Table of contents

These special focus articles in New Journal of Physics include selected articles focusing on the recently claimed discovery of the QCD (quantum chromo dynamics) phase transition in CERN-SPS heavy ion experiments, which continues to be investigated at higher energies at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and at the Large Hadron Collider (LHC) at CERN. RHIC, which started in June 2000, is the world's highest energy heavy ion collider with a maximum energy of 100+100 GeV per nucleon. Since then, four experiments have started data collection, and the first results have already been published. In 2005, the LHC will be commissioned and heavy ion collisions in the energy range 2.76+2.76 TeV per nucleon are expected to be mainly studied with the ALICE experiment.

The experimental program on heavy ion collision has the outstanding goal of searching for the QCD phase transition. This transition from deconfined quark and gluon matter, the so-called quark-gluon plasma (QGP), to colourless hadrons is believed to have happened in the early universe, a few microseconds after the Big Bang. Among the many phase transitions which occurred in the early universe, the QCD phase transition is the only one which is experimentally reproducible today. This is because the temperatures and energy densities needed for the transition can be reached in ultrarelativistic heavy ion collisions.

In collisions of heavy nuclei, for example lead or gold, one expects to create a multiparticle system at high enough initial energy density, temperature and volume, which undergoes a transition to a quark and gluon phase and subsequently hadronizes back to particles, which can be detected by experiments.

In recent years, compelling evidence for a QCD phase transition arose from experiments at the CERN SPS accelerator. They are summarized in the CERN press release (nucl-th/0002042) of 10 March 2000 on the CERN SPS results (experiments NA44, NA45, NA49, NA50, NA52, WA97/NA57 and WA98), where it is stated that 'a common assessment of the collected data leads us to conclude that we now have compelling evidence that a new state of matter has indeed been created ... in heavy ion collisions at the CERN SPS ... which features many of the characteristics of the theoretically predicted quark-gluon plasma'.

Clearly, much more experimental and theoretical work is needed in order to substantiate these findings. For example, no experimental data exist on open charm production, or on measurements near the expected critical energy density of 1 GeV fm^-3, or above the largest energy density reached at CERN of 3 GeV fm^-3. These topics will be addressed at low energies at the CERN SPS and at high energies at RHIC and LHC.

These focus papers contribute to the discussion and interpretation of the experimental data in view of the QCD phase transition. A wide spectrum of different points of view gives a 'dynamical picture' of the ongoing QGP searches today and their theoretical understanding.

Focus on Quark Gluon Plasma searches in heavy ion collisions contents

Hyperon enhancement in the dual parton model

A Capella and C A Salgado

On the role of energy conservation in high-energy nuclear scattering

H J Drescher, M Hladik, S Ostapchenko, T Pierog and K Werner

Simple predictions from ALCOR_c for rehadronization of charmed quark matter

P Lévai, T S Biró, T Csörgo and J Zimányi

Mapping out the QCD phase transition in multiparticle production

Sonja Kabana and Peter Minkowski

Microscopic coloured quark dynamics in the soft non-perturbative regime - description of hadron formation in relativistic S + Au collisions at CERN

S Scherer, M Hofmann, M Bleicher, L Neise, H Stöcker and W Greiner

Search for QGP and thermal freeze-out of strange hadrons

Giorgio Torrieri and Johann Rafelski

Charm and strangeness in nuclear reactions ats^1/2⩽ 19 GeV

Sonja Kabana

Klaus Pretzl and Sonja Kabana Laboratory for High Energy Physics, University of Bern, Switzerland

We study the production of charmed hadrons with the help of ALCOR_c, the algebraic coalescence model for rehadronization of charmed quark matter. Mesonic ratios are introduced as factors connecting various anti-baryon to baryon ratios. The resulting simple relations could serve as tests of quark matter formation and coalescence type rehadronization in heavy ion collisions.

