# Gilles Brassard

Wolf Prize Laureate in Physics 2018

**Gilles Brassard**

#### Affiliation at the time of the award:

**University of Montreal, Canada**

#### Award citation:

**“for founding and advancing the fields of Quantum Cryptography and Quantum Teleportation”.**

#### Prize Share:

**Gilles Brassardand**

**Charles H. Bennett**

Gilles Brassard, born in Montreal (1955), received a PhD from Cornell University in the study of theoretical computer science, and more specifically cryptography. He has been a faculty member at the Université de Montréal ever since (1979) and Canada Research Chair in Quantum Information Science since 2001. Fellow of the Royal Society of London (2013), the International Association for Cryptologic Research (2006), the Canadian Institute for Advanced Research (2002) and the Royal Society of Canada (1996), he was made Officer in the Order of Canada (2013) and in the Ordre national du Québec (2017). Among his numerous honours, professor Brassard has been awarded the Killam Prize for Natural Sciences (2011) and the Gerhard Herzberg Canada Gold Medal for Science and Engineering (2009), which are the two highest scientific awards given by Canada, as well as the Prix d’excellence du FRQNT (2013) and the Prix Marie-Victorin (2000), which are the two highest scientific awards given by Québec. Together with Charles H. Bennett and Stephen Wiesner, he was also awarded the Rank Prize in Opto-electronics (2006) for research on the original concept of quantum cryptography.

The information revolution, which continues to transform every aspect of life in the 21st century, grew out of two discoveries made at Bell Laboratories in 1948. One was the transistor, which launched decades of amazing miniaturization of electronics. The other was Claude Shannon’s revolutionary paper on information theory. Nowadays even non scientists understand the gist of it: anything one wishes to communicate—words, sounds, pictures, shapes, movements and maybe someday even smells—can be coded into bits—zeros and ones—transmitted through a channel such as radio or optical fiber to a remote location and then reassembled into an arbitrarily good approximation of the original, for the benefit of the recipient. Shannon’s theory was an idealization of the robust behavior of macroscopic objects then used as information carriers, like punch cards, cog wheels and electrical switches. Such information can be accurately read and copied without disturbing the original. But chemists and physicists have long known that the information in tiny objects behaves in subtler ways. One cannot learn the exact state of an atom of matter, or a photon of light, because attempting to do so disturbs it; and two atoms or photons, that have once interacted but subsequently move too far apart to influence one another, can exist in a so-called entangled state, where the particles each behave randomly, but in ways that are too strongly correlated to be explained by supposing that each particle is in some (perhaps unknown) state of its own. These phenomena (called “quantum” in distinction to the ordinary “classical” behavior of macroscopic objects) have been reasonably well understood since the 1930’s, and have even excited a certain amount of interest among philosophers, but were considered to be part of the disciplines of physics and chemistry, with little relevance to information processing except as a nuisance, for example making tiny transistors noisier and less reliable than their larger cousins

Twenty years after Shannon’s paper, Stephen Wiesner noticed that quantum effects could be used to do some intriguing things not covered by Shannon’s theory, for example combining two messages into a single transmission from which the receiver could recover either one, but not both. Wiesner made little effort to publish or publicize these ideas, but he did tell a few friends. Charles Bennett from IBM Research and Gilles Brassard from the Université de Montréal were respectively the first physicist and first computer scientist to take Wiesner’s ideas seriously and develop them, thereby launching the discipline now known as quantum information science. Their first discovery, which became the first practical application of quantum information, was quantum key distribution. Quantum key distribution allows two users, communicating via a public classical channel (such as radio) and a quantum channel susceptible to eavesdropping (such as faint flashes of light sent through empty space or an optical fiber), to agree on a body of shared secret information, a so-called cryptographic key, with high confidence based on laws of physics that it is unknown to anyone else. With their students François Bessette, Louis Salvail and John Smolin they built a working demonstration in 1989, along the way overcoming other problems needed to make the scheme practical, such as compensating for transmission and measurement errors and partial information leakage to an eavesdropper. In the early 1990’s, in collaboration with Wiesner, Claude Crépeau, Richard Jozsa, Asher Peres and William Wootters, Bennett and Brassard showed that entanglement was not just an intriguing phenomenon, but a useful and quantifiable resource, despite having no ability to communicate by itself. In the technique called superdense coding, it doubles the amount of classical information that can be sent through a quantum channel, while in quantum teleportation it enables quantum information to be sent through a classical channel. Meanwhile, Artur Ekert showed that entanglement itself can be used for quantum key distribution. Also in the 1990’s they and other researchers, building on early work of David Deutsch and Richard Feynman in the 1980’s, showed that quantum notions provide the same kind of powerful generalization of Turing’s classical theory of computation as of Shannon’s classical theory of communication. This culminated in Peter Shor’s celebrated 1994 discovery of fast quantum algorithms for factoring and discrete logarithm, problems whose presumed intractability underlies the security of much of today’s electronic commerce, launching a worldwide effort to build a scalable quantum computer. Since then, quantum key distribution systems have become commercially available, and have been extended to ranges of hundreds of kilometers through optical fibers, and thousands of km in satellite-based systems. Practically, aside from their applications to communication, computation and modern kinds of information processing, which involve both communication and computation, quantum-information-inspired techniques have improved timekeeping and precision measurement. Theoretically, they have provided tantalizing hints about some of physics’s deepest mysteries, such the black hole information problem, quantum gravity and the origin of spacetime

The mechanics of quantum information processing, where qubits—the quantum generalization of Shannon’s bits—are acted on by the quantum generalizations of classical computing’s ANDs, ORs and NOTs, can be described in full detail using only a modest amount of secondary-school algebra, but it is harder to find words to explain how quantum information differs from the familiar classical kind. Though it is only a metaphor, one could say that whereas classical information is like the information in a book, quantum information is like the information in a dream. A dream cannot be copied or broadcast, and if you try to describe it to someone else, you eventually forget the dream and remember only what you said about it. But unlike dreams, this fragile dreamlike kind of information obeys well-understood laws, makes possible new kinds of communication and computing, and is improving our understanding of the universe in ways still being discovered

It is for their role in launching quantum information theory that Bennett and Brassard have received the 2018 Wolf Prize in Physics