Showing papers in "Contemporary Physics in 2008"
TL;DR: This review is an introduction to Bayesian methods in cosmology and astrophysics and recent results in the field, and presents Bayesian probability theory and its conceptual underpinnings, Bayes' Theorem and the role of priors.
Abstract: The application of Bayesian methods in cosmology and astrophysics has flourished over the past decade, spurred by data sets of increasing size and complexity. In many respects, Bayesian methods have proven to be vastly superior to more traditional statistical tools, offering the advantage of higher efficiency and of a consistent conceptual basis for dealing with the problem of induction in the presence of uncertainty. This trend is likely to continue in the future, when the way we collect, manipulate and analyse observations and compare them with theoretical models will assume an even more central role in cosmology. This review is an introduction to Bayesian methods in cosmology and astrophysics and recent results in the field. I first present Bayesian probability theory and its conceptual underpinnings, Bayes' Theorem and the role of priors. I discuss the problem of parameter inference and its general solution, along with numerical techniques such as Monte Carlo Markov Chain methods. I then review the th...
962 citations
TL;DR: A review of the recent theoretical and experimental advances in this exciting new field of quantum optical metrology, focusing on examples that exploit a particular two-mode entangled photon state, the High-N00N state, is given in this article.
Abstract: Quantum states of light, such as squeezed states or entangled states, can be used to make measurements (metrology), produce images, and sense objects with a precision that far exceeds what is possible classically, and also exceeds what was once thought to be possible quantum mechanically. The primary idea is to exploit quantum effects to beat the shot-noise limit in metrology and the Rayleigh diffraction limit in imaging and sensing. Quantum optical metrology has received a boost in recent years with an influx of ideas from the rapidly evolving field of optical quantum information processing. Both areas of research exploit the creation and manipulation of quantum-entangled states of light. We will review some of the recent theoretical and experimental advances in this exciting new field of quantum optical metrology, focusing on examples that exploit a particular two-mode entangled photon state – the High-N00N state.
605 citations
TL;DR: In this paper, the spectral phase of the harmonics of the photoelectron spectra at 2.6 and 3.6 µm has been studied and it has been shown that strong field interaction still keeps some secrets after more than a decade.
Abstract: The recent development of intense, ultrashort, table top lasers in the mid-infrared opens new avenues for research in strong field atomic physics. Electrons submitted to such radiation acquire huge quiver energies, even at moderate intensity and interesting properties arise: first, the wavelength offers a convenient experimental knob to tune the ionisation regime by controlling the Keldysh parameter. Second, many processes like above-threshold ionisation or high harmonic generation, whose characteristics depend directly on this energy, can be pushed to unprecedented limits. Third, the wavelength controls the spectral phase of the harmonics and hence the possibility to improve the generation of pulses in the attosecond regime. Recent studies of rare gas and alkali atoms' photoelectron spectra and harmonic generation at 2 and 3.6 μm have begun to confirm the theoretical predictions. However, unexpected features have also been found showing that strong field interaction still keeps some secrets after more th...
68 citations
TL;DR: In this paper, a fully quantum-mechanical formalisation of the recollision model in terms of the lowest-order S-matrix element for this process is introduced and its limitations are discussed.
Abstract: Double and multiple ionisation cannot be understood without the contribution of a non-sequential mechanism, which is due to electron–electron correlation. This paper discusses the experimental manifestations of this effect, from its first footprint in the total yields of multiply charged ions to the highly differential cross-sections for ion and electron production, and the theoretical models that have been set up for their description. Except for the ab initio solution of the time-dependent Schrodinger equation, most models implement the recollision mechanism. The kinematical constraints imposed by the former are extensively discussed. A fully quantum-mechanical formalisation of the recollision model in terms of the lowest-order S-matrix element for this process is introduced and its limitations are discussed. Various simplifications are proposed that drop more and more of the quantum features. This is contrasted with purely classical trajectory descriptions with no approximations to the dynamics.
56 citations
TL;DR: In this article, the authors discuss the properties of the interface and the physical mechanisms leading to the anomalous transparency of a water-air interface and show that the power flux in the wave transmitted into air increases dramatically when a compact underwater sound source approaches the interface within a fraction of wavelength.
Abstract: A water–air interface is usually an almost perfect reflector of acoustic waves. It was found recently that the interface becomes anomalously transparent and the power flux in the wave transmitted into air increases dramatically when a compact underwater sound source approaches the interface within a fraction of wavelength. The phenomenon is robust with respect to the roughness of the interface and to the variation of air and water parameters and may have significant geophysical and biological implications. This review article discusses the properties of the interface and the physical mechanisms leading to the anomalous transparency.
49 citations
TL;DR: This article gives a simple introduction to feedback control in quantum systems, and then describes verification theorems and viscosity solutions in simple language and illustrates their use with a concrete example of current interest.