We argue that the most commonly used models for nuclear scattering at ultra-relativistic energies do not treat energy conservation in a consistent fashion. Demanding theoretical consistency as a minimal requirement for a realistic model, we provide a solution for the above-mentioned problem, the so-called 'parton-based Gribov-Regge theory'.

In order to keep a clean picture, we do not consider secondary interactions. We provide a very transparent extrapolation of the physics of more elementary interactions towards nucleus-nucleus scattering, without considering any nuclear effects due to final state interactions. In this sense we consider our model a realistic and consistent approach to describe the initial stage of nuclear collisions.

We review the two sources of hyperon enhancement in the dual parton model: strings originating from diquark-antidiquark pairs in the nucleon sea and net baryons containing two or three sea quarks with a yield controlled by the observed stopping. We show that by adding final state interactions (including strangeness exchange reactions as well as the inverse reactions required by detailed balance) with a single averaged cross section, σ = 0.2 mb, we can explain the observed hyperon enhancement in PbPb collisions at CERN SPS.

We simultaneously measured the distance dependence of the force and the tunnelling current between a W(111) tip and a Au(111) sample in an ultrahigh vacuum atT = 150 K. The tip was characterized by field ion microscopy. Even at atomically close contact no evidence of structural instabilities was found. The scaling of the force curves show an unexpectedly long distance scaling parameter of λ = 0.2 nm. We conclude that not only the apex atoms contribute to the adhesion forces, but the first three layers play an almost equal role. Using a model that correlates the force and the tunnelling current, we are able to extract the tip density of states. Possible reasons for the long scaling length are discussed.

Novel methods are discussed for the state control of atoms coupled to single-mode and multi-mode cavities and microspheres. (1) Excitation decay control: The quantum Zeno effect, i.e. inhibition of spontaneous decay by frequent measurements, is observable in high-Q cavities and microspheres using a sequence of evolution-interrupting pulses or randomly-modulated CW fields. By contrast, in 'bad' cavities or open space, frequent measurements can only accelerate the decay, causing the anti-Zeno effect. (2) Location-dependent interference of decay channels: Control of two metastable states is feasible via resonant single-photon absorption to an intermediate state, by engineering spontaneous emission in a multimode cavity. (3) Decoherence control by conditionally interfering parallel evolutions: An arbitrary internal state of an atomic wavepacket can be protected from decoherence by interference of its interactions with the cavity over many different time intervals in parallel, followed by the detection of appropriate atomic-momentum observables. The arsenal of control methods described above can advance the state-of-the-art of quantum information storage and manipulation in cavities.

We study the plasmon resonances for small two-dimensional silver particles (nanowires) with elliptical or triangular shapes in the 20 nm size range. While the elliptical particle has only two resonances, a well known fact, we demonstrate that the triangular particle displays a much more complex behaviour with several resonances over a broad wavelength range. Using animations of the field amplitude and field polarization, we investigate the properties of these different resonances. The field distribution associated with each plasmon resonance can be related to the polarization charges on the surface of the particles. Implications for the design of plasmon resonant structures with specific properties, for example, for nano-optics or surface enhanced Raman scattering are discussed.

In this paper, we study in detail the intensity noise characteristics of vertical-cavity surface-emitting lasers (VCSELs). We demonstrate the possibility of generating intensity squeezed light with free-running or injection-locked VCSELs. Sub-shot noise operation results from very strong anticorrelations between the transverse modes. These anticorrelations have also been analysed through the transverse spatial distribution of the intensity noise. In the case of two transverse modes above threshold, our experimental results are found to be in good agreement with the predictions of a phenomenological model.

We have developed a waveguide for atoms based on magnetic confinement within a hollow glass fibre. Weak-field-seeking atoms are transported along a central hole and are prevented from hitting the wall by the magnetic field due to four current-carrying wires embedded in the fibre. A 'pinch' coil wound around the fibre plugs the guide at one end with a magnetic field strong enough to reflect the weak-field-seeking atoms. We have demonstrated that all the positive m_F sublevels of the ⁸⁵Rb F = 3 ground state are guided and have made a movie of the atom dynamics.

