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Toshiba extends limits of secure intercity comms

02 May 2018

Twin-Field QKD allows secure quantum key distribution over 500km of standard optical fiber.

The Cambridge Research Laboratory of Toshiba Research Europe (CRL) has devised a new protocol for quantum key distribution (QKD) that its says will extend its range to over 500 km of standard telecom fiber.

This development, called Twin-Field QKD, enables the protection of sensitive data - such as financial transactions - transmitted in optical networks between cities. CRL says it would allow a secure link between cities, for critical communications such as financial transactions.

The details of the breakthrough are published in Nature. The article's lead author is Toshiba CRL's Marco Lucamarini.

QKD can be used to distribute the secret digital keys important for protecting our personal data, such as bank statements, health records, and digital identity.The longest demonstration of QKD to date, over 404km of special low loss fibre (equivalent to 323.2 km of standard fiber), had a secure key rate of 0.32 milli-bit/sec (1.15 bit/hour). For the new protocol a bit rate in excess of 100 bits/sec is expected for the same channel loss.

Its security relies on encoding each bit of the key upon a single photon transmitted, for example, through an ordinary optical fiber. As any attempt to read the photons alters their encoding, this allows the secrecy of each key to be tested and guaranteed.


Unlike other existing security solutions, quantum cryptography is secure from all future advances in mathematics and computing, even from the number crunching abilities of a quantum computer. It is therefore expected to be an essential tool for protecting communication infrastructure from cyber-attacks in the future and putting businesses on the front foot when it comes to protecting operation-critical information.

Up until now, the typical range of QKD has been limited to a few hundred kilometers of optical fiber. This is because the photons carrying the information can be scattered and thereby lost from the fiber, reducing the rate at which secret keys can be formed.

500km links in prospect

Now, Toshiba has discovered a way to enhance key rate and transmission distance of QKD, potentially allowing fibre links beyond 500 km for the first time. The final secure key rate can be orders of magnitude greater than that obtainable with existing protocols.1 In fact the key rate and distance achievable with the new method exceeds the secret key capacity previously thought to be a limit that cannot be overcome without using a quantum repeater.

In conventional QKD, single photons are sent from one end of the fiber to the other end. In contrast, for Twin-Field QKD, light pulses are sent from both ends of the fiber to a central location, where a photon is detected. Provided it is impossible to tell which end of the fiber the photon came from, this technique effectively doubles the transmission distance at a given rate.

No protection

Although conventional systems may be daisy-chained together to increase the total transmission distance, this requires that the intermediate stations are in a secure location. In contrast, no physical protection in the central location is necessary for the security of Twin-Field QKD.

“Twin-Field QKD would enable a bank in London to connect to a data centre in Leeds via a link that can guarantee the secure transmission of customer data,” comments Dr. Andrew Shields, Assistant Managing Director at the Cambridge Research Laboratory.

“At present, the bank would have to place trust in intermediate nodes at secure, guarded locations between London and Leeds. Our breakthrough means that businesses can create a QKD network that connects their sites across the country for the first time.”

Professor Tim Whitley, head of research for BT, and MD of Adastral Park, said: “This is a further important step in the commercial development of quantum cryptography. Our secure quantum communication showcase at Adastral Park already demonstrates how financial institutions can secure links between offices and branches and off-site data centers, which are prevalent throughout the financial sector.

"The application of twin-field QKD would extend that capability to sites anywhere in Europe, for example, making it possible to securely network an organisation at a national and international level.” The CRL team intends to demonstrate the viability of the new protocol experimentally in the coming year.

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