Quantum Communications

Construction of quantum secure communication networks is taking place in numerous countries across the world, with notable and well-publicised examples already operational in China and in various EU countries as parts of the European Quantum Communication Infrastructure (EuroQCI). In the UK, the first national Quantum Network(UKQN) connects the cities of Cambridge and Bristol with quantum equipment installed over the National Dark Fibre Facility (or NDFF – specialist dedicated fibre allowing researchers to develop the technologies necessary for a future internet).UKQN is a research-focused network, extending across 410kmand includes a range of connecting technologies, from city-scale ‘metro’ (or metropolitan, multiple inter-connected nodes in urban environments), to inter-city ‘backbone’ (connecting ‘metro’ networks along a common spine). Central to the network is Quantum Key Distribution (QKD – see below), an established quantum technology for the secure distribution of secret keys for data encryption. The encrypted data may then be transmitted over conventional communications infrastructure - typically standard optical fibre – and securely stored. The UK Quantum Network has and continues to contribute to the commercialisation of quantum security and its incorporation into the country’s existing telecommunications infrastructure.

Modern society is becoming increasingly reliant on the security of communications, be it sensitive data or transmitted information. Data security is currently largely delivered through cryptography, using cryptographic keys (e.g. set of random numbers) to encrypt and decrypt the data. So, the protection of sensitive information is ultimately determined by the security of these keys.

Symmetric cryptography involves the use of a shared secret key to both encrypt and decrypt messages between legitimate users; both parties use the same key which must be secret and known only to them. Therefore, the security of their subsequent communications is determined by the security of the key distribution mechanism.

This is where Quantum Key Distribution (QKD) comes in. Quantum mechanics dictates that when any of us try to measure or interact with a quantum system, e.g. by observing it, to learn about what it is doing, we inevitably and irreversibly disturb it. This relationship between disturbance and information gained is fundamental. It is not something that can be overcome by building better measurement devices and probes in the future – it is built into Nature. Taken into the context of communications scenarios, it simply means that where the distribution of encryption keys for securing sensitive messages is implemented with quantum light signals, anyone covertly attempting to gain information about the keys will necessarily disturb some of the light signals as they do so. Thus, Nature ensures that eavesdroppers cannot avoid being detected – in fact even if they try to use other quantum technologies to examine the light signals. Quantum communication systems use these quantum effects to securely distribute encryption keys with quantum light signals. These keys are then utilised to secure all manner of sensitive data transmissions, such as bank transactions or personal health records.

Quantum communications technologies such as Quantum Key Distribution (QKD) employ the use of standard optical fibre already used for conventional telecommunications to securely connect users and transmit encrypted information. This setup though clearly poses challenges when the distance between communicating parties is too large (across big land masses) or involves fibre installed underwater (e.g. under oceans and across continents). The longer the fibre, the greater the chance of the delicate quantum light signals deteriorating, and the greater the “noise” in the communication channel that can swamp these quantum signals. The solution to these problems comes in the form of satellites. Installing quantum communications technologies onto orbiting satellites means that quantum security can be guaranteed at global scales.

Researchers in the UK are working on the launch of small low-earth-orbit (LEO) cube satellites (so-called CubeSats measuring just 20 x20 x 35cm) into space in 2025, to demonstrate how quantum communications can be transmitted to a specialist telescope (or Optical Ground Station, or OGS). One such station is currently being installed outside Edinburgh, in Scotland. Specialist space research missions such as these typically include: the satellite itself, the quantum signal transmitter (or payload) on the satellite, and the OGS telescope with appropriate quantum signal receivers attached, on the ground. Successful operation of quantum satellite missions will enable us to establish the crucial next steps towards future commercial quantum secure services inspace.