We study the dynamics of an elastic rod-like filament in two dimensions, driven by internally generated forces. This situation is motivated by cilia and flagella which contain an axoneme. These hair-like appendages of many cells are used for swimming and to stir surrounding fluids. Our approach characterizes the general physical mechanisms that govern the behaviour of axonemes and the properties of the bending waves generated by these structures. Starting from the dynamic equations of a filament pair in the presence of internal forces we use a perturbative approach to systematically calculate filament shapes and the tension profile. We show that periodic filament motion can be generated by a self-organization of elastic filaments and internal active elements, such as molecular motors, via a dynamic instability termed Hopf bifurcation. Close to this instability, the behaviour of the system is shown to be independent of many microscopic details of the active system and only depends on phenomenological parameters such as the bending rigidity, the external viscosity and the filament length. Using a two-state model for molecular motors as an active system, we calculate the selected oscillation frequency at the bifurcation point and show that a large frequency range is accessible by varying the axonemal length between 1 and 50 µm. We discuss the effects of the boundary conditions and externally applied forces on the axonemal wave forms and calculate the swimming velocity for the case of free boundary conditions.

Atomic hydrogen lines and line ratios are being used for diagnostics of technical plasmas in hydrogen or of edge plasmas in fusion research. In the presence of hydrogen molecules, dissociative excitation also contributes to this radiation. The H Lyman lines become optically thick quite easily, which modifies the excited-state population and ionization balance. Line ratios are then a function of electron temperature and density, but also of molecular densities and opacity. To quantify these effects, collisional-radiative population calculations were carried out for the conditions of technical low-pressure plasmas using the most recent hydrogen cross sections and population escape factors. The model for computing opacity is described and results are shown as a function of optical depth. Various spatial emission profiles and spectral line profiles can be included. These results allow the analysis of hydrogen lines from low-temperature plasmas. Measurements are presented, which were carried out in microwave discharges in mixtures of hydrogen or of deuterium and helium. Atom densities and dissociation degrees were determined from absolute Balmer line intensities and from line ratios. The effects of non-Maxwellian electron energy distributions are briefly discussed. The results demonstrate the influence of dissociative excitation and opacity. Taking into account these processes, very consistent results were obtained within the experimental error limits, thus confirming the analysis methods and the rate coefficients used. Dissociation degrees of 0.1-10% were measured depending on pressure and hydrogen concentration. For standard diagnostics, a suitable method can be chosen according to the experimental conditions.

Light has always been the most important carrier of information. Most of the information received by a human is visual. Technological breakthroughs have been made by changing from copper networks to networks based on optical data processing using laser light. In the last fifteen years, however, research has gone much further. We can now generate and control new sources of nonclassical light which are constructed from single atoms in a resonator. The systems for generating this new light are the micromaser in the microwave regime and the microlaser in the optical regime. Fundamental properties as well as possible applications have been investigated in a common European effort within the TMR networkMicrolasers and Cavity QED. Funded by the European Commission seventeen teams have been working together sharing their knowledge about the fundamental interaction of light with matter. These Focus articles will highlight several important findings of this collaboration.

From the selection of papers one can immediately see the wide range of research that has been done in this European project. New quantum effects have been found in the interaction of a single light mode with single atoms and new properties of the corresponding field states have been explored. The fundamental forces acting on atoms in a cavity field have been simulated. This will open new ways to cool an ensemble of atoms in a cavity. In addition it has been shown that cold atoms can be transported in a waveguide. Regarding semiconductor microlasers the quantum noise of vertical surface emitting lasers (VCSELs) has been understood experimentally as well as theoretically.

The focus articles also demonstrate that the ability to manipulate quantum characteristics of light will have direct applications in information technology. Systems based on cavity QED are candidates for future processing of quantum information. One of the most challenging tasks therefore is to understand the dissipative effects and to actively control the decay of atomic states. A clear understanding of all these decoherence effects then allows for the development of error avoiding quantum codes that stabilize quantum algorithms within the realm of cavity QED.

