Final Report Summary - MULTIPLE-ACCESS QKD (Multiple-Access Quantum Key Distribution Networks)
Quantum key distribution (QKD) offers future-proof solutions for data security. It enables two users to exchange a secret key, which can be used in a variety of cryptographic applications, such as in our daily bank transactions and online shopping. QKD relies on the transmission of quantum signals (e.g. single photons), hence, its implementation over long distances, because of path loss, can be challenging. Nevertheless, over 30 years since its introduction, QKD has reached an exciting milestone in its development in that now one can plan for its widespread deployment. Over the past few years, secure communications systems relying on various QKD techniques have reached new distance records exceeding 300 km, been demonstrated alongside strong conventional channels, and supported multiple users up to around 50. New protocols have also made their implementations more reliable and secure against possible attacks. Commercial deployment of QKD has also begun with plans to build large scale quantum networks in the UK, US, South Korea and China.
Within the above context, Multiple-Access QKD took several initiatives to enable QKD networks, over existing infrastructure, alongside classical communications systems. This is a crucial step to bring QKD technologies to the doorsteps of home users. We investigated immediate and future generations of such networks and developed new solutions for each. We, in particular, looked at the first-generation of such networks, which will rely on a trusted set of relay nodes, as well as the second generation, where the issue of trust is partially relaxed by using today’s imperfect quantum memories, all the way to future trust-free generations, where quantum repeaters are in use. In each case, we found out what our proposed systems could offer in terms of key exchange rate versus the number of users and distance.
The main feature of a network is to support multiple users and to enable them to communicate with each other. Until very recently, QKD had only been implemented between two users. This project developed new multiple-access and multiplexing techniques necessary for the development of QKD at the network level. In the multiple-access scenario, multiple users wish to exchange a secret key with a single network node, or mutually with other network users via the network infrastructure. Their signals, however, will be mixed with each other and we need mechanisms to separate different signals from each other at the receiver ends. In this project, we looked at separating QKD signals in code, time, and frequency spaces, and combinations thereof, and showed how they perform in comparison with each other. By multiplexing, the capacity at the core of a network can be shared among many users/service providers. Here, we developed highly efficient techniques to make the best use of the allocated bandwidth for as many users as possible.
Another important issue in developing QKD networks is the level of trust required for the intermediate nodes run by the service providers. Although at early stages of the development, we may need to trust all such nodes, we can gradually reduce, and then completely remove, the trust requirement as the technology advances. This project proposes architectures that even with current imperfect technologies one can improve the secret key generation rates at moderately long distances (~500 km) with no middle trusted nodes. This will roughly increase the current distance records by a factor of two, and it is a necessary development toward long-distance QKD networks. The main enabling ingredient in our scheme is a quantum memory unit, which can store quantum states of transmitted photons. Such devices are still being developed in the laboratories and they often cannot meet all the requirements needed for proper deployment of quantum repeaters (the main solution to long-distance QKD). In our scheme, we, however, relax some of these requirements and that would allow us to get advantage out of a single repeater link with our even imperfect technology for quantum memories.
Finally, one cannot build a huge quantum network without accounting for the issue of cost and the required resources. Via this project, we have made a comparison between some of the candidates for long-haul QKD networks and found interesting results for the dependence of the optimum resource allocation to the number of available memories. It turns out that the more quantum memories that we have, sparser nodes are more favoured in the studied protocols. This is a desirable property as it makes the integration of quantum networks with classical infrastructure easier. This may not hold if we move to new proposals for memory-less quantum networks, which typically require very closely spaced network nodes. A full value for money comparison between all possible solutions needs to be undertaken in the continuation of this work.
