Showing papers in "Reviews of Modern Physics in 2020"

Quantum computational chemistry

Abstract: One of the most promising suggested applications of quantum computing is solving classically intractable chemistry problems. This may help to answer unresolved questions about phenomena such as high temperature superconductivity, solid-state physics, transition metal catalysis, and certain biochemical reactions. In turn, this increased understanding may help us to refine, and perhaps even one day design, new compounds of scientific and industrial importance. However, building a sufficiently large quantum computer will be a difficult scientific challenge. As a result, developments that enable these problems to be tackled with fewer quantum resources should be considered important. Driven by this potential utility, quantum computational chemistry is rapidly emerging as an interdisciplinary field requiring knowledge of both quantum computing and computational chemistry. This review provides a comprehensive introduction to both computational chemistry and quantum computing, bridging the current knowledge gap. Major developments in this area are reviewed, with a particular focus on near-term quantum computation. Illustrations of key methods are provided, explicitly demonstrating how to map chemical problems onto a quantum computer, and how to solve them. The review concludes with an outlook on this nascent field.

Secure quantum key distribution with realistic devices

Abstract: Some years ago quantum hacking became popular: devices implementing the unbreakable quantum cryptography were shown to have imperfections which could be exploited by attackers. Security has been thoroughly enhanced, as a consequence of both theoretical and experimental advances. This review gives both sides of the story, with the current best theory of quantum security, and an extensive survey of what makes quantum cryptosystem safe in practice.

Sensitivity optimization for NV-diamond magnetometry

Abstract: Solid-state spin systems including nitrogen-vacancy (NV) centers in diamond constitute an increasingly favored quantum sensing platform. However, present NV ensemble devices exhibit sensitivities orders of magnitude away from theoretical limits. The sensitivity shortfall both handicaps existing implementations and curtails the envisioned application space. This review analyzes present and proposed approaches to enhance the sensitivity of broadband ensemble-NV-diamond magnetometers. Improvements to the spin dephasing time, the readout fidelity, and the host diamond material properties are identified as the most promising avenues and are investigated extensively. Our analysis of sensitivity optimization establishes a foundation to stimulate development of new techniques for enhancing solid-state sensor performance.

Nuclear effective field theory: status and perspectives

Abstract: The nuclear physics landscape has been redesigned as a sequence of effective field theories (EFTs) connected to the Standard Model through symmetries and lattice simulations of Quantum Chromodynamics (QCD). EFTs in this sequence are expansions around different low-energy limits of QCD, each with its own characteristics, scales, and ranges of applicability regarding energy and number of nucleons. We review each of the three main nuclear EFTs---Chiral, Pionless, Halo/Cluster---highlighting their similarities, differences, and connections. In doing so, we survey the structural properties and reactions of nuclei that have been derived from the ab initio solution of the few- and many-body problem built upon EFT input.

Colloquium: Spintronics in graphene and other two-dimensional materials

Abstract: Controlled spin transport in graphene and other two-dimensional materials has become increasingly promising for applications in devices. Of particular interest are custom-tailored heterostructures, known as van der Waals heterostructures, that consist of stacks of two-dimensional materials in a precisely controlled order. This Colloquium gives an overview of this developing field of spintronics and outlines the experimental and theoretical state of the art.

Evolution of shell structure in exotic nuclei

Abstract: The atomic nucleus is a quantum many-body system whose constituent nucleons (protons and neutrons) are subject to complex nucleon-nucleon interactions that include spin- and isospin-dependent components. For stable nuclei, several decades ago, emerging seemingly regular patterns in some observables could already be described successfully within a shell-model picture that results in particularly stable nuclei at certain magic fillings of the shells with protons and/or neutrons: N, Z=8, 20, 28, 50, 82, 126. However, in short-lived, so-called exotic nuclei or rare isotopes, characterized by a large N/Z asymmetry and located far from the valley of β stability on the nuclear chart, these magic numbers, viewed through observables, were shown to change. These changes in the regime of exotic nuclei offer an unprecedented view at the roles of the various components of the nuclear force when theoretical descriptions are confronted with experimental data on exotic nuclei where certain effects are enhanced. This article reviews the driving forces behind shell evolution from a theoretical point of view and connects this to experimental signatures.

