# Professor Charles H. Bennett, Doctor Honoris Causa of the Gdańsk University

Charles H. Bennett was born in 1943, the son of music teachers – Anne Wolfe and Boyd Bennett. He graduated from Croton-Harmon High School in 1960 and went on to study chemistry at Brandeis University in Waltham. In 1964 he started his doctoral studies in molecular dynamics at Harvard University, under the guidance of David Turnbull and Bernie Alder, and in 1970 received his PhD in computer simulation of molecular motion. Afterwards he continued this research under Aneesur Rahman at Argonne Laboratory. In 1972 Dr Bennett joined the IBM Research team where a renowned physicist, Rolf Landauer, was working on the issues concerning theoretical informatics. It was of fundamental significance to the future pioneer of quantum informatics. Here his interests regarding the connections between physics and information were crystallised. In 1973 Dr Bennett published a breakthrough paper on logical reversibility of computing, where he showed, contrary to Landauer's pessimism, that calculations can be performed in a reversible way. It was a prophetic outcome, since reversibility constitutes the basis for the idea of a quantum computer, introduced much later by Feymann and Deutsch. In 1982 Dr Bennett, using the information theory, proposed a reinterpretation of Maxwell's demon, showing that the finite capacity of its memory must result in destroying information, which is always a thermodynamically irreversible process. In the years 1983-85 Dr Bennett conducted classes in cryptography and computation physics as a visiting professor at the University of Boston.

In 1984 Dr Bennett, together with Gilles Brassard from the University of Montreal, introduced the first quantum protocol for encrypting information, based on Heisenberg's uncertainty principle, which consists in sending single, randomly polarised photons. Afterwards, with John Smolin, he built the first quantum key distributor, thus beginning the rapid development of quantum cryptography with the use of optical fibres, as well as in the open space. At the same time Dr Bennett was developing the algorithmic information theory, different from that by Kolmogorov, introducing the measure of intrinsic complexity of physical state (logical depth), based on computational complexity of the algorithm, which can reconstruct a given piece of information.

At the beginning of the 1990s Dr Bennett directed his attention to the unusual correlations discovered in the 1930s by Einstein, Podolsky, Rosen and Schrödinger, called quantum entanglement. In 1992, together with Stephen Wiesner, he published a paper which revolutionised the theory of communication. It turned out that, thanks to an entangled pair of particles, it is possible to send, with the help of one quantum bit (e.g. a photon with two polarisations) two bits of informations. This allows for circumventing Holevo's “ban”, in accordance with which only one bit of information can be sent with the use of one qubit.

In the same year, following William Wootters' seminar in Montreal, there was a heated discussion with Asher Peres, Giles Brasard, Claude Crepeau and Richard Jozsa, concerning the optimal transmission of a quantum state between two laboratories, distant from each other.

As Peres mentioned in his paper, dedicated to Dr Bennett on the occasion of his 60^{th} birthday, Bennett had asked a fundamental question: “What will happen if we give them a pair of entangled particles?”. That question opened the way to the discovery of quantum teleportation. In 1993 Physical Review Letters published a paper titled “Teleporting an Unknown Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels”, written by the six participants of the discussion in Montreal. It was shown in the paper that, having at one's disposal a pair of entangled particles and the possibility to communicate two bits of information, quantum information can be transferred from one particle to another one, in a distant laboratory, without violating the ban on cloning.

Quantum information is destroyed on one particle and then reconstructed on another one with the use of non-directed resource – a pair of entangled particles. A few years later both non-intuitive quantum effects – dense coding and quantum teleportation were realised in pioneer experiments performed by Anton Zeilinger's team.

In the years 1995-97 Dr Bennett's team created the quantitative theory of entanglement and introduced various techniques for faithful transmission of classical and quantum information through noisy channels. That result, together with the discovery of quantum teleportation and dense coding, constituted a significant contribution to the theory of quantum communication and quantum computing. In particular, the famous quantum entanglement distillation protocol, elaborated by Dr Bennett and his collaborators was an inspiration for the Gdańsk team. Namely, in 1996, so-called bound entanglement, was discovered in Gdańsk. Such entanglement cannot be distilled to a pure form with the use of the “distillation machine” created by Dr Bennett and his collaborators. This peculiar entanglement was the subject of intensive research conducted by Dr Bennett's team, as well as by the Gdańsk team. It initiated cooperation, which resulted, among others, in the discovery of the so-called information blocking effect, and in the elaboration of the basis for the theory of multi-particle quantum correlations.

Charles H. Bennett has also significantly contributed to the theory of quantum channels. In particular, his paper on the connection between capacities of quantum channels supported by entanglement and Shannon's opposite theorem became a classic title in this field of science.

Dr Bennett's achievements have formed the basis for a new field of science – the quantum information theory. They have been stimulating the dynamic development of experimental techniques with regard to transforming and controlling quantum systems, i.e. the so-called quantum technology. They have also revolutionised our view on the basis for the quantum description of Nature.

Charles H. Bennett is the author / co-author of papers cited more than 28,300 times, including 10 papers cited over 1,000 times. His work on quantum teleportation has been cited almost 7,000 times.

In the year 2006 the Senate of the University of Gdańsk awarded Charles H. Bennett with the title of doctor honoris causa for “**fundamental and comprehensive contribution to the development of a new branch of science – the quantum information theory as the basis for quantum technology and new understanding of Nature**”.

Charles H. Bennett is, among others, an IBM Fellow, a Fellow of the American Physical Society and a member of the American Academy of Sciences. He is married with three grown-up children. His hobbies are photography and music.

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