Highlighted Papers / Colloquium PapersHighlighted Papers are EPJ D articles selected by the Editorial Board to increase the visibility of particularly important papers. The full PDF of the articles selected can be viewed and downloaded from this page free of charge. Every Highlighted Paper is introduced by a short paragraph explaining the novelty and particular importance of the published work. Highlighted Papers are added continously to this page and will be kept for several months at least before being removed again.
A further subset of Highlighted Papers is "Colloquium Papers". They describe the development of new areas of research or the impact of new and promising experimental or theoretical methods in the fields that are within the spectrum of topics covered by the journal. While not as extensive and complete as reviews in the usual sense, they are intended to suitably introduce new research directions and techniques in their early stage of development to a wider audience.
May 2010
Compact Stark slower for polar molecules
Stark deceleration has emerged over the last decade as a leading technique for obtaining packets of quantum-state-selected molecules whose velocity can be tuned all the way down to zero. Here, a new compact, ultrahigh-vacuum-compatible Stark decelerator is described and demonstrated. The deceleration stages are fashioned out of tantalum wires, reducing the total length to about a tenth of that of a conventional Stark decelerator with the same number of electrode pairs.
The significantly lower cost of assembling and operating the wire decelerator makes it an attractive source of cold molecules, for use in applications ranging from trapping experiments to cold collisions to sympathetic cooling.
[A. Marian et al., Eur. Phys. J. D (2010)]
April 2010
The two-loop self-energy: diagrams in the coordinate-momentum representation
Calculations of QED effects to all orders in the nuclear field play an important role in testing fundamental theories, both in systems with a
strong binding field (e.g., in hydrogen-like and lithium-like uranium) and in weakly bound systems (particularly, in the hydrogen atom). While
investigations of the one-loop QED corrections have a long history, nontrivial two-loop QED effects became accessible to calculation only
relatively recently, made possible by advances in both theory and computing power. A characteristic feature of the two-loop effects
treated to all orders in the nuclear field is the presence of Feynman diagrams containing both the ultraviolet divergent subgraphs and the
bound-electron propagators. For such diagrams, neither coordinate nor momentum-space representation suits the purposes of numerical
evaluation. The present investigation reports a new calculation technique developed for the treatment of such diagrams in the mixed
coordinate-momentum representation. When applied to the two-loop self-energy correction, this technique advances the numerical accuracy
and sheds some light on the previously reported discrepancy between the perturbative and the all-order calculations for hydrogen.
[V.A. Yerokhin, Eur. Phys. J. D (2010)]
April 2010
Effect of a magnetic field in photodetachment microscopy
Photodetachment of a negative ion produces a nearly free electron, hardly perturbed by the residual atomic core. Applying an external electric field does not only concentrate the photoelectron current in a round spot, but also gives rise to an electron interference pattern, due to the existence of a pair of possible trajectories bound to every point of the spot. This very fundamental matter-wave interferometer appears to produce extraordinarily robust interferograms. Though magnetic fields, even in the sub-µT range, are able to produce fluxes between the interfering trajectories that can be huge when compared to the quantum unit of magnetic flux, a magnetic perturbation of the system appears to only produce a global deviation of the spot, without any detectable modification of the internal interference pattern. The main result of the present work is that even in higher magnetic fields (typically 100 µT) able to refocus the electron spot, the electron interference phase, or number of interference rings, still appears unperturbed. This comforts photodetachment microscopy as a sub-µeV accurate electron spectrometry method and the most accurate one for electron affinity measurements.