Advances and challenges in single-molecule electron transport

Abstract: Electronic transport properties of single-molecule junctions have been widely measured by several techniques, including mechanically controllable break junctions, electromigration break junctions, and by means of scanning tunneling microscopes. In parallel, many theoretical tools have been developed and refined for describing such transport properties and for obtaining numerical predictions. Most prominent among these theoretical tools are those based upon density functional theory. In this review, theory and experiment are critically compared, and this confrontation leads to several important conclusions. The theoretically predicted trends nowadays reproduce the experimental findings well for series of molecules with a single well-defined control parameter, such as the length of the molecules. The quantitative agreement between theory and experiment usually is less convincing, however. Two main sources for the quantitative discrepancies can be identified. Experimentally, the atomic structure of the junction typically realized in the measurement is not well known, so simulations rely on plausible scenarios. In theory, correlation effects can be included only in approximations that are difficult to control for experimentally relevant situations. Therefore, one typically expects qualitative agreement with present modeling tools; in exceptional cases a quantitative agreement has already been achieved. For further progress, benchmark systems are required that are sufficiently well defined by experiment to allow quantitative testing of the approximation schemes underlying the theoretical modeling. Several key experiments can be identified suggesting that the present description may even be qualitatively incomplete in some cases. Such key experimental observations and their current models are also discussed here, leading to several suggestions for extensions of the models toward including dynamic image charges, electron correlations, and polaron formation.

The physics of climate variability and climate change

Abstract: The climate system is a forced, dissipative, nonlinear, complex and heterogeneous system that is out of thermodynamic equilibrium. The system exhibits natural variability on many scales of motion, in time as well as space, and it is subject to various external forcings, natural as well as anthropogenic. This paper reviews the observational evidence on climate phenomena and the governing equations of planetary-scale flow, as well as presenting the key concept of a hierarchy of models as used in the climate sciences. Recent advances in the application of dynamical systems theory, on the one hand, and of nonequilibrium statistical physics, on the other, are brought together for the first time and shown to complement each other in helping understand and predict the system's behavior. These complementary points of view permit a self-consistent handling of subgrid-scale phenomena as stochastic processes, as well as a unified handling of natural climate variability and forced climate change, along with a treatment of the crucial issues of climate sensitivity, response, and predictability.

Modes and states in quantum optics

Abstract: Quantum states of light are at the same time endowed with two superposition principles: the one of the classical Maxwell waves and the one of the quantum states occupying these waves. This article reviews the interplay between these two aspects of quantum optics. A summary of the description of multimode quantum states is presented along with an example of the characterization of correlations and entanglement with applications in metrology and quantum computation.

Strong-field nano-optics

Abstract: The present status and development of strong-field nano-optics, an emerging field of nonlinear optics, is discussed. A nonperturbative regime of light-matter interactions is reached when the amplitude of the external electromagnetic fields that are driving a material approach or exceed the field strengths that bind the electrons inside the medium. In this strong-field regime, light-matter interactions depend on the amplitude and phase of the field, rather than its intensity, as in more conventional perturbative nonlinear optics. Traditionally such strong-field interactions have been intensely investigated in atomic and molecular systems, and this has resulted in the generation of high-harmonic radiation and laid the foundations for contemporary attosecond science. Over the past decade, however, a new field of research has emerged, the study of strong-field interactions in solid-state nanostructures. By using nanostructures, specifically those made out of metals, external electromagnetic fields can be localized on length scales of just a few nanometers, resulting in signficantly enhanced field amplitudes that can exceed those of the external field by orders of magnitude in the vicinity of the nanostructures. This leads not only to dramatic enhancements of perturbative nonlinear optical effects but also to significantly increased photoelectron yields. It resulted in a wealth of new phenomena in laser-solid interactions that have been discovered in recent years. These include the observation of above-threshold photoemission from single nanostructures, effects of the carrier-envelope phase on the photoelectron emission yield from metallic nanostructures, and strong-field acceleration of electrons in optical near fields on subcycle timescales. The current state of the art of this field is reviewed, and several scientific applications that have already emerged from the fundamental discoveries are discussed. These include, among others, the coherent control of localized electromagnetic fields at the surface of solid-state nanostructures and of free-electron wave packets by such optical near fields, resulting in the creation of attosecond electron bunches, the coherent control of photocurrents on nanometer length and femtosecond timescales by the electric field of a laser pulse, and the development of new types of ultrafast electron microscopes with unprecedented spatial, temporal, and energy resolution. The review concludes by highlighting possible future developments, discussing emerging topics in photoemission and potential strong-field nanophotonic devices, and giving perspectives for coherent ultrafast microscopy techniques. More generally, it is shown that the synergy between ultrafast science, plasmonics, and strong-field physics holds promise for pioneering scientific discoveries in the upcoming years. (Less)