At this point we take the opportunity to thank the authors and the reviewers of these articles for preparing and ensuring high-quality presentations. We are convinced that this cluster of articles will give the readers a broad insight into an inspiring piece of modern physics.

Focus on Microlaser and Cavity QED Contents

Coherent states sometimes look like squeezed states and vice versa: the Paul trap Michael Martin Nieto and D Rodney Truax

Error avoiding quantum codes and dynamical stabilization of Grover's algorithm Michael Mussinger, Aldo Delgado and Gernot Alber

Schrödinger-cat entangled state reconstruction in the Penning trap Michol Massini, Mauro Fortunato, Stefano Mancini, Paolo Tombesi and David Vitali

Nonlocality of the Schrödinger cat Krzysztof Wódkiewicz

Driving atoms into decoherence-free states Almut Beige, Daniel Braun and Peter L Knight

Transport of cold atoms in a miniature guide M Key, W Rooijakkers and E A Hinds

Quantum noise in VCSELs J-P Hermier, I Maurin, E Giacobino, P Schnitzer, R Michalzik, K J Ebeling, A Bramati and A Z Khoury

A physical explanation of excess quantum noise due to non-orthogonal modes A M van der Lee, M P van Exter, N J van Druten and J P Woerdman

Collective light forces on atoms in a high-finesse cavity T Fischer, P Maunz, T Puppe, P W H Pinkse and G Rempe

Tunable whispering gallery modes for spectroscopy and CQED experiments Wolf von Klitzing, Romain Long, Vladimir S Ilchenko, Jean Hare and Valérie Lefèvre-Seguin

Matthias Freyberger and Wolfgang Schleich Abteilung für Quantenphysik, Universität Ulm, Germany

We describe the decoherence-free subspace of N atoms in a cavity, in which decoherence due to the leakage of photons through the cavity mirrors is suppressed. We show how the states of the subspace can be entangled with the help of weak laser pulses, using the high decay rate of the cavity field and strong coupling between the atoms and the resonator mode. The atoms remain decoherence-free with a probability which can, in principle, be arbitrarily close to unity.

The nonlocality of the Schrödinger cat is established. A correlation measurement that exhibits quantum interference in the form of the Wigner function of the cat state is introduced. The relation of this correlation to quantum interference in phase space and the nonlocal character of the Schrödinger cat are discussed. It is shown that the Schrödinger cat correlation function leads to violation of the Bell inequality for macroscopically distinguishable states of the cat.

We present a tomographic method for the reconstruction of the full entangled quantum state for the cyclotron and spin degrees of freedom of an electron in a Penning trap. Numerical simulations of the reconstruction of several significant quantum states show that the method turns out to be quite accurate.

Dynamical stabilization properties of error avoiding quantum codes are investigated beyond the perturbative regime. As an example Grover's search algorithm and its behaviour under a particular class of coherent errors are studied. Numerical examples which demonstrate that error avoiding quantum codes may be capable of stabilizing quantum algorithms well beyond the regime for which they were designed originally are presented.

Using the Paul trap as a model, we point out that the same wavefunctions can be variously coherent or squeezed states, depending upon the system to which they are applied.

The cross sections for the photodetachment from a weakly bound positronium negative ion Ps^- below the threshold for the formation of Ps(n = 4) are calculated using the hyperspherical close-coupling method, and are compared with the corresponding spectra for the H^- ion. Detailed resonance structures in the spectra near the Ps(n = 2, 3 and 4) thresholds are reported for the first time. The off-resonance cross section below the Ps(n = 2) threshold differs appreciably from that obtained by a variational calculation (Ward et al 1987 J. Phys. B: At. Mol. Phys.20 127), but agrees well with the recent close-coupling calculations with a B-spline expansion (Igarashi et al 2000 Phys. Rev. A 61 032 710). The resonance energies and widths of the ¹P^o symmetry are generally in good agreement with the results of the complex-coordinate rotation calculation.

