Improving the security of quantum key distribution by solving the problem of incomplete randomization of light pulses


The research groups of Prof. Marcos Curty of the University of Vigo (Spain) and Prof. Norbert Lütkenhaus of the University of Waterloo (Canada), in collaboration with Prof. Kiyoshi Tamaki of Faculty of Engineering, University of Toyama have proposed a method to improve the security of actual quantum key distribution systems.

Quantum key distribution is a next-generation cryptography that is expected to be the ultimate cryptography that protects communication between users from any kind of eavesdropping by utilizing the principles of quantum mechanics. In order to achieve this ultimate security, the actual quantum key distribution devices must satisfy the conditions imposed by the security theory of quantum key distribution. The problem is that the conditions are so stringent that it is very difficult for the actual quantum key distribution devices to meet. Especially in high-speed communications, an effect called phase correlation has been a major problem, in which the phase modulation applied by an optical modulator to an optical pulse affects the one of subsequent optical pulses.

In this research, we proposed a new method for secure quantum key distribution under the presence of imperfections such as phase correlation in the decoy state method used in typical quantum key distribution protocols such as BB84, and rigorously proved the security of its security. This research will further improve the security of quantum key distribution in practice.

Our result was published in the scientific journal, Quantum Science and Technology, on December 22, 2023.

Background of the Research

The Internet has become very convenient and indispensable in modern life, but at the same time, there is a risk that information exchanged over the Internet can be illegally stolen and misused by a third party (eavesdropper). For example, when shopping on the Internet, important personal information such as credit card information and address information is transmitted, and if this information is leaked out, it could be a serious problem. This risk of information leakage can be avoided by using cryptography, and quantum key distribution (or quantum cryptography) is expected to be the most secure and ultimate in cryptography. Unlike conventional cryptography, where security is based on the difficulty of mathematical problems, quantum key distribution protects information by making use of a mysterious property of nature of very small objects such as light particles called photons.

In this way, quantum key distribution holds promise as the next generation cryptography, but the actual devises for quantum key distribution entail inevitable imperfections such as noises, and security cannot be guaranteed if these imperfections are exploited by an eavesdropper. Due to the progress made in our work, actual security of quantum key distribution is expected to further increase.

Research Contents and Achievements

With the above background, our research focused on the decoy state method, which is widely used in typical quantum key distribution protocols such as BB84. The decoy state method requires that the phase of each optical pulse be independently and completely randomized by a phase modulator, but this independent and complete randomization is very difficult to achieve in practice. In high-speed communications, in particular, the devices manipulate (modulate) the state of the light operate at very high speeds, so that manipulation of one optical pulse affects the following optical pulse, resulting in loss of independence.

We solve this problem by developing a new security theory based on the existence of these imperfections, rather than taking the approach of how to remove them.

Future Developments

Our work is a theoretical proposal, we plan to conduct future research to demonstrate and implement this experimentally.

Original article information


Quantum Science and Technology

Journal link

Paper title

Security of quantum key distribution with imperfect phase randomisation


Guillermo Currás-Lorenzo, Shlok Nahar, Norbert Lütkenhaus, Kiyoshi Tamaki, Marcos Curty