Colloquium : Statistical mechanics and thermodynamics at strong coupling: Quantum and classical

Abstract: The question of how classical systems approach thermal equilibrium is as old as the foundations of thermodynamics and statistical mechanics. How quantum systems decohere and thermalize is even more puzzling. This Colloquium provides an account of how the thermal equilibrium of a system is influenced by the presence of a thermal bath. It also gives a view of both classical and quantum aspects providing an understanding on the particularities of the quantum case. Moreover, a description of the challenges in the definition of heat from the perspective of fluctuating thermodynamical potentials is given. An old subject, perhaps, but definitely fundamental.

Colloquium: Linear in temperature resistivity and associated mysteries including high temperature superconductivity

Abstract: Even after much research and heated debate, high temperature superconductivity remains one of the most challenging and controversial problems in condensed matter. Apart from that at a phenomenological level some accepted and successful paradigms for the behavior of electrons fail, even the model relevant for the observed properties has been under debate, as well as methods to solve any model satisfactorily. In this Colloquium a perspective is provided into issues involved and results of a systematic solution on a model are reviewed and compared with experiments on an anomalous metallic state which is the basis for the emerging superconducting state.

Colloquium : Multiconfigurational time-dependent Hartree approaches for indistinguishable particles

Abstract: The problem of many interacting particles in classical mechanics is notoriously difficult. In quantum mechanics, indistinguishability makes the theoretical description of many-particle systems even more challenging. In this Colloquium a theory and associated practical numerical approaches to treat the time-dependent Schr\"odinger equation for identical interacting particles from first principles and their use in atomic and molecular systems are discussed.

Colloquium: Unusual dynamics of convection in the Sun

Abstract: The Sun, Earth's star, is of fundamental interest for life on our planet and remains a source of many scientific mysteries. The motion in its interior is complex and involves diverse physical phenomena at many scales, from nuclear to astronomical. In this Colloquium the unusual flow of mass and energy inside the convective region of our yellow star is discussed.

Hadronic structure in high-energy collisions

Abstract: Parton distribution functions (PDFs) describe the structure of hadrons as composed of quarks and gluons. They are needed to make predictions for short-distance processes in high-energy collisions and are determined by fitting to cross-section data. Definitions of the PDFs and their relations to high-energy cross sections are reviewed. The focus is on the PDFs in protons, but PDFs in nuclei are also discussed. The standard statistical treatment needed to fit the PDFs to data using the Hessian method is reviewed in some detail. Tests are discussed that critically examine whether the needed assumptions are indeed valid. Also presented are some ideas of what one can do in case tests indicate that the assumptions fail.

Colloquium : Bell’s theorem and locally mediated reformulations of quantum mechanics

Abstract: Quantum mechanics and relativity are two of the most profound theoretical developments of the 20th century. In 1964, John Bell proposed a test for quantum mechanics that shocked the theoretical community, showing that quantum mechanics implies a violation of locality, that is, action at a distance beyond the light-speed limits of relativity. In this Colloquium this fundamental problem is reviewed in a framework wider than the usual hidden-variable formulation, indicating an allowable ``continuous action'' option in addition to the standard action-at-a-distance approaches.