Abstract: While feedback control has many applications in quantum systems, finding optimal control protocols for this task is generally challenging. So-called ‘verification theorems’ and ‘viscosity solutions’ provide two useful tools for this purpose: together they give a simple method to check whether any given protocol is optimal, and provide a numerical method for finding optimal protocols. While treatments of verification theorems usually use sophisticated mathematical language, this is not necessary. In this article we give a simple introduction to feedback control in quantum systems, and then describe verification theorems and viscosity solutions in simple language. We also illustrate their use with a concrete example of current interest.
38 citations
TL;DR: Luminescence data offer delicate probes of changes in structure and local environment in insulating materials as discussed by the authors, and therefore they have long been employed in studies of imperfections and characterisation of lattice distortions.
Abstract: Luminescence data offer delicate probes of changes in structure and local environment in insulating materials. Therefore they have long been employed in studies of imperfections and characterisation of lattice distortions. Luminescence techniques are inherently very sensitive so respond to small concentrations of intrinsic defects and impurities, and intentionally added probe ions, such as rare earth or chromium ions, are particularly effective at displaying responsive changes linked to modified structural environments. Parameters of interest are variations in luminescence efficiency, the details of the emission spectra, polarisation, temperature dependence and changes in excited state lifetimes. These are suitable properties to monitor variations in both short and long range lattice structure, composition, pressure and temperature. For modern photonic materials such probes are powerful tools to follow changes introduced by processing to make waveguides, surface layers and new materials. The use of differ...
34 citations
TL;DR: LSA brings the principles of biometric identification currently used in human fingerprint- or iris-recognition to the world of inanimate objects such as paper documents, plastic cards, high value branded products and a range of other things that are commonly counterfeited or smuggled.
Abstract: A laser-based security technique called Laser Surface Authentication (LSA) is described in which diffuse laser scattering is used as a probe of the naturally-occurring randomness present in virtually all surfaces. LSA brings the principles of biometric identification currently used in human fingerprint- or iris-recognition to the world of inanimate objects such as paper documents, plastic cards, high value branded products and a range of other things that are commonly counterfeited or smuggled.
33 citations
TL;DR: In this paper, the authors developed a model for mechanical erosion that explains the main field observations, and opens alternative ways to describe erosion by flowing lava, and found a lava channel whose features cannot be explained in the frame of thermal erosion.
Abstract: Hot lava is a viscous fluid that, driven by gravity, moves along the Earth's surface. Intuitively, one attributes constructional properties to lava–it accumulates in volcanic landforms, compound lava fields and, in the end, entire mountains. On the other hand, there are also examples of the erosive power of lava: on Earth and especially on other planets in the Solar System, there exist channels incised by flowing lava. The origins of these erosive features have long been debated among volcanologists and planetologists. The dominant paradigm is thermal erosion, although it leaves many questions open. After the 2001 eruption on Mount Etna we found a lava channel whose features cannot be explained in the frame of thermal erosion. On the basis of our observations, we have developed a model for mechanical erosion that explains the main field observations, and opens alternative ways to describe erosion by flowing lava.
24 citations
TL;DR: The main characteristic of a granular gas is its tendency to form clusters, i.e. to spontaneously separate into dense and dilute regions as discussed by the authors, which can be interpreted as a separation in cold and hot regions, meaning that Maxwell's demon is at work.
Abstract: The main characteristic of a granular gas, which makes it fundamentally different from ordinary molecular gases, is its tendency to form clusters, i.e. to spontaneously separate into dense and dilute regions. This can be interpreted as a separation in cold and hot regions, meaning that Maxwell's demon is at work: this demon – notoriously powerless in any system in thermodynamic equilibrium – makes clever use of the non-equilibrium state of affairs that reigns in a granular gas, with on the one hand an external energy source and on the other a continuous loss of energy due to the inelastic particle collisions. We focus on vibrated compartmentalised systems, because these give a particularly clear-cut view of the clustering process and also because they resemble the typical machinery used in industrial applications to sort and transport granular materials. We discuss how the clustering can be exploited to build a Brownian motor, a fountain, a granular clock, and how it gives insight into a related clusteri...
22 citations
TL;DR: Femtosecond laser ablation is a powerful tool in basic biological research as discussed by the authors, where the high peak light intensity generated by a tightly focused, ultrashort, pulse of infrared laser light enables versatile submicron ablation deep within biological samples.
Abstract: Femtosecond lasers are emerging as a powerful tool in basic biological research. The high peak light intensity generated by a tightly focused, ultrashort, pulse of infrared laser light enables versatile submicron ablation deep within biological samples. Recent studies have begun to exploit these capabilities to conduct meticulous laser surgery experiments within single cells, as well as within intact organisms. This review will discuss the basic physical mechanisms behind femtosecond laser ablation in biological samples. It will then examine a series of prominent applications in biology and how they are opening new possibilities in a range of research fields. The interface between physics and biology has been exceptionally fruitful over recent years and femtosecond laser ablation is proving to be another prime example of this.
TL;DR: In this article, the use of laser-induced breakdown spectroscopies for the analysis and restoration of artworks is discussed, in particular of painted artworks and parchments.