[W. Chaibi et al., Eur. Phys. J. D 58 (2010)]
April 2010 / Colloquium Paper
Quantum memories set to go a long way
Quantum memories are essential elements for many potential applications of quantum technology, including quantum information processing, quantum repeaters and entanglement-enhanced precision metrology. The development of such memories is, therefore, currently a very active field of research with a particular emphasis on memories that can interface with photons, which are the best carriers of quantum information over long distances. This paper reviews a number of different approaches to this challenge, with a focus on the approaches that were represented in the large European Union Integrated Project "Qubit Applications". While we make no claim for this review to be exhaustive, this is a field in which the European research community is particularly strong, making a primarily European research centred review highly relevant on a global perspective. In particular we discuss solid-state atomic ensembles, nitrogen-vacancy centres, quantum dots, single atoms and atomic gases. Since the considered approaches are very diverse, an important part of our work was to establish criteria that allow a meaningful comparison. We discuss both the current experimental state of the art and the potential long-term performance of the various systems in significant detail. The results of our comparison are summarized in a compact table.
March 2010
Anomalous photon diffusion in atomic vapors
In contrast to Gaussian random phenomena, characterized by a mean value and a variance, Lévy processes have their dynamics dominated by a few out-of-scale events (Lévy flights), whose statistical distribution lacks a second moment. In some cases, even the distribution's first moment diverges, meaning that it is not possible to characterize a typical scale of the phenomenon through a mean value. The interest in systems exhibiting such a non-normal statistical behavior has been growing since the 1990s. On the other hand, an understanding of the way photons propagate in multiple scattering media is fundamental in systems such as stars, gas lasers or discharge lamps. This paper describes the experimental observation of the step size distribution of photons scattered in a hot, resonant Rb vapor and the characterization of this stochastic process as a Lévy process as a consequence. This anomalous behavior has its origin in spectral inhomogeneities that spread through a frequency redistribution at each scattering process. A quantitative description of the frequency redistribution through Monte Carlo simulations supports the experimental findings.
[M. Chevrollier et al., Eur. Phys. J. D (2010)]
March 2010
The 1S+1S asymptote of Sr2 studied by Fourier-transform spectroscopy
Currently, there is a high interest in ultracold strontium atoms and molecules, because strontium is a candidate for a precise optical frequency standard using ultracold atom ensembles, for which the interaction properties play an important role. The present work offers a significant improvement in the experimental knowledge of the atomic long-range van der Waals coefficients and for modeling cold collisions highly precise scattering lengths for all naturally abundant atomic Sr pairs. The spectroscopic observations and the derived potential energy curves point clearly to a successful coherent population transfer in the reverse direction as opposed to the present excitation path. It would yield ultracold Sr2 molecules in the rovibrational ground state from molecules in highly excited vibrational levels populated e.g. by photoassociation.
[A. Steina et al., Eur. Phys. J. D (2010)]
March 2010
Quantum theory of synchronously pumped type I optical parametric oscillators: characterization of the squeezed supermodes
In recent years, the generation and manipulation of non-classical states of light possessing more than one degree of freedom has witnessed a growing interest, since multi-modal complexity has a great potential in many applications concerning the quantum treatment of information and the generation of a variety of non-classical states. On the other hand, multi-mode optical systems can also improve the precision measurements in the domain of quantum imaging and metrology. Hence, mastering quantum fluctuations and correlations in complex optical systems represents a crucial point in this context. This paper presents the quantum model for an optical parametric oscillator synchronously pumped by a mode locked laser. To cope with the complexity of a system that usually involves about 105 modes, it is advantageous to introduce new physical objects that we call supermodes, which are proper combinations of standard modes. Their dynamics is studied from both a classical and a quantum point of view with respect to the experimental condition considered. It is revealed that a SPOPO is a suitable and malleable source of highly multimode non-classical light in the temporal domain.
[G. Patera et al., Eur. Phys. J. D 56, 123-140 (2010)]
December 2009
Atom interferometry in free flight
Philippe Bouyer and co-workers in France performed a range of impressive experiments with an atom interferometer in free flight, onboard an Airbus aircraft making "micro-gravity jumps". These experiments successfully demonstrate that when atoms are sufficiently cooled and controlled, their wave properties can be used to perform interferometry in a way analogous to standard interferometry with light. This moves the field closer to the implementation of a range of sensors for e.g. gravity, rotation and inertia with unprecedented accuracy. Such devices will be potentially very useful in satellites and in space missions.