Colloquium: Heavy-electron quantum criticality and single-particle spectroscopy

Abstract: Quantum critical behavior occurs at low temperatures where quantum fluctuations become more important than thermal fluctuations. Of particular current interest is unconventional quantum criticality, which lacks a classical thermal counterpart. Angle resolved photoemission and scanning tunneling microscopy are experimental techniques that measure single-particle excitations, and recently been used to probe strongly correlated metals near quantum critical points Here, how the single-particle properties reflect the unconventional quantum criticality is discussed, and the prospect for further measurements to unearth new physical behavior.

Grand unified neutrino spectrum at Earth: Sources and spectral components

Abstract: We briefly review the dominant neutrino fluxes at Earth from different sources and present the Grand Unified Neutrino Spectrum ranging from meV to PeV energies. For each energy band and source, we discuss both theoretical expectations and experimental data. This compact review should be useful as a brief reference to those interested in neutrino astronomy, fundamental particle physics, dark-matter detection, high-energy astrophysics, geophysics, and other related topics.

Colloquium: Neutrino detectors as tools for nuclear security

Abstract: In 1930, Wolfgang Pauli proposed that conservation of energy and momentum in beta decay required the existence of a new particle: the neutrino. Since then experimental methods have matured to the point that neutrino detection now has an important practical application, namely, enhancing nuclear security. In this Colloquium the challenges and advances in the field of neutrino detection and their use in national security are discussed.

Theoretical perspectives on biological machines

Abstract: By operating out of equilibrium, nanoscale biological machines execute many functions that do not take place in abiotic systems. Statistical physics, physical chemistry, and polymer physics principles are needed in elucidating the rules and constraints governing large-scale structural changes that occur when metabolizing molecular fuel. This paper reviews the advances in theories rooted in coarse graining the complex systems while reminding us that molecular details must be an integral part of a deeper understanding of processes in living systems.

Light rays, singularities, and all that

Abstract: This article is an introduction to causal properties of General Relativity. Topics include the Raychaudhuri equation, singularity theorems of Penrose and Hawking, the black hole area theorem, topological censorship, and the Gao-Wald theorem. The article is based on lectures at the 2018 summer program Prospects in Theoretical Physics that was held at the IAS as well as the New Zealand Mathematical Research Institute summer school held in Nelson in January, 2020.

Models of polymer solutions in electrified jets and solution blowing

Abstract: Fluid flows hosting electrical phenomena are the subject of a fascinating and highly interdisciplinary scientific field. In recent years, the extraordinary success of electrospinning and solution-blowing technologies for the generation of polymer nanofibers has motivated vibrant research aiming at rationalizing the behavior of viscoelastic jets under applied electric fields or other stretching fields including gas streams. Theoretical models unveiled many original aspects in the underpinning physics of polymer solutions in jets and provided useful information to improve experimental platforms. This review examines advances in the theoretical description and numerical simulation of polymer solution jets in electrospinning and solution blowing. Instability phenomena of electrical and hydrodynamic origin, which play a crucial role in the relevant flow physics, are highlighted. Specifications leading to accurate and computationally viable models are formulated. Electrohydrodynamic modeling, theories on jet bending instability, recent advances in Lagrangian approaches to describe the jet flow, including strategies for dynamic refinement of simulations, and effects of strong elongational flow on polymer networks are reviewed. Finally, the current challenges and future perspectives in the field are outlined and discussed, including the task of correlating the physics of the jet flows with the properties of relevant materials, as well as the development of multiscale techniques for modeling viscoelastic jets.

Colloquium: Quantum limits to the energy resolution of magnetic field sensors

Abstract: Magnetometry, that is, the measurement of magnetic fields, has applications that range from brain imaging to exploration of the outer Solar System. The best magnetometry technologies achieve a sensitivity close to Planck's constant, the number that appears in the Heisenberg uncertainty relation. This Colloquium reviews what is known about quantum mechanical limits on field sensing, and identifies new sensing approaches that may break the current impasse in magnetic sensitivity.

