Physics

Rafi Bistritzer

Allan H. MacDonald 

Pablo Jarillo-Herrero

Since the 2004 groundbreaking experiments regarding the two-dimensional material -graphene, several research groups were soon studying the properties of twisted bilayer graphene. Graphene is a significant foundation for an entirely new generation of technologies. The hope is that graphene-based applications will benefit the environment and reduce costs. Electronic and computer industry requires materials whose conductance can be controlled.

The work of Jarillo-Herrero, MacDonald and Bistrizer has shown that the conductance properties of graphene interfaces can be controlled via the spatial misfit angle between the layers and then at certain angles the electrons exhibit surprising physical behavior. This physical discovery has the potential of leading to an energy revolution.

In 2011, a group led by Allan Macdonald, a theoretical physicist from the University of Texas, researched an intriguing behavior of twisted bilayer graphene, where the atomic lattices of two stacked graphene layers are laterally rotated with respect to each other by a small misfit angle. According to the calculations of MacDonald and Bistrizer (who did his post-doctoral thesis under the supervision of MacDonald at that time), the tunneling velocity of electrons between the layers depends on the misfit angle and completely vanishes at the “magic angle” of 1.1 degrees. It was hoped that this discovery would lead to the creation of a new type of super-conductor, namely a material that allows electrical current to pass with no impedance and with no energy loss.

The original paper by MacDonald and Bistrizer, which describes their discovery, was not received with enthusiasm by the scientific community and was even forgotten for several years.

At the same time, Jarillo-Herrero was working on twisted bilayer graphene in his lab at MIT. He became convinced that the ideas expressed by Macdonald and Bistrizer had substance.

His research team therefore invested considerable efforts in creating and measuring twisted bilayer graphene of various twist angles. The experiments proved successful in 2017 when it was found that positioning the layers at an angle of 1.1 degrees relative to one another (“the magic angle”) resulted in unusual electrical properties, precisely as MacDonald and Bistrizer have suggested. In this position, at sufficiently low temperatures, the electrons move from one layer to the other, creating a lattice with unusual qualities. The paper that described the phenomenon, which was published in Nature in 2018, revolutionized physics and triggered a flood of additional papers.

The discovery opens the door to building a super-conductor from bilayer graphene, in which electron movement is completely controlled by external electrical current. This electrical behavior resembles the behavior of copper-based superconductors called Cuprates. Cuprates demonstrate electrical conductivity with no resistance in relatively high temperatures compared with other super-conductors. For this reason, Cuprates now form a source of hope for realizing the dream of electrical conductivity with no energy loss at temperatures close to room temperature. If this mission is achieved, it would lead to a far-reaching energy revolution. However, one obstacle that prevents this revolution is that we do not yet have a theory that explains the behavior of superconductors at high temperatures. In the absence of a solid theoretical foundation, it is difficult to develop new, better materials. This is one of the reasons for the excitement around the discovery of bilayer graphene and the magic angle, which allows us to understand better what happens on the microscopic level when transitioning from a conductor to a superconductor state.

Rafi Bistritzer (1974, Israel) received his bachelor’s degree in physics from Tel Aviv University and his M.Sc. and Ph.D. degrees in physics from the Weizmann Institute of Science. In 2007, Bistritzer moved to Austin, where he completed a postdoctoral fellowship at the University of Texas, under the guidance of Professor McDonald. In 2012, he returned to Israel and worked on research and development in the fields of electromagnetsm and algorithms. Bistritzer is currently the manager of an algorithm group at Applied Materials. Bistritzer’s group specializes in computer-vision and machine learning algorithms.

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

Charles H. Bennett

Gilles Brassardand

Charles Bennett, born in New York City (1943), received a PhD from Harvard University in the study of molecular dynamics (computer simulations of molecular movements). He then joined IBM’s research labs where he helped develop the theoretical basis for pioneering research called Quantum Computation. In 1984 he began to develop, along with Gilles Brassard, the quantum encryption system named BB84, after, its two creators

Bennett is a member of the National Academy of Sciences of the United States (elected in 1997), a member of the American Physical Society (1998), a Laureate of the Rank Award for Electro-Optics (2006) a Laurete of the Harvey Award of the Technion in Haifa (2009), a Laureate of Japan’s Okawa Prize (2011), a Laurate of The Dirac Medal of the International Center of Theoretical Physics (2017)Iand has honorary doctorates from Masaryk U., U. of Gdansk, U. of Bristol, and ETH-Zurich

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

Michel Mayor 

Didier Queloz

In 1995, Mayor and Queloz were the first to discover a planet outside the solar system orbiting a solar-type star. The discovery of this planet, 51 Pegasi b, was the result of a continuous improvement of cross-correlation spectrographs by Mayor (completed in collaboration with Queloz) over a period of 20 years in order to obtain more accurate radial velocities. The discovery of 51 Pegasi b led to a revolution in the theory of planetary systems, since it is a Jovian planet having a very short orbital period of only 4.2 days, orders of magnitude smaller than the 12-year period of Jupiter in the solar system. This short period is attributed to orbital migration of planets during their formation in an accretion disk.

