Table of contents

In 2005 we celebrate the hundredth anniversary of the publication of five seminal papers by Albert Einstein establishing the basis of three fundamental fields of physics: the theory of relativity, quantum theory and the theory of Brownian motion. This happened at a time when the reality of atoms and molecules was still far from certain. The microscopic view Einstein took of molecular motion led to the calculation of Avogadro's number and the size of molecules by studying the motion of small particles in liquids. Combining kinetic theory and classical thermodynamics finally led Einstein to the conclusion that the displacement of a microparticle under Brownian motion varies as the square root of time. The experimental demonstration of this law three years later was considered as the striking proof that atoms and molecules are physically real.

Today single atoms are probed directly in experiments, and we are able to observe the individual paths of colliding or dissociating particles. These new techniques are described in some of the papers of this issue in a very impressive way. An even more exciting development was initiated with the demonstration of Bose–Einstein condensation of dilute gases of ultracold atoms, a phenomenon first predicted by Einstein in papers published between 1916 and 1924.

The first paper of Einstein's famous series of 1905, 'On an heuristic point of view about the creation and conversion of light', was an explanation of the photoelectric effect, demonstrated in several experiments over the previous few years. The explanation was based on the quantum hypothesis introduced by Max Planck five years earlier, and was considered as an additional and important proof of Planck's hypothesis. Today quantum theory and photons are the basis for much of modern science and technology. We have learned to experiment with single photons, and we have demonstrated the advantages of information transfer by single photons. Photons permit new and incredibly precise time and frequency standards. These are not only technically important, but may also lead to even more stringent tests of relativity and other fundamental laws and concepts of physics. Examples of these applications and many other uses of modern optics are discussed in this issue.

Special relativity, the third of Einstein's 1905 topics, has always been closely connected with atomic, molecular and optical physics. Today atomic physics provides some of the most stringent tests of special relativity. Furthermore, fast electron beams are used in advanced light sources such as synchrotron radiation generators and free electron lasers. Those aspects and related applications are also discussed here.

We hope that this special issue will be of great interest to the reader by highlighting recent advances in atomic, molecular and optical physics. This field continues to provide one of the most fertile areas for research one hundred years after it first emerged from the work of Einstein published in 1905, his 'miraculous year'.

The article discusses Einstein's famous papers of 1905—his miraculous year—and deals with their physical and historical context as well as their fundamental impact on modern physics. It shows that the papers are not isolated, but connected with each other by Einstein's deep-seated conviction of physical atomism and his criticism of an ether. They are concerned with specific problems that can be characterized as 'borderline problems' since they go beyond the traditional divisions between mechanics, electrodynamics, and thermodynamics.

This paper describes advances in microwave frequency standards using laser-cooled atoms at BNM-SYRTE. First, recent improvements of the ¹³³Cs and ⁸⁷Rb atomic fountains are described. Thanks to the routine use of a cryogenic sapphire oscillator as an ultra-stable local frequency reference, a fountain frequency instability of 1.6 × 10⁻¹⁴ τ^−1/2 where τ is the measurement time in seconds is measured. The second advance is a powerful method to control the frequency shift due to cold collisions. These two advances lead to a frequency stability of 2 × 10⁻¹⁶ at 50 000 s for the first time for primary standards. In addition, these clocks realize the SI second with an accuracy of 7 × 10⁻¹⁶, one order of magnitude below that of uncooled devices. In a second part, we describe tests of possible variations of fundamental constants using ⁸⁷Rb and ¹³³Cs fountains. Finally we give an update on the cold atom space clock PHARAO developed in collaboration with CNES. This clock is one of the main instruments of the ACES/ESA mission which is scheduled to fly on board the International Space Station in 2008, enabling a new generation of relativity tests.

For more than 100 years, optical atomic/molecular frequency references have played important roles in science and technology, and provide standards enabling precision measurements. Frequency-stable optical sources have been central to experimental tests of Einstein's relativity, and also serve to realize our base unit of length. The technology has evolved from atomic discharge lamps and interferometry, to narrow atomic resonances in laser-cooled atoms that are probed by frequency-stabilized cw lasers that in turn control optical frequency synthesizers (combs) based on ultra-fast mode-locked lasers. Recent technological advances have improved the performance of optical frequency references by almost four orders of magnitude in the last eight years. This has stimulated new enthusiasm for the development of optical atomic clocks, and allows new probes into nature, such as searches for time variation of fundamental constants and precision spectroscopy.

