantum internet. The quantum internet is a global network of quantum devices that exchange information over long distances based on the laws of quantum mechanics. What seems like not so long ago, it was purely sci-fi, but today the quantum internet is a real and key ambition for many countries across the globe. The realisation of such an internet would enable complex optimisation problems to be swiftly solved in the cloud. It would also allow precise global timing and highly secure communications worldwide. However, before the quantum internet can become a reality, a number of obstacles need to be overcome, one of the most difficult being the problem of transmitting quantum bits, or qubits, over long distances on optical fibres. As the fundamental building block for quantum information processes, qubits have great potential but are also extremely fragile and error-prone because of their interactions with the surroundings. So when optical fibres expand and contract as a result of small changes in ambient conditions (e.g. temperature fluctuations), this scrambles the fragile qubits that are encoded as a phase delay of a weak optical pulse passing through the fibre.
Two wavelengths to the rescue
The Toshiba Europe researchers have now shown that quantum communication is possible over a record distance of 600 km through the introduction of a dual-band stabilisation method. In this method, two optical reference signals are sent at different wavelengths to minimise phase fluctuations on long fibres (hence the term dual-band). The first signal cancels out the varying fluctuations, while the second (sent at the same wavelength as the qubits) is used for fine adjustment of the phase. This has allowed the researchers to keep the optical phase of a quantum signal constant to within a fraction of a wavelength, even over hundreds of kilometres of optical fibre. As reported in a news item posted on ‘Fiber Optics Online’, the first application of dual-band stabilisation is in long-distance quantum key distribution (QKD). QKD secures communication using cryptography and allows users to detect any eavesdropping attempts. The Toshiba Europe team has now demonstrated QKD over 600-km-long optical fibres for the first time. “This is a very exciting result,” remarked Mirko Pittaluga of Toshiba Europe in the same news item. “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,” continued Pittaluga, who is also the first author of the relevant study published in ‘Nature Photonics’ and funded by OPENQKD (Open European Quantum Key Distribution Testbed) and QCALL (Quantum Communications for ALL). With this latest advance, it will be possible to connect quantum devices across countries and continents, doing away with intermediate nodes and ushering in the quantum internet of the future. For more information, please see: OPENQKD project website QCALL project website
OPENQKD, QCALL, quantum internet, quantum communication, optical fibre, qubit, quantum key distribution, QKD