This discovery opened the floodgates for subsequent observations revealing an incredible diversity of exoplanets, some with large and quite eccentric elliptical orbits, unlike the nearly circular orbits in our own solar system. The team led by Mayor and Queloz has contributed to the discovery of more than 250 additional exoplanets, including several multi-planetary systems having up to 7 planets. Thanks to the most recent spectrograph (HARPS) developed by their team and installed at La Silla Observatory, they were able to reveal the very rich subpopulation of super-Earth planets on tight orbits and to conduct statistical research on exoplanets. Mayor participated in the first detection of an exoplanet transiting its host star, opening the way to the study of the composition of exoplanets. Soon after, Queloz, Mayor and their collaborators were able to measure the first Rossiter-McLaughlin effect for a transiting planet, which allowed the measurement of the projected angle between the stellar spin axis and the planets orbital axis. Subsequent observations show a large variety of angles, which cannot be explained solely by planetary migration.

Didier Queloz

Michel Mayor

In 1995, Mayor and Queloz were the first to discover a planet outside the solar system orbiting a solar-type star. The discovery of this planet, 51 Pegasi b, was the result of a continuous improvement of cross-correlation spectrographs by Mayor (completed in collaboration with Queloz) over a period of 20 years in order to obtain more accurate radial velocities. The discovery of 51 Pegasi b led to a revolution in the theory of planetary systems, since it is a Jovian planet having a very short orbital period of only 4.2 days, orders of magnitude smaller than the 12-year period of Jupiter in the solar system. This short period is attributed to orbital migration of planets during their formation in an accretion disk.

This discovery opened the floodgates for subsequent observations revealing an incredible diversity of exoplanets, some with large and quite eccentric elliptical orbits, unlike the nearly circular orbits in our own solar system. The team led by Mayor and Queloz has contributed to the discovery of more than 250 additional exoplanets, including several multi-planetary systems having up to 7 planets. Thanks to the most recent spectrograph (HARPS) developed by their team and installed at La Silla Observatory, they were able to reveal the very rich subpopulation of super-Earth planets on tight orbits and to conduct statistical research on exoplanets. Mayor participated in the first detection of an exoplanet transiting its host star, opening the way to the study of the composition of exoplanets. Soon after, Queloz, Mayor and their collaborators were able to measure the first Rossiter-McLaughlin effect for a transiting planet, which allowed the measurement of the projected angle between the stellar spin axis and the planets orbital axis. Subsequent observations show a large variety of angles, which cannot be explained solely by planetary migration.

Robert P. Kirshner

James D. Bjorken 

This year’s Wolf prize in physics is awarded to two researchers who made fundamental contributions toward understanding the structure of the Universe at the very smallest and the very largest sizes.

Robert Kirshner has devoted his professional life to cutting-edge research on cosmology and supernovae. He created the group, environment and directions that allowed his graduate students and postdoctoral fellows to uncover the acceleration in the expansion of the universe. This discovery is a landmark in fundamental physics, as well as in astronomy, and presents a profound challenge to theorists.

In 1974, Prof. Kirshner invented, with John Kwan, a method to measure the expansion rate of the universe based on observations of supernovae. Many key roadblocks, especially the effects due to reddening by interstellar dust, had to be overcome before supernovae could be used as a standard candle of sufficient accuracy to detect any change in the expansion rate. In the 1980s, Kirshner’s program of monitoring supernova explosions in a suite of wavelengths was the world’s most extensive and led to SNIa becoming widely accepted as the best for cosmological investigations. This was an essential step for the later discovery of the acceleration of the expansion.

Kirshner also led a program in which ultraviolet spectra of SNIa’s were measured with the Hubble Space Telescope. The results enabled the effects of redshift on the light (“photometry”) from supernovae at different distances to be properly corrected. All supernova cosmology teams now use these essential data.

Prof. Kirshner guided the formation of the High Z Supernova Team, one of the two teams widely credited with the discovery of cosmic acceleration. He gathered a first-rate group of students, such as Brian Schmidt and Adam Reiss, and led them toward practical and effective steps to find and follow SN Ia in a way that could reliably reveal cosmic deceleration, as was then almost the universally-expected result. Especially important was Kirshner’s insistence that the data at more than one colour be obtained to allow separation of dust from cosmic-motion effects in the photometry data.

James D. Bjorken 

Robert P. Kirshner

This year’s Wolf prize in physics is awarded to two researchers who made fundamental contributions toward understanding the structure of the Universe at the very smallest and the very largest sizes.