Experiments aimed at searching for gravitational waves from astrophysical sources have been under development for the last 40 years, but only now are sensitivities reaching the level where there is a real possibility of detections being made within the next 5 years. In this paper, a history of detector development will be followed by a description of current detectors such as LIGO, VIRGO, GEO 600, TAMA 300, Nautilus and Auriga. Preliminary results from these detectors will be discussed and related to predicted detection rates for some types of sources. Experimental challenges for detector design are introduced and discussed in the context of detector developments for the future.

We present a detailed, (hopefully) pedagogical, analysis of amplitude and intensity correlations from two two-level atoms. We discuss various schemes to examine interference in scattering from two two-level systems, both with semiclassical treatment and quantum mechanical treatment. We show realization of quantum eraser and subensemble sorting in two different schemes of two two-level systems. We also show how the two-photon correlation can be useful in quantum imaging.

Einstein often performed thought experiments with 'photon boxes', storing fields for unlimited times. This is yet but a dream. We can nevertheless store quantum microwave fields in superconducting cavities for billions of periods. Using circular Rydberg atoms, it is possible to probe in a very detailed way the quantum state of these trapped fields. Cavity quantum electrodynamics tools can be used for a direct determination of the Husimi Q and Wigner quasi-probability distributions. They provide a very direct insight into the classical or non-classical nature of the field.

On the occasion of the hundredth anniversary of Albert Einstein's annus mirabilis, we reflect on the development and current state of research in cavity quantum electrodynamics in the optical domain. Cavity QED is a field which undeniably traces its origins to Einstein's seminal work on the statistical theory of light and the nature of its quantized interaction with matter. In this paper, we emphasize the development of techniques for the confinement of atoms strongly coupled to high-finesse resonators and the experiments which these techniques enable.

This paper summarizes some important achievements of quantum information processing with trapped ions or neutral atoms. In particular, we describe the storage of information and realization of two-qubit gates with ions, as well as the creation of entanglement and quantum simulation with cold atoms in optical lattices.

Einstein, together with Podolski and Rosen (EPR), tried to point out inconsistencies of standard quantum mechanics using an effect which is now called entanglement. The experiments testing EPR's hypothesis laid the basis for the new field of quantum information processing, which in turn gave rise to impressive progress in methods to observe and to analyse the phenomenon of entanglement. Here we give an overview of the various systems useful for the novel applications of quantum communication and quantum computation.

We present an overview of recent theoretical and experimental work on the control of the propagation and quantum properties of light using electromagnetically induced transparency in atomic ensembles. Specifically, we discuss techniques for the generation and storage of few-photon quantum-mechanical states of light as well as novel approaches to manipulate weak pulses of light via enhanced nonlinear optical processes.

We study resonator-induced light forces arising from cooperative atom–light interaction. For such collective processes, the force on the sample can be orders of magnitude larger than the sum of conventional light forces on individual atoms. Since resonator-induced light forces can be dissipative even when the incident light is far detuned from atomic transitions, they may be applicable to target particles with a complex level structure.

We briefly review some recent developments in nonlinear atom optics. Basic principles, as well as some of the early effects predicted and observed in the nonlinear optics of bosonic atoms, are presented in general terms. Recent results on fermionic four-wave mixing and on the matter-wave analogue of optical second-harmonic generation are discussed in detail.

Seventy years after Einstein's prediction, the seminal achievement of Bose–Einstein condensation in dilute atomic gases in 1995 has provided us with a new form of quantum matter. Such quantum matter can be described as a single giant matter wave. By loading it into an artificial periodic potential formed by laser light—a so-called optical lattice—it has become possible to probe matter far beyond the wave-like description. In a review of a series of experiments with ultracold quantum gases in optical lattices, we show that the granularity of the matter wave field, caused by the discreteness of atoms, gives rise to effects going beyond the simple single matter wave description. Bose–Einstein condensates in optical lattices have thereby opened novel possibilities for investigating strongly correlated many-particle phenomena of condensed matter physics and have opened new opportunities for quantum information processing with neutral atoms

We give a brief overview of recent studies of weakly bound homonuclear molecules in ultracold two-component Fermi gases. It is emphasized that they represent novel composite bosons, which exhibit features of Fermi statistics at short intermolecular distances. In particular, the Pauli exclusion principle for identical fermionic atoms provides a strong suppression of collisional relaxation of such molecules into deep bound states. We then analyse heteronuclear molecules which are expected to be formed in mixtures of different fermionic atoms. We show how an increase in the mass ratio for the constituent atoms changes the physics of collisional stability of such molecules compared to homonuclear ones. We discuss Bose–Einstein condensation of these composite bosons and consider prospects for future studies.