We utilized a local density functional potential, the linear combination of atomic orbital (LCAO) method, and the BZW procedure to study the electronic structure of 3C- and 4H-SiC. We present the calculated energy bands, band-gaps, effective masses of n-type carriers, and critical point transition energies. There is good agreement between the calculated electronic properties and experimental results. Our preliminary total energy calculations for 3C-SiC found an equilibrium lattice constant of a = 4.35 Å, which is in agreement with the experimentally measured value of 4.348 Å. The calculated charge transfers indicate that each silicon atom loses about 1.4 electrons that are gained by a carbon atom in both 3C- and 4H-SiC.

This `Focus on Dark Matter' cluster of articles is the very first in a series of featured topics to appear in New Journal of Physics that will be of great interest to the physics community. The idea is to bring together contributions from leading researchers in topical fields to provide insights into the key issues for both experts and non-specialists alike.

Investigations of dark matter in the universe are of fundamental interest to astronomers, astrophysicists, cosmologists, and nuclear and particle physicists. Recent developments have provided new insight into the matter budget of the universe. Using supernovae of type 1a as standard candles, distance measurements indicate an accelerating expansion of the universe. This result suggests a non-zero cosmological constant, and the resulting vacuum energy density contributes 70% of the critical mass of the universe. Gravitational lensing, originally proposed by Einstein and later utilized for mass determination of galactic clusters by F Zwicky and others, has recently been used to map dark matter in clusters and large-scale structures. These experiments show that 30% of the critical mass is contained in these structures, of which only 3% is visible. Since the baryonic component of matter is only 6% of the critical mass, assuming a Hubble constant of 60 km s^-1 Mpc^-1, there is five times more dark matter than baryonic matter in the universe. Prime candidates for making up this deficit are neutrinos with mass (hot dark matter) and weakly interacting massive particles, the so-called WIMPs (cold dark matter). Cosmologists and particle physicists favour the latter to be the lightest supersymmetric particle, the so-called neutralino, since if it does exist it would solve two problems at the same time, namely cold dark matter and grand unification theories. Cosmic background microwave radiation data and large-scale structure distributions, however, favour a cocktail of 30% hot and 70% cold dark matter.

Treumann's group investigates matter distributions in galactic clusters, including massive neutrinos and collisional intracluster gas emitting x-ray radiation. The issue of baryonic dark matter in our galactic halo is explored in the contribution from De Paolis and coworkers. They show that the observed diffuse gamma-ray emission from the galactic halo can be explained by cosmic ray proton scattering on clouds of dark clusters of brown dwarfs and cold self-gravitating H₂ clouds in the halo. New experimental results on terrestrial WIMP searches via WIMP-Xe inelastic scattering are presented by Bernabei's group. The contribution from Cebrián's team describes present and future WIMP experiments using conventional and cryogenic detectors in the Canfranc underground laboratory. An interesting novel large-mass, low-background, superheated droplet detector for WIMP searches is described by Collar and colleagues.

Focus on Dark Matter Contents

Neutrino dark matter in clusters of galaxies

R A Treumann, A Kull and H Böhringer

Gamma ray emission from a baryonic dark halo

F De Paolis, G Ingrosso, Ph Jetzer and M Roncadelli

Cold dark matter searches at the Canfranc underground laboratory

S Cebrián, E García, D Gonzalez, I G Irastorza, A Morales, J Morales, A Ortiz, A Peruzzi, J Puimedon, M L Sarsa, S Scopel and J A Villar

Prospects for SIMPLE 2000: a large-mass, low-background superheated droplet detector for WIMP searches

J I Collar, J Puibasset, T A Girard, D Limagne, H S Miley and G Waysand

Improved limits on WIMP-¹²⁹Xe inelastic scattering

R Bernabei, P Belli, R Cerulli, C J Dai, G Ignesti, A Incicchitti, F Montecchia and D Prosperi

Klaus Pretzl Laboratory for High Energy Physics, University of Bern, Switzerland

Additional Information on Dark Matter The Focus on Dark Matter articles address one of the most important questions in modern cosmology, astronomy, astrophysics, nuclear and particle physics. These articles bring together contributions from leading researchers, providing an overview for experts as well as for non specialists. There is a wealth of additional information also available. A selection of related information from IOPP books, magazines and other IOPP journals is shown below.