[Ph. Bouyer et al., Eur. Phys. J. D 53, 353-357 (2009)]
October 2009
Time-dependent delayed electron spectra: a direct measurement of total decay rate as a function of internal energy
In cluster physics and molecular science in general, there has been a growing interest in studying how increasingly complex systems "cool down" after an initial excitation step. In such experiments it is of primary importance to determine the emission rate constants and to extract accurate information on the competing decay channels. This article describes a new and general method to measure internal energy dependant decay rates in the case of systems emitting thermal electrons.
The approach is based on the measurement of the time evolution of electron kinetic energy spectra using a velocity map imaging spectrometer. In other words the authors measure directly the temperature of the molecules decaying in a well defined time-window after excitation. This method is illustrated in the case of C_60 molecules for which ionization and dissociation are the most favorable competing processes. More specifically this paper reports quantitative information on the decay channels of the molecule especially on the rate constant for the dominant neutral C_2 emission process. This method is absolutely general and may be used to map out the total emission rate for intricate decay mechanisms in complex molecules.
[F. Lépine et al., Eur. Phys. J. D 55, 627-635 (2009)]
October 2009
Time stamping in EPRB experiments: application on the test of non-ergodic theories
The Einstein-Podolsky-Rosen paradox is one of the most enduring and challenging problem in the history of Physics. It implies a contradiction between Quantum Mechanics and basical notions on the locality of natural phenomena. Many experiments measuring stationary probabilities have confirmed the quantum mechanical predictions, but "the vital time factor", as John S. Bell named it, has been left relatively unexplored. The so-called "non ergodic theories" reconcile Quantum Mechanics with the intuitive view of the world by hypothesizing that the apparent randomness at the quantum level is the consequence of some complex (eventually chaotic) underlying dynamics. These dynamics are not detectable by measuring probabilities, but they may be revealed through the analysis of the series formed by the time of observation of each single event (the detection of a photon, in our case). A time resolved record of the data, or "time stamping", may be therefore essential to elucidate the EPR mystery. In this paper, we show how a time stamped setup of satisfactory performance can be built up with accessible means. We use it, jointly with the raw data of the experiment performed under strict Einstein locality more than ten years ago, to study a class of non-ergodic theories that cannot be disproved by measuring average rates, even in ideally perfect setups, and that had remained untested until now.
[M.B. Agüero et al., Eur. Phys. J. D 55, 705-709 (2009)]
October 2009
A miniature electron beam pumped laser
Electron beam pumped lasers are traditionally large devices operating in the MV and kA regime. We succeeded in building a miniature version of transversely electron beam pumped lasers by using extremely thin ceramic entrance foils for the electron beam, whose energy could be kept as low as 12 keV. The volume of the gain medium was on the order of only 1 mm3. Although the overall pumping- and output power levels of the laser are small, the power densities are similar to those found in much larger devices. This allows studies of laser gas mixtures, gas kinetics, and gain parameters conveniently in table top experiments. The well-known 1.73 μm Ar-Xe laser system was used for demonstrating the experimental concept. Both pulsed and fully continuous operation of this laser were observed at gas pressures between 200 and 800 mbar for comparatively low pumping power levels.
[C. Skrobol et al., Eur. Phys. J. D 54, 103-109 (2009)]
August 2009 / Colloquium Paper
Physisorption kinetics of electrons at plasma boundaries
[F.X. Bronold et al., Eur. Phys. J. D 54, 519-544 (2009)]
May 2009
Light-pulse atom interferometry in microgravity
Atom Interferometry involves the possibility of using beams of atoms in the same way as a laser beam in a photon interferometer, a type of device that is used for example in high precision telemetry, rotation sensing or gravity mapping. The recent progresses in cold atom interferometry have led to a new way of sensing inertial forces, where matter waves could be used to detect accelerations and rotations much more accurately than it is possible at present. In addition, significant performance enhancement is anticipated when operated in the microgravity environment of space. An atom interferometer based accelerometer could then be used for satellite-based global gravity field mapping, deep-space navigation or for testing the fundamental laws of physics such as gravitation or general relativity. In their letter, G. Stern et al. present one step towards satellite- based atom inertial sensors by demonstrating long-baseline light-pulse atom interferometer that would not operate on ground. The prototype they used in the zero-G from planes is still a laboratory product, far from the spatial constraints that exist during launching as well as in orbit, but it is a first and necessary step to create a device which, at the end of several development stages, will finally be launched.