We review the dominant processes that shape the optical nonlinearities of mesoscopic composite materials with particular emphasis on the photo-induced modifications of their spectral features that are related either to the electron or photon confinement in metal and semiconductor nanoparticles embedded in a transparent dielectric in low concentration. We also derive fluctuation–dissipation relations for the nonlinear regime in such systems. These modifications are related to the strong charge-photon coupling regime in confined space and can be exploited in different ways in applications and in particular as photonic nanoprobes with outstanding spatio-temporal resolution.

The first rare-earth-doped fibre lasers were operated in the early 1960s and produced a few milliwatts at a wavelength around 1 µm. For the next several decades, fibre lasers were little more than a low-power laboratory curiosity. Recently, however, fibre lasers have entered the realm of kilowatt powers with diffraction-limited beam quality. In this paper we review the reasons for this power evolution. Beyond this, we will discuss how the next generation of fibres, so-called photonic crystal fibres, enable upward power scaling and therefore open up the avenue to new performance levels of solid-state lasers.

Stimulated emission, predicted by Albert Einstein in 1917, not only prepared the grounds for the invention of the laser, but also for a far-field fluorescence microscopy with diffraction-unlimited resolution. In stimulated emission depletion (STED) microscopy, stimulated emission is not used for light amplification but for a saturated quenching of the fluorescence emission. After demonstrating a five-fold improvement of the lateral (x and y) resolution over the diffraction barrier, we apply STED microscopy to nanostructures of stained PMMA. For the first time, periodic line structures of 80 nm width and 40 nm gaps are resolved with focused visible light.

Recent progress in the study of the photon emission from highly charged heavy ions is reviewed. These investigations show that high-Z ions provide a unique tool for improving the understanding of the electron–electron and electron–photon interaction in the presence of strong fields. Apart from the bound-state transitions, which are accurately described in the framework of quantum electrodynamics, much information has also been obtained from the radiative capture of (quasi-) free electrons by high-Z ions. Many features in the observed spectra hereby confirm the inherently relativistic behaviour of even the simplest compound quantum systems in nature.

Recent progress in generation and control of intense optical fields has given rise to isolated soft x-ray pulses with durations significantly below 1 fs. These constitute a tool of unprecedented temporal definition for attosecond physics—the study and manipulation of electronic motion on a time scale approaching the atomic unit of time. A key mechanism in such experiments is the well-defined momentum transfer between a quasi-free electron, released from an atom following irradiation by a short x-ray pulse, and a precisely controlled strong visible light field. The electrons' final kinetic energy thus sensitively depends on the timing of electron release with respect to the field oscillations and reveals the ejected electrons' confinement in time with sub-cycle, i.e. attosecond, resolution. Experiments resulting in electron emission of different durations can be interpreted in terms of a particle-like or wave-like electron, depending on whether the emission duration is considerably shorter or longer than the wave period of the probing light.

We describe the consequences of the theory of special relativity on particle accelerators and present a historical overview of their evolution and contributions to science and the present limitations of existing accelerator technology. We report recent results of our experiment where we succeeded in accelerating relativistic electrons with visible light in vacuum. The experimental demonstration is the first of its kind and is the proof of principle for future linear laser-driven particle acceleration schemes in vacuum that may lead to the realization of electron–positron colliders beyond the TeV scale.

Multiple ionization of atoms by an ultrashort intense laser pulse is a process in which the few-body problem is closely interrelated with the highly nonlinear interaction between the electrons and the external field. We review recent advances in unveiling the mechanisms behind the unusually large ion yields for double and multiple ionization observed in a strong laser pulse. Its study requires on one hand the combination of highly differential experimental techniques with laser systems having high repetition rates and on the other the development of new theoretical methods to simultaneously account for the long-ranged Coulomb interaction between the particles and the field nonlinearity. Different mechanisms are analysed diagrammatically and quantitatively in comparison with experimental data for the total ion yields. Distributions for the electron and ion momenta of coincidence measurements are discussed along with predictions of the various theoretical methods.