Books ... Dark Matter in Astrophysics and Particle Physics 1998: Proceedings of the Second International Conference on Dark Matter in Astro and Particle Physics, held in Heidelberg, Germany, 20-25 July 1998, ed H V Klapdor-Kleingrothaus and L Baudis. This book contains the invited papers presented at the second international conference which brought together leading researchers from around the world to review recent progress and future directions for research in the different approaches to the dark matter problem. Read more about this book at bookmark.iop.org. Special Offer - 25% discount (whilst stocks last) - available only to readers of the NJP 'Focus on Dark Matter' articles. To obtain this discounted price please state on your order form that you read about the book in NJP.

News items from Physics World ... Boomerang backs flat universe: The faint microwave glow left over from the big bang has been measured with unprecedented precision, giving astronomers a new insight into the nature of the universe...

The search for dark matter: Experiments housed deep underground are searching for new particles that could simultaneously solve one of the biggest mysteries in astrophysics and reveal what lies beyond the Standard Model of particle physics...

Physics gets dark and exotic: Two of the outstanding challenges in physics identified in our millennium survey last month were the nature of "dark matter" and a proper understanding of nuclear structure. This month we look at these challenges in greater detail...

Dark-matter dispute intensifies: Recent results from a dark-matter experiment in Italy suggest that the elusive weakly interacting massive particle or WIMP has finally been detected ­ but a rival experimental collaboration in the US disagrees...

Calling all cosmophysicists: [20 Apr 2000] Over 100 astrophysicists, particle physicists and space scientists attended a workshop on fundamental physics in space organized by the European Space Agency (ESA) and CERN in Geneva last month. The aim of the meeting was to encourage greater interactions between the different communities with a view to generating proposals to test the fundamental laws of physics in space...

Dark matter claim meets resistance: [25 Feb 2000] Physicists working at the Gran Sasso underground laboratory claim to have observed the first direct evidence for dark matter particles in experiments. The so-called weakly interacting massive particle or WIMP has a mass about 50 times the mass of the proton. If confirmed the findings will have immense implications for particle physics and cosmology...

Articles from other IOPP journals ...Non-baryonic dark matter: observational evidence and detection methods. Lars Bergström 2000 Rep. Prog. Phys. 63 793-841 The evidence for the existence of dark matter in the universe is reviewed. A general picture emerges, where both baryonic and non-baryonic dark matter is needed to explain current observations...

Improved limits on WIMP-¹²⁹Xe inelastic scattering have been obtained by analysing a statistics of ≃2500 kg day, collected by the low radioactivity ≃6.5 kg DAMA liquid xenon scintillator deep underground in the Gran Sasso National Laboratory of the INFN.

The Superheated Instrument for Massive Particle searches (SIMPLE 2000) will consist of an array of 8-16 large active mass (≃15 g) superheated droplet detectors (SDDs) to be installed in the new underground laboratory of Rustrel-Pays d'Apt. Several factors make the use of SDDs an attractive approach for the detection of weakly interacting massive particles (WIMPs), namely their intrinsic insensitivity to minimally ionizing particles, high fluorine content, low cost and operation at near ambient pressure and temperature. We comment here on the fabrication, calibration and already-competitive first limits from prototype SDDs for SIMPLE, as well as on the expected immediate increase in sensitivity of the programme, which aims at an exposure of > 25 kg day during 2000. The ability of modest-mass fluorine-rich detectors to investigate regions of neutralino parameter space beyond the reach of the most ambitious cryogenic projects is pointed out.

