Gilles Brassard

Gilles Brassard (born April 20, 1955, Montreal, Quebec, Canada) is a Canadian computer scientist known for his work in quantum cryptography and quantum teleportation.

Brassard received bachelor’s (1972) and master’s (1975) degrees in computer science from the University of Montreal and then a doctorate (1979) in theoretical computer science from Cornell University. He returned to the University of Montreal as an assistant professor in 1979 and then became a full professor in 1988.

After Brassard received his doctorate, he attended a conference in San Juan, Puerto Rico, where he was swimming when he was approached by American physicist Charles Bennett, who knew of his interest in cryptography. Bennett told Brassard about his friend Stephen Wiesner’s idea of using quantum mechanics to create money that could not be counterfeited. Such a banknote would contain twenty two-state quantum systems in which any attempt to measure the states and thus duplicate them would alter the states from their original configuration.

Inspired by Wiesner’s idea, Brassard and Bennett came up with the first quantum cryptography scheme, BB84, named after their initials and the year the work was published. In BB84, Alice wishes to communicate securely with Bob. To represent bits, she sends him single polarized photons. Half of all the photons are polarized in a horizontal-vertical basis; that is, bits with value 0 are horizontally polarized, and bits with value 1 are vertically polarized. The other half of the photons are polarized in a diagonal basis, with 0s being polarized at 45 degrees from horizontal and 1s being polarized at 135 degrees. Bob measures half the individual photons in the horizontal-vertical basis and half in the diagonal basis. If Alice sends a horizontal photon and Bob measures it in the diagonal setting, the reading will be either 45 or 135 degrees. After Bob measures all the photons, he tells Alice which basis he used. Alice tells him when they used the same basis. They throw out the bits for which they used a different basis and keep those that had the same basis as their key. To make sure no one is eavesdropping, Alice sends a piece of the key to Bob. If there were an eavesdropper, that person could not know which basis the photons were being transmitted in and thus could sometimes send a wrongly polarized photon to Bob. Sending the wrong photons would alter the key, and Bob and Alice would know someone was listening in. Bennett, Brassard, and their collaborators first experimentally tested BB84 in 1989, and it has even been used over a distance of 1,200 km (about 745 miles) between an artificial satellite and Earth.

Brassard, Bennett, and their collaborators published work in 1993 describing how to “teleport” a quantum state from one subatomic particle to another by using quantum entanglement. Two particles can be in a single entangled state such that measuring a property of one particle instantly determines that same property in another particle. For example, two particles are in a state in which one is spin-up and the other is spin-down. Since the second particle must have the opposite value of the first particle, measuring the first particle results in a definite state for the second particle, notwithstanding that the two particles may be millions of kilometers apart and are not interacting with each other at the time. The teleportation is done by entangling one particle in an entangled pair with a third particle. The third particle then has the properties of the other particle in the original entangled pair. Austrian physicist Anton Zeilinger and his collaborators successfully demonstrated quantum teleportation in 1997, and he won the 2022 Nobel Prize for Physics for his experiments.

Brassard has received many honors for his work, including the Herzberg Medal (2009), the Order of Canada (2013), and the Wolf Prize (2018). He has written the books Algorithmics: Theory and Practice (1988, with Paul Bratley), Modern Cryptology: A Tutorial (1989), and Fundamentals of Algorithmics (1995, with Bratley).