Synchrotron radiation (SR) is having a very large impact on interdisciplinary science and has been tremendously successful with the arrival of third generation synchrotron x-ray sources. But the revolution in x-ray science is still gaining momentum. Even though new storage rings are currently under construction, even more advanced rings are under design (PETRA III and the ultra high energy x-ray source) and the uses of linacs (energy recovery linac, x-ray free electron laser) can take us further into the future, to provide the unique synchrotron light that is so highly prized for today's studies in science in such fields as materials science, physics, chemistry and biology, for example. All these machines are highly reliant upon the consequences of Einstein's special theory of relativity. The consequences of relativity account for the small opening angle of synchrotron radiation in the forward direction and the increasing mass an electron gains as it is accelerated to high energy. These are familiar results to every synchrotron scientist. In this paper we outline not only the origins of SR but discuss how Einstein's strong character and his intuition and excellence have not only marked the physics of the 20th century but provide the foundation for continuing accelerator developments into the 21st century.

In a free-electron laser (FEL) the lasing medium is a high-energy beam of electrons flying with relativistic speed through a periodic magnetic field. The interaction between the synchrotron radiation that is produced and the electrons in the beam induces a periodic bunching of the electrons, greatly increasing the intensity of radiation produced at a particular wavelength. Depending only on a phase match between the electron energy and the magnetic period, the wavelength of the FEL radiation can be continuously tuned within a wide spectral range. The FEL concept can be adapted to produce radiation wavelengths from millimetres to Ångstroms, and can in principle produce hard x-ray beams with unprecedented peak brightness, exceeding that of the brightest synchrotron source by ten orders of magnitude or more. This paper focuses on short-wavelength FELs. It reviews the physics and characteristic properties of single-pass FELs, as well as current technical developments aiming for fully coherent x-ray radiation pulses with pulse durations in the 100 fs to 100 as range. First experimental results at wavelengths around 100 nm and examples of scientific applications planned on the new, emerging x-ray FEL facilities are presented.

A review of resonant and non-resonant electron spectroscopy on atoms and molecules at third generation synchrotron radiation facilities is given. The high brilliance of the soft x-ray radiation has made possible new types of experiments giving information on the fundamental behaviour of photoionization. The relevance of Einstein's photoelectric law, and notably the question of when electron energies disperse or do not disperse with the photon energy, is given special attention.

A brief account of the developments in photodissociation studies of free molecules by VUV and soft x-ray sources is given as an extended introduction. Then two typical experimental setups are described for multiple-ion coincidence momentum imaging and high-resolution Auger electron–ion coincidence momentum imaging. Finally, some individual cases of molecular dissociation following core-hole creation are examined, to illustrate how the complex multidimensional data produced can be represented and interpreted. The new experimental techniques based on Coulomb explosion are shown to allow direct characterization of the initial nuclear motions which follow electronic excitation.

The study of the emission of two electrons from an atom by absorption of a single energetic photon has become of much current interest because it provides the most detailed information on the interaction of the electrons between themselves. Its investigation in the simplest two-electron system, the He atom, in the last ten years has challenged experimentalists and theorists alike. By using selected examples from electron–electron and electron–recoil ion coincidence experiments the main achievements in the field are reviewed. It is shown that the dynamics of the electron pair is strongly constrained by its own symmetry and the Coulomb repulsion. The further aspects brought in by the experiments in diatomic molecules (H₂, HD and D₂) as well as in the heavier rare gases are also illustrated. Future perspectives which involve other processes which result in an electron pair in the continuum are considered.

Synchrotron radiation is a good mimic of solar radiation and therefore has been widely used to study photo-induced physics and chemistry in the terrestrial atmosphere. In this paper we review how synchrotron radiation is being used as a tool for investigating atmospheric physics and chemistry with particular emphasis on studies related to ozone depletion, global warming and ionospheric phenomena. The paper concludes with a discussion of the new possibilities that the next generation of synchrotron-based light sources will provide.