Institute for Quantum Computing (IQC) reserachers have develop a new tool for evaluating the security of a broad class of protocols that can take advantage of existing telecom infrastructure to deploy large quantum-secured communication networks in the near future.
Quantum computing poses a threat to our current cryptographic infrastructure. Quantum key distribution (QKD) is expected to be an important part of the solution to this problem by allowing two distant parties to establish long strings of digits, called keys, which are secured using the laws of quantum mechanics.
These secret keys can then be used in other cryptographic systems for secure communication, authentication and other cryptographic applications.New research by Institute for Quantum Computing (IQC) PhD student Jie Lin, Master’s student Twesh Upadhyaya and professor in the Faculty of Science's physics and astronomy department Norbert Lütkenhaus demonstrates the security of a class of QKD protocols that could be both practical and affordable for widespread adoption on our current telecommunications networks.
The main theoretical problem in QKD is calculating the key rate of QKD protocols, which reveals how efficiently they operate. Many protocols that have already been established as theoretically secure are so-called discrete-variable QKD protocols. They encode quantum information in distinct quantum states of photons, and require advanced single photon detectors to receive the key. These protocols, while easier to analyze theoretically, have a higher hurdle to be adopted widely: they would require augmenting existing telecommunications infrastructure with specialized, separate technology.
An alternative to discrete-variable QKD is continuous-variable QKD (CVQKD), which allows the information in a secret key to be encoded in continuous variables like the amplitude of an optical signal sent through a fiber cable. The technology to receive this kind of quantum information is already widely used in the high-speed telecommunications networks that keep us connected, thus lowering the threshold for CVQKD compared to the discrete-variable protocols. This makes CVQKD a promising candidate for the widespread adoption of secure quantum networks.
Previous studies of CV protocols required these encoding quantum states to be randomly chosen from a huge set of states according to a continuous distribution in order to guarantee the security. However, the accurate and fast preparation of the states, the demand on the amount of randomness generated, and the computational and communication power required for this type of protocols poses a challenge to widespread adoption.
In this new research, IQC researchers studied a class of CVQKD protocols that only require a small number of quantum states. These protocols combine the advantages of discrete variable protocols (low requirement on random number generation, low computational power for processing of data) with that of CVQKD, namely sharing the optical technology with modern optical communication equipment that is deployed today. So far, however, the theoretical security and rate analysis of these protocols has been missing as they are more challenging to do than previously studied protocols.
The researchers developed a method to prove the security of this type of CVQKD protocols and determine the secure key rate. Their method of analysis is versatile, meaning that it can be used to evaluate a variety of useful CVQKD protocols.
The work builds on previous research by Lütkenhaus’ Optical Quantum Communication Theory Group that developed an easy-to-use software tool to evaluate many QKD protocols. The new research extends the tool to highly promising CVQKD protocols compatible with mainstream telecommunications infrastructure.
The combination of security, versatility and practicality means that CVQKD protocols are a cost-effective way to secure our vital information with secret keys in future quantum communications networks.
“Asymptotic Security Analysis of Discrete-Modulated Continuous-Variable Quantum Key Distribution” was published in Physical Review X on December 30, 2019.