Quantum communication networks: A game of quantum cats and mice

Cryptography is the art of secret. It has been used since the antiquity to ensure the confidentiality of diplomatic and military communication, a use that was still dominant during world war II. In the 1970s, new methods were developed to secure digital information, which enabled the age of information.

Throughout its history, cryptography has been a game of cats and mice between code designers and code breakers. The methods used to secure communications are often based on mathematical conjectures and it is very important to keep analyzing them to detect potential flaws and weaknesses. On the other hand, the search for new methods, systems and cryptographic constructions still has a big disruptive potential.

One of the most surprising development of cryptography is based on the use of quantum physics. The birth of quantum cryptography is a well-documented story. In 1979 on a beach in Puerto Rico, Gilles Brassard was approached by Charles Bennet who claimed to know how to secure communications using quantum states of light. In their protocol, the quantum states are used to establish a shared secret key, and the security follows from the fact that quantum information cannot be copied. Therefore, the legitimate parties can detect an eavesdropper trying to extract information from the quantum communication channel.

Building on the work of Stephen Wisner on quantum money, Bennett and Brassard, a physicist and a computer scientist, found a way to achieve something impossible using only classical digital information. Quantum key distribution (QKD) enables secret key establishment over an insecure communication channel. It even reaches the holy grail of encryption, which is unconditional security. No matter the effort spent by an eavesdropper, data encrypted through quantum key distribution remain secure forever.

Bennet and Brassard’s protocol is undoubtedly a great scientific discovery. It opened a new field of research, and provided new insight to quantum information. Arguably, the original motivation for QKD was to investigate the mysterious way information is processed at the quantum scale. They spent years trying to convince the scientific community of the relevance of this approach, eventually publishing it in 1984.