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07 Jun 2023
Multiplexing approach allows quantum key distribution signals to co-exist with classical signals at distances up to 70 km.
New research by Toshiba Europe and telecoms giant Orange suggesting that quantum key distribution (QKD) can be deployed on existing optical networks is being hailed by the two firms as a “huge step forward” for quantum-secure communications.
Revealed in a newly published paper based around a presentation made at this year’s Optical Fiber Communication (OFC) conference, the work relates to a QKD system operating at 1310 nm, coupled with 1550 nm data channels transmitted over 50 kilometers of standard single mode fiber.
Co-propagation
UK-based Toshiba Europe, which is an acknowledged leader in the development of QKD, says the results of work at Orange Labs in Lannion, France, show how the technology can be deployed on a provider’s existing fiber network, alongside current data services.
“These findings could help network operators reduce the cost of implementing QKD by removing the need to invest in dedicated quantum fiber infrastructure,” points out the firm.
Until now, the deployment of QKD has required operators to invest in so-called “dark fiber” across their networks specifically for sending quantum information, increasing the cost and time to adoption.
Previous studies have indicated issues compromising the viability of such deployments, including the possible number and optical power of the data channels used and the effective network distance, as well as the speed of the QKD element.
But Toshiba Europe now says that wavelength division multiplexing (WDM) should enable QKD to operate on existing optical fibers thanks to spectral separation.
“Through the tests, researchers from Toshiba and Orange demonstrated the effective co-propagation of the classical and quantum signals with high secret bit rates, allowing them to co-exist while still being capable of effectively delivering keys at distances of up to 70 km, showing great promise for deployments in metro networks in built-up areas,” the two firms reported.
High secret key rate
While quantum computers capable of cracking public key encryption used to protect sensitive data are yet to appear, there is a widespread fear that the current approach could be rendered insecure in the future.
QKD presents one way to protect such data, because it relies on the physical properties such as the polarization of an individual photon - meaning that any attempt to intercept the key would immediately be detected.
However, the same characteristics mean that - so far - dedicated fiber and highly sensitive, expensive photonic components have been required for QKD deployment.
Paulette Gavignet from Orange Innovation commented: “This work shows that we can have co-propagation of the quantum channel with WDM data channels in the same fiber, without changing the engineering rules of the operational WDM links.
“The high secret key rate obtained in this configuration is very promising for the introduction of QKD in operator’s networks.”
Orange group CTO Laurent Leboucher added: “Together with Toshiba, we showed that it is possible to introduce new security functions in the operators’ networks without requiring the use of dedicated fibers. With this cost-effective approach, we pave the way towards a digital fortress, guaranteeing the security of our customers’ most valuable data.”
Andrew Shields, a QKD pioneer who heads up Toshiba’s Quantum Technology Division in Cambridge, said: “Validating the ability of our QKD technology to protect transmissions while using existing fiber networks is a huge step forward in making quantum-secure communications accessible for today’s organisations.”
New metric
The work detailed in the paper includes evaluation of a 1310 nm quantum channel multiplexed with up to 60 data channels, each carrying a 100 Gb/s bit rate in the telecommunication “C band” across a commercially available Toshiba QKD system.
“The ability to multiplex classical data while retaining excellent QKD performance was enabled by the system’s novel design, which included high-extinction spectral filters and time-domain gating used to help isolate the quantum signal and reduce noise introduced from the classical channels,” explains the Toshiba-Orange team.
Tests were run with both 30 and 60 multiplexed channels over 20 km, 50 km and 70 km fiber lengths, with the secure bit rate measured over each distance.
Results showed that the high number of WDM channels had little impact on the secure bit rate, which was more influenced by the optical launch power of the aggregated data channels used in the system.
That finding has enabled Orange and Toshiba to propose a new metric - which they call the “co-propagation Efficiency (CE)” - to estimate the performance of the QKD system in such a deployment while considering the total power of the data channels and transmission distance.
“These findings have two key implications for the viability of using QKD to secure communications against attack by quantum computers at a commercial level,” says Toshiba.
“Firstly, it shows that the commercially available equipment evaluated by Toshiba and Orange is successful at allowing QKD to be more effectively deployed on current fiber networks.
“Secondly, the new metric developed by the researchers, which acknowledges that power - and not the number of channels - has the most impact on efficiency, may aid operators in network and service planning.”
In their research paper, the team concludes: “These results show the possibility to deploy commercial QKD system[s] on currently existing fully filled WDM links with 100 Gb/s and 400 Gb/s channels in data center interconnection (DCI) applications.”
ID Quantique joins space-QKD effort
In related news the Swiss firm ID Quantique - another developer of QKD systems - says that it is now participating in the “EAGLE-1” initiative to deploy space-based quantum encryption.
First revealed last year, the project is funded via Horizon Europe and the European Space Agency and is aiming to launch a satellite into low-Earth orbit as soon as next year.
Germany’s TESAT is due to manufacture a QKD payload to establish a secure optical link from space to the ground, as well as the QKD module of the satellite, and has selected ID Quantique to provide its quantum random number generator (QRNG) and chipset technology.
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