An overview of the searches for weak interacting massive particles (WIMPs) through detection of their scattering off various target nuclei carried out in the Canfranc Tunnel Astroparticle Laboratory (at 675 and at 2450 metres of water equivalent (m.w.e.)) is given. The main experimental results both for conventional (COSME, IGEX and NaI-32) and for cryogenic detectors (ROSEBUD) are sketched, and a briefing on the forthcoming experiment ANAIS is also presented. The results of a solar axion search are also reported.

A re-analysis of EGRET data by Dixon et al has led to the discovery of a statistically significant diffuse γ-ray emission from the galactic halo. We show that this emission can naturally be accounted for within a previously proposed model for baryonic dark matter, according to which dark clusters of brown dwarfs and cold self-gravitating H₂ clouds populate the outer galactic halo and can show up in microlensing observations. Basically, cosmic-ray protons in the galactic halo scatter on the clouds clumped into dark clusters, giving rise to the observed γ-ray flux. We derive maps for the corresponding intensity distribution, which turn out to be in remarkably good agreement with those obtained by Dixon et al. We also address future prospects to test our predictions.

We present a model calculation for the radial matter density and mass distribution in two clusters of galaxies (Coma and A119) including cold dark matter, massive though light (≃2 eV) neutrino dark matter and collisional intra-cluster gas which emits x-ray radiation. The calculation uses an extension of the Lynden-Bell statistics to the choice of constant masses instead of constant volume. This allows proper inclusion of mixtures of particles of various masses in the gravitational interaction. When it is applied to the matter in the galaxy cluster the radial ROSAT x-ray luminosity profiles can be nicely accounted for. The result is that the statistics identifies the neutrino dark matter in the cluster centre as being degenerate in the sense of Lynden-Bell's spatial degeneracy. This implies that it is distributed in a way different from the classical assumption. The best fits are obtained for the ≃2 eV neutrinos. The fraction of these and their spatial distribution are of interest for understanding cluster dynamics and may have cosmological implications.

Much of the current understanding of logic gates for quantum computation relies on the application of one- and two-qubit gates in order to implement a universal set of logic gates, as well as other gates. Such ideas stem from the notion that the Hamiltonians available for quantum computation are one or two-body processes. However, present day NMR implementations of quantum information processing (QIP) rely on the internal Hamiltonian of a liquid state system in which there are many two-body processes that occur simultaneously. Such use of the internal Hamiltonian allows for the creation of `multiqubit' logic gates, i.e. logic gates that operate on many qubits simultaneously and are more efficient than a sequence of one- and two-qubit rotations that effect the same operation. Such larger qubit operations offer a universal set of gates (even when not all couplings within the internal Hamiltonian are readily accessible) as well as more convenient and efficient implementations of the Hadamard transform, the controlled-NOT gate, and a quantum Fourier transform that scales linearly (assuming all couplings can be turned on and off at will).

Magnetic impurity effects on metallic carbon nanotubes are studied theoretically. The resolvent method for the multichannel Kondo effect is applied to the band structure of the k · p perturbation Hamiltonian in the limit of infinite onsite repulsion at the impurity site. We discuss the local non-Fermi liquid behaviour at temperatures below the Kondo temperature T_K. The density of states of localized electrons has a singularity at about |ω|^1/2, which gives rise to a pseudogap at the Kondo resonance at low temperatures. The temperature dependence of the electronic resistivity is predicted to be T ^1/2, and the imaginary part of the dynamical susceptibilities is dependent on |ω|^1/2. Possible experimental observations are discussed.

In order to investigate the electronic characteristics of ferroelectric thin films that behave as semiconductors, we derive an equation to calculate the band bending in ferroelectrics, to replace the Poisson equation. We find that the space-charge density term, , in the Poisson equation should be replaced by - P_r , where P_ris the remanent polarization. In order to simplify the band bending calculation, only one dimension is considered here. For example, in the case of the 3 V band bending in a metal-semiconductor Schottky barrier junction with a 1 µm thick ferroelectric semiconductor, the depletion region extends over the whole semiconductor, even with a dielectric constant of 10 and a donor density of 1 × 10¹⁷cm^-3due to dP_r/dx . However, the depletion layer width in a semiconductor without remanent polarization is only about 0.18 µm under the same conditions.