Abstract: The study, preservation and restoration of cultural heritage materials and objects are crucial for obtaining important historical information, maintaining their aesthetic appeal or understanding early technologies. The unique, highly diverse character of cultural heritage items impose the need for rigorous scientific approaches, often incorporating non-invasive, mobile or remote technologies and the valorisation of objects and preservation methodologies. Here, we overview the fundamental aspects in the use of lasers for the analysis and restoration of artworks. For the former purpose, emphasis is placed on the use of Raman and Laser-Induced-Breakdown spectroscopies, both of which have been of increasing importance in the field of Cultural Heritage. The use of lasers for the restoration of artworks, in particular of painted artworks and parchments are described. Studies on model polymeric systems, as well as realistic artefacts, have permitted detailed assessment of the mechanisms and of the effects of the...
TL;DR: In this article, the authors discuss the role of coronal mass ejections (CMEs) in the Sun-Earth connection, and predict the ever changing environmental conditions in outer space due to solar eruptive events.
Abstract: Ejections of magnetised plasma from the Sun, commonly known as coronal mass ejections (CMEs), are one of the most stunning manifestations of solar activity. These ejections play a leading role in the Sun–Earth connection, because of their large-scale, energetics and direct impact on the space environment near the Earth. As CMEs evolve in the solar corona and interplanetary space they drive shock waves, which act as powerful accelerators of charged particles in the inner solar system. Some of these particles, known as solar energetic particles (SEPs), can strike our planet, and in doing so they can disrupt satellites and knock out power systems on the ground, among other effects. These particles, along with the intensive X-ray radiation from solar flares, also endanger human life in outer space. That is why it is important for space scientists to understand and predict the ever changing environmental conditions in outer space due to solar eruptive events – the so-called space weather. To enable the develop...
TL;DR: A review of The Science of Low Energy Nuclear Reaction: A Comprehensive Compilation of Evidence and Explanations about Cold Fusion, by E.C. McKubre and D.J. Nagel, Sin... as mentioned in this paper.
Abstract: A review of The Science of Low Energy Nuclear Reaction: A Comprehensive Compilation of Evidence and Explanations about Cold Fusion, by E. Storms with Forewords by M.C.H. McKubre and D.J. Nagel, Sin...
TL;DR: The axion is a light pseudoscalar particle predicted to exist as a consequence of the Peccei-Quinn solution to the strong-CP problem as discussed by the authors.
Abstract: The axion is a light pseudoscalar particle predicted to exist as a consequence of the Peccei–Quinn solution to the strong-CP problem. Its abundant production in the early Universe along with its stability and insignificant interaction cross-section make it a prime dark matter candidate. This report summarises the search for dark matter axions using resonant microwave cavities as well as searches for axions produced in the Sun and in the laboratory. All of these current experiments focus on the axion–photon interaction, as coherent axion–photon mixing in strong magnetic fields of large spatial extent can make up for the extraordinary weakness of the the coupling of axions to photons.
TL;DR: In this article, the experimental realisation of low energy antihydrogen, via the mixing of carefully prepared clouds of positrons and antiprotons, and some of the progress that has been made in the last few years in characterising properties of the nascent anti-atoms are discussed.
Abstract: The creation of atoms of antihydrogen under controlled conditions has opened up a new era in physics with antimatter. We describe the experimental realisation of low energy antihydrogen, via the mixing of carefully prepared clouds of positrons and antiprotons, and some of the progress that has been made in the last few years in characterising properties of the nascent anti-atoms. Ongoing efforts aimed at trapping the anti-atoms in magnetic field minima are discussed. Some of the motivations for undertaking experiments with antihydrogen are presented.
TL;DR: In this article, the authors proposed an NMR method to implement such exponential sums and presented a brief summary and an outlook on the main idea of factorization with Gauss sums, which relies on the truncated Gauss sum.
Abstract: idea of factorizing numbers with Gauss sums. Motivated by the questionof resources we then turn in Section 3 to exponential sums. Finally, inSection 4 we propose an NMR method to implement such exponential sums.We conclude in Section 5 by presenting a brief summary and an outlook.2. Factorization with Gauss sums: the central ideaOur factorization algorithm relies on the truncated Gauss sum
TL;DR: Professor Abdus Salam, Nobel Laureate in Physics 1979, attends the Symposium on Contemporary Physics to celebrate the opening of the new International Centre for Physics at Quaid-I-Azam University, Islamabad.
Abstract: First I should like to record how happy and honoured I am to be able to attend the Symposium on Contemporary Physics to celebrate the opening of the new International Centre for Physics at Quaid-I-Azam University, Islamabad. This imaginative project, now made concrete with superb buildings for both science and accommodation of scientists, should promote new activities in Pakistan and internationally. It is a venture, which would have given great pleasure to Professor Abdus Salam, Nobel Laureate in Physics 1979, who throughout his scientific career fought to enhance scientific research and its applications in the developing world.