[G. Stern et al., Eur. Phys. J. D 53, 353-357 (2009)]
February 2009
Special Analytical Properties of Ultrastrong Coherent Fields
Advances in ultrastrong lasers suggest the possibility of employing them to study the properties of the vacuum. The awareness that very strong fields can create particles possessing mass out of "nothingness" - the vacuum - has been a fascinating but largely unrealized goal for more than half a century. The lone experiment that succeeded in creating electron-positron pairs from the collision of a strong laser beam with an energetic photon was, as shown in this paper, undervalued because it was misconstrued as simply a high-order weak-field effect when it was actually a true example of the strongly nonlinear processes that are being sought. This paper also points out that, although quantum electrodynamics (QED) has been proven to be accurate to the limits of laboratory measurement capability, there remains still the unresolved problem that QED has a fundamental analytical flaw. Additional, previously unmentioned goals can also be pursued, such as the study of the remarkable predicted strong-field property of a free electron that it no longer behaves as a simple electron, but as a new kind of particle that has an intensity-dependent spin.
[H.R. Reiss, Eur. Phys. J. D (2009)]
February 2009
Strong Laser Fields as a Probe for Fundamental Physics
Rapid progress in laser technology at highest intensities has created a wealth of novel fundamental and applied applications of lasers in atomic and plasma physics. Upcoming laser systems in the Peta Watt regime and beyond even have the potential to probe the fundamental structure of the quantum vacuum. Compatible with Heisenberg's uncertainty principle, fluctuations of particles and anti-particles form our picture of the quantum vacuum which has been established already in the 1930s in the early days of quantum field theory. Future high-intensity lasers will have the potential to probe the quantum vacuum, e.g., by rendering the quantum vacuum birefringent just like the refractive properties of a uniaxial crystal. Moreover, as the quantum vacuum knows about all degrees of freedom that possibly exist, strong laser fields can also look for new particles that have evaded detection so far owing to a potentially very feeble coupling to light or matter. This article explores the potential of upcoming laser systems for a discovery of new fundamental degrees of freedom and discusses possible optical signatures to be searched for. Optical experiments involving strong lasers can thereby contribute to the quest for the fundamental structure of nature in a manner complementary to accelerator experiments or astrophysical and cosmological observations.
[H. Gies, Eur. Phys. J. D (2009)]
January 2009
Possibility of Resonant Capture of Antiprotons by Highly Charged Hydrogenlike Ions
An antiprotonic atom is an exotic system where one of the atomic electrons has been replaced with an antiproton. It offers a fascinating mixture of matter and anti-matter well suited for tests of fundamental symmetries, as demonstrated in series of experiments on antiprotonic helium, produced when antiprotons are stopped in liquids or gaseous helium. New experimental facilities are now under construction that will provide cooled antiprotonic beams of intensities many orders of magnitude larger than today, opening for production of new types of antiprotonic systems in novel ways. In this paper the authors investigate antiprotonic capture in highly charged ions through electronic excitation followed by photon emission. The produced systems would provide excellent possibilities for high precision spectroscopy, since the capture occurs in a well defined state that can be controlled by tuning the collision energy.
[M. Genkin et al., Eur. Phys. J. D 51, 205-212 (2009)]
January 2009
Measurement of the Trapping Lifetime Close to a Cold Metallic Surface on a Cryogenic Atom-Chip
In atom-chip experiments, cold atoms are trapped in magnetic field gradients created by microfabricated current-carrying wires. By adjusting these fields, the atomic cloud can be manipulated in a very versatile manner. The large achievable oscillation frequencies in the traps pave the way to the quantum control of the atomic motion.