Calculations of the hydrogen molecular continuum , where the latter is repulsive) have been carried out in the wavelength range 120 - 600 nm, based on potential curves and reduced masses. They give transition probabilities, lifetimes and spectral intensities as a function of vibrational level population in the upper state . Absolute radiation measurements in H₂ /He and D₂ /He low-pressure ECR plasmas in the wavelength range 170 - 300 nm are compared with results from these calculations. Using the Franck-Condon principle and the corona model for excitation, the relative vibrational population in the upper state is correlated with the population in the ground state, characterized by the vibrational temperature T_vib . The absolute intensities depend on the electron temperature T_evia the electron excitation rate coefficients. Therefore, the shape of the continuum radiation reflects T_vib , and the absolute value is a function of T_e . The results are in good agreement with those from other spectroscopic techniques (T_efrom He line intensities, T_vibfrom Fulcher band radiation) in the visible spectral range. This demonstrates that emission spectroscopy of the continuum radiation of H₂and D₂is a good tool for diagnostics of low-pressure plasmas. The calculations have also been carried out for other hydrogen isotopes (T₂ , HD, DT).

We implement an ensemble quantum counting algorithm on three NMR spectrometers with ¹ H resonance frequencies of 500, 600 and 750 MHz. At higher frequencies, the results deviate markedly from naive theoretical predictions. These systematic errors can be attributed almost entirely to off-resonance effects, which can be substantially corrected for using fully compensating composite rotation pulse sequences originally developed by Tycko. We also derive an analytic expression for generating such sequences with arbitrary rotation angles.

A first-order perturbative theory of the motion of a harmonic oscillator interacting with a weak arbitrary force field is presented, as it pertains to dynamic force microscopy. In essence the theory corresponds to a Born approximation for the scattering of standing waves trapped in a perturbed parabolic potential. In particular, it is shown that the scattering amplitudes are related to corresponding moments, involving Chebyshev polynomials and associated metrics of the conservative interaction force, and of a generalized friction coefficient accounting for irreversible interactions. Implications for dynamic force microscopy are discussed.

The dynamics of many different quantum systems is characterized by a regular net of minima and maxima of probability stretching out in a spacetime representation. We offer an explanation to this phenomenon in terms of the Wigner function. This approach illustrates very clearly the crucial role played by interference.

Using approximate techniques we study the final moments of the collision of two (individually non-spinnning) black holes which inspiral into each other. The approximation is based on treating the whole spacetime as a single distorted black hole. We obtain estimates for the radiated energy, angular momentum and waveforms for the gravitational waves produced in such a collision. The results can be of interest for analysing the data that will be forthcoming from gravitational wave interferometric detectors, like the LIGO, GEO, LISA, VIRGO and TAMA projects.

We present measurements on microwave spectroscopy on a double quantum dot with an on-chip microwave source. The quantum dots are realized in the two-dimensional electron gas of an AlGaAs/GaAs heterostructure and are weakly coupled in series by a tunnelling barrier forming an `ionic' molecular state. We employ a Josephson oscillator formed by a long Nb/Al-AlO_x /Nb junction as a microwave source. We find photon-assisted tunnelling sidebands induced by the Josephson oscillator, and compare the results with those obtained using an externally operated microwave source.

A combined statistical analysis of the experimental results of the LSND and KARMEN _{µ e}oscillation search is presented. LSND has evidence for neutrino oscillations that is not confirmed by the KARMEN experiment. However, there is a region in the (sin² (2 ), ) parameter space where the results of both experiments are statistically compatible. This joint analysis is based on likelihood functions for both data sets. A frequentist approach to creating Monte Carlo samples analogous to the experimental outcome is applied to deduce correct confidence limits. Different schemes of combination can be chosen to provide correct coverage which leads to slightly different confidence regions in (sin² (2 ), ).