Hybrid Quantum-Classical Communication Networks

Information security is a necessity of today's complex society. Let it be our personal information, a bank transaction, or some confidential military correspondence, they all rely on cryptographic techniques that guarantee secure communication between the transmitter and the intended receiver. This assurance, however, does not necessarily last forever. Technological advancements have already made many older cryptosystems obsolete, and it is anticipated that future discoveries will make the current secure communication methods unreliable as well. In particular, the development of new computational paradigms based on the laws of quantum mechanics is a threat to the security of the widely used public-key cryptosystems. Fortunately, what quantum mechanics may take by one hand, it gives back with the other. Secure communication, facilitated by the use of quantum key distribution (QKD) protocols, is the most imminent application of the developing field of quantum information. QKD provides unbreakable, future-proof, security safe from the vulnerabilities of most cryptosystems currently in operation. To this point, QKD has been implemented over dedicated channels and between two parties. Before current communication vulnerabilities are exploited, it is essential to facilitate the use of QKD technology for any two public users at any distance, via a network. This unsolved problem lies at the intersection of quantum physics and optical communications engineering, as all known QKD protocols rely on light transmission.

This proposal focuses on the problems that arise when multiple users wish to utilise the same infrastructure, namely, optical fibre, for both classical and quantum communication applications. This is in essence similar to a classical multiple-access problem, such as mobile communication, where multiple users communicate via a shared communication channel. In hybrid quantum-classical networks, this feature must be extended to include QKD applications, where we are dealing with optical signals as weak as a single photon.

In this project, I aim at undertaking a theoretical study of a range of network configurations and different multiple-access techniques for hybrid quantum-classical networks. This project will shed light on the necessary steps that underpin future implementations. I will also look at compatibility issues regarding the integration of present optical communication networks, which solely support classical applications, and future hybrid networks, which will offer both data transmission services as well as QKD-driven secure communications. That will enable long-distance classical-quantum communication at a national scale.

There are many groups in public and private sectors who, within relevant timescales, will benefit from this research. Here, I provide a summary of such impacts.

Private sector

The private firms, who will benefit from this research, can be categorised into two groups:

- Group 1: R&D companies in quantum technologies and the service-provider group, who will deliver these technologies to potential users;
- Group 2: The potential customers of such technology.

Group 1
Within the first group, those who are active in developing quantum cryptography systems, such as ID Quantique, QinetiQ, Telcorida Technologies, Toshiba, Mitsubishi, and MagiQ Technologies, will be initially most interested in this research as the proposed project improves and expands the types of services they offer to their customers. Currently, the only commercially available product is that of a point-to-point quantum communication link over a dedicated channel. This proposal improves and enhances this product in several ways:

- It removes the requirement of using dedicated channels;
- It enables both quantum and classical communication applications to be run over the same infrastructure;
- It provides such a secure service not just between two partners, but any two users in a wider network; and
- It reduces the total cost per user.

The second subgroup within Group 1 are large communication service providers, such as British Telecom (BT) or Virgin Media, who have access to the infrastructure, such as the underground optical fibre or the optical routers along the way. Their interest in this project can be over a longer term, when the technology reaches the point of becoming accessible to the public. They need, however, to follow the progress in the field and prepare themselves for that time.

Group 2
At the early stages of commercialisation, the main customer of quantum cryptography is the financial sector. Such firms annually spend millions of pounds for information security, and they will be willing to upgrade their security level to the one being offered by quantum technology. The quantum networks proposed here are of practical importance to such institutes. For instance, they will enable ultra secure communication between all branches of a bank within a metropolitan area at a price and quality not available today.

Public Sector

The security offered by quantum cryptography is of extreme importance to all sections in the public sector. Government agencies, such as military or defence departments, who deal with confidential information, as well as any agency who handles private/personal information of citizens will benefit from the results of this research.

With the extensive research being pursued worldwide to build the first functional quantum computer, it will be possible to crack some of the most important cryptosystems in daily use, including bank transactions, military information, and confidential government correspondence. From a policy-making standpoint, it is essential to invest in technologies that will guarantee security of information in the quantum era. Multiple-access QKD networks, as proposed here, take us one step closer to achieving that goal, thereby enhancing UK competitiveness globally. From an economic standpoint, those countries to first offer quantum technology will enjoy a competition-free market and establish themselves as the sole providers of the new technology. It is essential for the UK to be among this elite group: it will lead to security benefits and financial advantages for its citizens.

Last but not least, and common to private and public sectors, this project will result in training highly skilled workforce, including research staff and project students, who will increase the public awareness and contribute to the UK economy.