Large D limit of Einstein’s equations

Abstract: Recent progress in taking the large dimension limit of Einstein’s equations is reviewed. Most of the analysis is classical and concerns situations where there is a black hole horizon, although various extensions that include quantum gravitational effects are discussed. The review consists of two main parts: the first is a discussion of general aspects of black holes and effective membrane theories in this large dimension limit, and the second is a series of applications of this limit to interesting physical problems. The first part includes a discussion of quasinormal modes that leads naturally into a description of effective hydrodynamiclike equations that describe the near-horizon geometry. There are two main approaches to these effective theories, a fully covariant approach and a partially gauge-fixed one, which are discussed in relation to each other. In the second part the applications are divided up into three main categories: the Gregory-Laflamme instability, black hole collisions and mergers, and the anti–de Sitter/conformal field theory correspondence (AdS/CFT). AdS/CFT posits an equivalence between a gravitational theory and a strongly interacting field theory, allowing the spectrum of applications to be extended to problems in hydrodynamics, condensed matter physics, and nuclear physics. The final, shorter part of the review describes further promising directions where there have been, as yet, few published research articles.

Thirty years of H 3 + astronomy

Abstract: This review covers the work of the three decades since the first spectroscopic identification of the H + 3 molecular ion outside of the laboratory in 1988, in the auroral atmosphere of the giant planet Jupiter. These decades have seen the astronomy related to this simple molecular ion expand to such an extent that a summary and evaluation of some 450 refereed articles is provided in the review. This enormous body of work has revealed surprises and illuminated the extensive role played by H + 3 in astrophysical environments in our Solar System and beyond. At the same time the physical chemistry and chemical physics of the molecule that has been revealed and studied during this time has proved to be fascinating and enabled high-resolution spectroscopy to benchmark its achievements against equally high-precision calculations. This review includes a brief look at some of the key foundational articles from before the original 1988 Jupiter detection (including the original 1911 ion discharge tube detection by J. J. Thomson and the key laboratory spectroscopy and quantum mechanics calculations on H + 3 structure and spectrum). The review explains the original detection and its serendipitous nature and looks at the astronomy that followed, all the way up to the latest results from NASA’s Juno mission. Also covered are the major advances in our understanding of the interstellar medium (known as ISM) that have resulted from the detection of H + 3 absorption lines there in 1996. The review closes by examining claims for the ion’s presence in other astrophysical environments and its potential role in the atmospheres of exoplanets and brown dwarfs.

Neutrinoless double-electron capture

Abstract: Efforts to understand the character of the neutrino, and searches for physics beyond the standard model, motivate several ongoing experiments to detect neutrinoless double-beta decay. The complementary process of double-electron capture has received less attention. Currently the limits on capture measurements are not competitive with the limits on decay measurements. With a look to future experiments, this review covers the current status, emphasizes the significant enhancements that can occur when a resonance condition exists, and provides a road map for future progress.

Colloquium : Quantum crystallizations of He 4 in superfluid far from equilibrium

Abstract: Bosonic helium is one of the most quantum mechanical materials ever studied in physics. It shows the famous Bose-Einstein condensation at extremely low temperatures. Although the equilibrium properties of boson liquids are now well understood, little is known about their nonequilibrium properties, especially the interfacial properties between superfluid and crystal phases far from equilibrium. In this Colloquium nonequilibrium crystal shapes in superfluid are discussed based on high-speed visualization techniques and the challenges related to the theoretical interpretation of the interfacial dynamics are presented.

Nobel Lecture: Plurality of worlds in the cosmos: A dream of antiquity, a modern reality of astrophysics

Abstract: The 2019 Nobel Prize for Physics was shared by James Peebles, Michel Mayor, and Didier Queloz. These papers are the text of the address given in conjunction with the award.

Nobel Lecture: 51 Pegasi b and the exoplanet revolution

Abstract: The 2019 Nobel Prize for Physics was shared by James Peebles, Michel Mayor, and Didier Queloz. These papers are the text of the address given in conjunction with the award.