Scientists at Toshiba Europe's Cambridge Research Laboratory managed to demonstrate quantum communications over a world-first distance of 600 km on June 8, 2021.
The achievement is a step toward long-distance quantum-secured information transfer between metropolitan areas, and is a major advance towards building the future Quantum Internet.
One of the most difficult technological challenges in building the quantum internet, is the problem of how to transmit quantum bits over long optical fibers. Small changes in the ambient conditions, such as temperature fluctuations, cause the fibers to expand and contract, thereby scrambling the fragile qubits, which are encoded as a phase delay of a weak optical pulse in the fiber.
Scientists managed the record distances for quantum communications by introducing a novel ‘dual band’ stabilization technique. This sends two optical reference signals, at different wavelengths, for minimizing the phase fluctuations on long fibers. 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. After deploying these new techniques, 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 100s of km of fiber. Without cancelling these fluctuations in real time, the fiber would expand and contract with temperature changes, scrambling the quantum information.
