By IQT News posted 08 Jun 2021

(EurekaAlert) The Cambridge Research Laboratory of Toshiba Europe today announced the first demonstration of quantum communications over optical fibres exceeding 600 km in length. The breakthrough will enable 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 fibres. Small changes in the ambient conditions, such as temperature fluctuations, cause the fibres to expand and contract, thereby scrambling the fragile qubits, which are encoded as a phase delay of a weak optical pulse in the fibre.
Now Toshiba has demonstrated record distances for quantum communications by introducing a novel ‘dual band’ stabilisation technique. 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. 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 fibre. Without cancelling these fluctuations in real time, the fibre would expand and contract with temperature changes, scrambling the quantum information.

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