The strong force is responsible for the existence of protons and neutrons and for holding them together in atomic nuclei. It is also responsible for over 99% of the atoms’ mass. Bjorken made a crucial contribution for elucidating the nature of the strong force. In 1967 Bjorken predicted that electrons scattering violently off protons would exhibit the so-called scaling behavior, as if they were interacting with pointlike, charged and quasi-free particles inside the nucleon. At the time this was a very counterintuitive and radical idea. Yet, subsequent experiments, carried in 1968/69 at the Stanford Linear Accelerator Center (SLAC), provided a stunning confirmation for Bjorken’s scaling prediction. The leaders of the SLAC experiments, Jerome Friedmann, Henry Kendall and Richard Taylor were recognized by the 1990 Nobel Prize in Physics, for providing the experimental proof for the existence of quarks – the pointlike constituents of the nucleon.

Following the experimental success of Bjorken’s scaling laws, theorists embarked on a quest for a fundamental quantum theory which exhibits scaling. In 1973 David Gross, Frank Wilczek and H. David Politzer discovered that a theory now known as Quantum Chromodynamics (QCD) possesses the required property, namely that the force between quarks goes down as they get closer, so that at small distance they behave as if they were free. This property is now known as “asymptotic freedom”. In 2004 Gross, Wilczek and Politzer were awarded the Nobel Physics Prize for their discovery. QCD was validated in detail by extensive experiments as the theory of Strong Interactions.

The prevailing view today is that all fundamental interactions in Nature, with possible exception of gravity, are described by theories whose mathematical structure is analogous to QCD. Such theories are known as non-abelian gauge theories. Thus in retrospective, Bjorken’s scaling not only led to the discovery of quarks, but also pointed the direction toward the mathematical framework governing all fundamental interactions.

Peter Zoller

Juan Ignacio Cirac 

Peter Zoller and Ignacio Cirac are undoubtedly recognized as one of the most prominent theorists in quantum optics, quantum information science and the theory of quantum gases. Their impact on these fields of research cannot be overestimated and is outstanding by all means used to evaluate them.

Among their numerous common works, two specific works stand alone and opened new fields of research. In 1995, Cirac and Zoller proposed a model for a quantum computer, which could be practically implemented with the help of trapped ions. The very concrete nature of their proposal led numerous groups worldwide to successful experiments and has inspired many researchers both theorists and experimentalists. Such a quantum computer would be able to solve problems currently beyond the abilities of classical computers, such as the factorization of large numbers, which currently requires exponentially large computing time.

Their second outstanding contribution came as an outcome of the realization of gaseous Bose-Einstein condensates. They proposed to use such cold atoms as a general versatile toolbox to probe new regimes of many-body physics and to simulate condensed matter problems such as strongly correlated electronic systems. In their most famous work, Cirac and Zoller showed that an optical lattice can simulate a tight binding regime where the on-site interaction energy becomes comparable to the tunneling energy between neighboring sites. This paper had a tremendous impact and was soon followed by experimental realization of this quantum phase transition from a superfluid to a Mott insulator. Since then, a whole new interdisciplinary community has emerged exploring other condensed matter problems such as e.g. superconductivity, quantum magnetism, Quantum Hall effects, and Anderson localization. There is no doubt that these quantum simulators using cold atoms, will and already have had a huge impact on the whole fields of quantum physics, condensed matter physics and material science.

Juan Ignacio Cirac 

Peter Zoller

Juan Ignacio Cirac and Peter Zoller are undoubtedly recognized as one of the most prominent theorists in quantum optics, quantum information science and the theory of quantum gases. Their impact on these fields of research cannot be overestimated and is outstanding by all means used to evaluate them.

Among their numerous common works, two specific works stand alone and opened new fields of research. In 1995, Cirac and Zoller proposed a model for a quantum computer, which could be practically implemented with the help of trapped ions. The very concrete nature of their proposal led numerous groups worldwide to successful experiments and has inspired many researchers both theorists and experimentalists. Such a quantum computer would be able to solve problems currently beyond the abilities of classical computers, such as the factorization of large numbers, which currently requires exponentially large computing time.

Their second outstanding contribution came as an outcome of the realization of gaseous Bose-Einstein condensates. They proposed to use such cold atoms as a general versatile toolbox to probe new regimes of many-body physics and to simulate condensed matter problems such as strongly correlated electronic systems. In their most famous work, Cirac and Zoller showed that an optical lattice can simulate a tight binding regime where the on-site interaction energy becomes comparable to the tunneling energy between neighboring sites. This paper had a tremendous impact and was soon followed by experimental realization of this quantum phase transition from a superfluid to a Mott insulator. Since then, a whole new interdisciplinary community has emerged exploring other condensed matter problems such as e.g. superconductivity, quantum magnetism, Quantum Hall effects, and Anderson localization. There is no doubt that these quantum simulators using cold atoms, will and already have had a huge impact on the whole fields of quantum physics, condensed matter physics and material science.