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Toshiba researchers demonstrated quantum key distribution on fibers of record 600km in length

Japanese tech giant Toshiba announced this week that they have successfully sent quantum information over 600-kilometer-long optical fibers. With that, they set a a new distance record and have heralded a new future for mammoth quantum networks that could send information securely between cities and even countries. 

The Toshiba researchers were working from the company’s R&D lab in Cambridge in the UK. These scientists have proven that they could transmit quantum bits (or qubits) over hundreds of kilometers of optical fiber without scrambling the fragile quantum data encoded in the particles. The reason they succeeded, they say, is due to a new technology that stabilizes the environmental fluctuations occurring in the fiber.  

What Quantum Internet means in real terms

The term “quantum internet” describes a global network of quantum computers connected by long distance quantum communication links. The idea is that this tech will allow the ultra-fast solution of complex optimization problems in the cloud. This will deliver a more accurate global timing system and highly secure communications across the globe, according to Toshiba. Several large government initiatives to build a quantum internet are already underway in the US, EU and China.

One major technological challenge in building the quantum internet is finding 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. This “scrambles” the fragile qubits, according to the researchers.

How Toshiba solved the stabilization problem

Now Toshiba has demonstrated 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 cancels the rapidly varying fluctuations. The second wavelength, which runs at the same wavelength as the optical qubits, performs the fine adjustment of the phase. Using these new techniques, the Toshiba team was able to hold the optical phase of a quantum signal constant to within a fraction of a wavelength.

The company says they can hold that phase with a precision of 10s of nanometers. Moreover, they can do this even over 100s of kilometers of fiber. Without cancelling these fluctuations in real time, the fiber would expand and contract with temperature changes, scrambling the quantum information.

Taro Shimada, Toshiba’s Corporate Senior VP and Chief Digital Officer, praised the breakthrough. “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.”