Toshiba’s Cambridge Research Laboratory demonstrated quantum communications over optical fibers exceeding 600 km in length. The work leverages a technique called dual-band stabilization, and it marks a significant advance in quantum-encrypted communication, particularly for the secure transfer of information between metropolitan areas. How to transmit quantum bits over long optical fibers is among the challenges in building a quantum internet; small changes in ambient conditions, such as fluctuations in temperature, cause the fibers to expand and contract. This scrambles the fragile qubits encoded as a phase delay of a weak optical pulse in the fiber. Diagram describing Toshiba’s dual-band stabilization method for quantum communication. Courtesy of Toshiba. Toshiba introduced a novel dual-band stabilization technique that sends two optical reference signals at different wavelengths to minimize phase fluctuations in long fibers. The first wavelength cancels the various fluctuations as they happen, and the second wavelength, which is the same wavelength as the optical qubits, is used for fine adjustment of the phase. With this method, the Cambridge team found that it’s possible to hold the optical pulse of a quantum signal constant to within a fraction of a wavelength, with a precision of tens of nanometers, even after propagation through hundreds of kilometers of fiber. The first application of this method will be in long distance quantum key distribution (QKD). Current commercially available QKD systems are limited to around 100 to 200 km of fiber. In 2018, Toshiba published a paper in Nature Photonics that proposed the Twin Field QKD protocol as a way to extend that distance, and tested its resilience to optical loss using short fibers and attenuators. With the introduction of the dual-band stabilization technique, the team demonstrated Twin Field QKD on longer fibers of more than 600 km. “With the new techniques we have developed, further extensions of the communication distance for QKD are possible, and our solutions can be applied to other quantum communications protocols and applications,” said Mirko Pittaluga, first author of the paper describing the results. The development follows news in 2020 of the first industrial quantum secure network in the U.K., built by U.K. communications company BT and Toshiba. The system transmits data between the National Composites Centre and the Centre for Modelling & Simulation. Toshiba’s multiplexing compatibility enables the data and quantum keys to be transmitted along the same fiber, eliminating the need for dedicated infrastructure for key distribution. The researchers believe the combination of multiplexed QKD using existing infrastructure for shorter distances and Twin Field QKD for longer distances could pave the way for a commercially viable global quantum-secure network. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-021-00811-0).