Researchers from Toshiba have successfully sent quantum information over 600-kilometer-long optical fibers, creating a new distance record and paving the way for large-scale quantum networks that could be used to exchange information securely between cities and even countries.
Working from the company's R&D lab in Cambridge in the UK, the scientists demonstrated 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, thanks to a new technology that stabilizes the environmental fluctuations occurring in the fiber.
This could go a long way in helping to create a next-generation quantum internet that scientists hope will one day span global distances.
The quantum internet, which will take the shape of a global network of quantum devices connected by long-distance quantum communication links, is expected to enable use-cases that are impossible with today's web applications. They range from generating virtually un-hackable communications, to creating clusters of inter-connected quantum devices that together could surpass the compute power of classical devices.
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But in order to communicate, quantum devices need to send and receive qubits – tiny particles that exist in a special, but extremely fragile, quantum state. Finding the best way to transmit qubits without having them fall from their quantum state has got scientists around the world scratching their heads for many years.
One approach consists of shooting qubits down optical fibers that connect quantum devices. The method has been successful but is limited in scale: small changes in the environment, such as temperature fluctuations, cause the fibers to expand and contract, and risk messing with the qubits.
This is why experiments with optical fiber, until now, have typically been limited to a range of hundreds of kilometers; in other words, nowhere near enough to create the large-scale, global quantum internet dreamed up by scientists.
To tackle the instable conditions inside optical fibers, Toshiba's researchers developed a new technique called "dual band stabilization". The method sends two signals down the optical fiber at different wavelengths. The first wavelength is used to cancel out rapidly varying fluctuations, while the second wavelength, which is at the same wavelength as the qubits, is used for finer adjustments of the phase.
Put simply, the two wavelengths combine to cancel environmental fluctuations inside the fiber in real time, which according to Toshiba's researchers, enabled qubits to travel safely over 600 kilometers.
Already, the company's team has used the technology to trial one of the most well-known applications of quantum networks: quantum-based encryption.
Known as Quantum Key Distribution (QKD), the protocol leverages quantum networks to create security keys that are impossible to hack, meaning that users can securely exchange confidential information, like bank statements or health records, over an untrusted communication channel such as the internet.
During a communication, QKD works by having one of the two parties encrypt a piece of data by encoding the cryptography key onto qubits and sending those qubits over to the other person thanks to a quantum network. Because of the laws of quantum mechanics, however, it is impossible for a spy to intercept the qubits without leaving a sign of eavesdropping that can be seen by the users – who, in turn, can take steps to protect the information.
Unlike classical cryptography, therefore, QKD does not rely on the mathematical complexity of solving security keys, but rather leverages the laws of physics. This means that even the most powerful computers would be unable to hack the qubits-based keys. It is easy to see why the idea is gathering the attention of players from all parts, ranging from financial institutions to intelligence agencies.
Toshiba's new technique to reduce fluctuations in optical fibers enabled the researchers to carry out QKD over a much larger distance than previously possible. "This is a very exciting result," said Mirko Pittaluga, research scientist at Toshiba Europe. "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."
When it comes to carrying out QKD using optical fiber, Toshiba's 600-kilometer mark is a record-breaker, which the company predicts will enable secure links to be created between cities like London, Paris, Brussels, Amsterdam and Dublin.
Other research groups, however, have focused on different methods to transmit qubits, which have enabled QKD to happen over even larger distances. Chinese scientists, for example, are using a mix of satellite-based transmissions communicating with optical fibers on the ground, and recently succeeded in carrying out QKD over a total distance of 4,600 kilometers.
Every approach has its pros and cons: using satellite technologies is more costly and could be harder to scale up. But one thing is certain: research groups in the UK, China and the US are experimenting at pace to make quantum networks become a reality.
Toshiba's research was partially funded by the EU, which is showing a keen interest in developing quantum communications. Meanwhile, China's latest five-year plan also allocates a special place for quantum networks; and the US recently published a blueprint laying out a step-by-step guide leading to the establishment of a global quantum internet.