Information security is a necessity of our today’s complex lives, and QKD offers a level of security that cannot be broken by technological advances. This project takes us one step closer to make the public QKD a reality. Our proposed systems enable cost-effective implementation of such systems over existing telecommunication infrastructure. This will impact the QKD industry, which develops QKD technologies, as well as the telecom industry, which will eventually provide the service to the end users. In addition to the advancement of knowledge, the results of this project can benefit policy makers and standardisation bodies in their developing proper plans for the future. Nevertheless, certain technological challenges remain to be solved before having an operational hybrid quantum-classical network. Some of these issues are now being addressed by the worldwide effort in exploiting quantum technologies, while others perhaps require more time and effort to be resolved.
Within the above context, Multiple-Access QKD took several initiatives to enable QKD networks, over existing infrastructure, alongside classical communications systems. This is a crucial step to bring QKD technologies to the doorsteps of home users. We investigated immediate and future generations of such networks and developed new solutions for each. We, in particular, looked at the first-generation of such networks, which will rely on a trusted set of relay nodes, as well as the second generation, where the issue of trust is partially relaxed by using today’s imperfect quantum memories, all the way to future trust-free generations, where quantum repeaters are in use. In each case, we found out what our proposed systems could offer in terms of key exchange rate versus the number of users and distance.
The main feature of a network is to support multiple users and to enable them to communicate with each other. Until very recently, QKD had only been implemented between two users. This project developed new multiple-access and multiplexing techniques necessary for the development of QKD at the network level. In the multiple-access scenario, multiple users wish to exchange a secret key with a single network node, or mutually with other network users via the network infrastructure. Their signals, however, will be mixed with each other and we need mechanisms to separate different signals from each other at the receiver ends. In this project, we looked at separating QKD signals in code, time, and frequency spaces, and combinations thereof, and showed how they perform in comparison with each other. By multiplexing, the capacity at the core of a network can be shared among many users/service providers. Here, we developed highly efficient techniques to make the best use of the allocated bandwidth for as many users as possible.
Another important issue in developing QKD networks is the level of trust required for the intermediate nodes run by the service providers. Although at early stages of the development, we may need to trust all such nodes, we can gradually reduce, and then completely remove, the trust requirement as the technology advances. This project proposes architectures that even with current imperfect technologies one can improve the secret key generation rates at moderately long distances (~500 km) with no middle trusted nodes. This will roughly increase the current distance records by a factor of two, and it is a necessary development toward long-distance QKD networks. The main enabling ingredient in our scheme is a quantum memory unit, which can store quantum states of transmitted photons. Such devices are still being developed in the laboratories and they often cannot meet all the requirements needed for proper deployment of quantum repeaters (the main solution to long-distance QKD). In our scheme, we, however, relax some of these requirements and that would allow us to get advantage out of a single repeater link with our even imperfect technology for quantum memories.
Finally, one cannot build a huge quantum network without accounting for the issue of cost and the required resources. Via this project, we have made a comparison between some of the candidates for long-haul QKD networks and found interesting results for the dependence of the optimum resource allocation to the number of available memories. It turns out that the more quantum memories that we have, sparser nodes are more favoured in the studied protocols. This is a desirable property as it makes the integration of quantum networks with classical infrastructure easier. This may not hold if we move to new proposals for memory-less quantum networks, which typically require very closely spaced network nodes. A full value for money comparison between all possible solutions needs to be undertaken in the continuation of this work.
Information security is a necessity of our today’s complex lives, and QKD offers a level of security that cannot be broken by technological advances. This project takes us one step closer to make the public QKD a reality. Our proposed systems enable cost-effective implementation of such systems over existing telecommunication infrastructure. This will impact the QKD industry, which develops QKD technologies, as well as the telecom industry, which will eventually provide the service to the end users. In addition to the advancement of knowledge, the results of this project can benefit policy makers and standardisation bodies in their developing proper plans for the future. Nevertheless, certain technological challenges remain to be solved before having an operational hybrid quantum-classical network. Some of these issues are now being addressed by the worldwide effort in exploiting quantum technologies, while others perhaps require more time and effort to be resolved.