High trapping frequencies are achieved when the atomic cloud is brought very close to the surface of the chip. In these conditions however, new loss mechanisms are observed. They originate mainly from Johnson-Nyquist and technical noise currents in the trapping structure. They produce magnetic-field fluctuations at the positions of the trapped cloud, inducing Zeeman transitions towards untrapped magnetic sublevels.
The cryogenic atom-chip group at ENS Paris has now shown that the Johnson-Nyquist noise is reduced by cooling down the atom-chip to cryogenic temperatures, resulting in significantly longer lifetimes.
Further improvements are expected to be achieved by using permanent supercurrents, in which case the Johnson-Nyquist noise can be reduced even more and the technical noise from current power supplies is completely removed.
[A. Emmert et al., Eur. Phys. J. D 51, 173-177 (2009)]
November 2008
Echo Spectroscopy of Atomic Dynamics in a Gaussian Trap via Phase Imprints
Nondestructive optical probing of cold and trapped two-level atoms holds promise as a means for state engineering of spin squeezed states for atomic clock applications. However, the light-atom interaction inevitably introduces decoherence of the atomic ensemble. The level of squeezing attainable grows with the strength of the nondestructive measurement, but unfortunately, so does the induced decoherence. It is thus of uttermost importance to gauge the various loss mechanisms, in order to pin down the optimal trade-off between information gained and damage inflicted on the atomic state. In this paper, the spatially dependent phase imprinted on Cs atoms in coherent superpositions of the two clock states is investigated, when probed by off-resonant light pulses with a Gaussian intensity profile. The inhomogeneous imprint on the internal atomic state becomes linked to the position and motion of each individual atom in the trap, limiting the efficiency of coherence restoring techniques, such as spin-echoes. Collapse-and-revival of the atomic coherence is observed, and an intimate connection with the radial trap period is established.
[D. Oblak et al., Eur. Phys. J. D 50, 67-73 (2008)]
November 2008
Precision measurement of the branching fractions of the 4P 2P3/2 decay of Ca II
Theorists devote considerable effort to precise structure calculations of singly charged alkaline ions. These efforts are motivated by the relevance of these ions in astrophysical observations and in parity non-conservation experiments. For a comparison with experimental data, theorists often turn to precision measurements of excited state lifetimes and decay branching ratios.
So far, measurements of these quantities in literature are often based on experiments in ion beams or trapped clouds of ions. This paper reports accurate measurements of the branching fractions in the 4P3/2 decay in a single trapped Ca+ ion by exploiting the precise state control available for single ions. Furthermore, the precision is improved by a new technique based on repetitive optical pumping. In this way, the branching fractions are determined with a precision of better than 1%, a forty-fold improvement with respect to the previously best known values.
[R. Gerritsma et al., Eur. Phys. J. D 50, 13-19 (2008)]
September 2008
Optical Characterization and Manipulation of Alkali Metal Nanoparticles in Porous Silica
The effect of atomic photodesorption and atomic confinement on nanoparticle formation/evaporation processes has been investigated. The experiment demonstrated that it is possible to grow or to evaporate Rb and Cs clusters inside nanoporous silica samples by proper selection of desorbing light frequency and intensity. Due to the confined geometry, this occurs by shifting atoms from surface layers to clusters and vice versa. The presence or absence of light does not substantially affect the size and shape of the nanoparticles that are formed. This result provides clear evidence that the structural properties of the cluster are determined by the interaction with the host matrix.
[A. Burchianti et al., Eur. Phys. J. D 49, 201-210 (2008)]
July 2008
Magnetic Interactions of Cold Atoms with Anisotropic Conductors
Material engineering meets atom optics.
Experiments with ultra-cold neutral atoms trapped in potentials generated by micro-structures integrated on a nearby surface (atom
chips) are evolving rapidly and may soon enable robust quantum devices.
This decade has seen atom chip based coherent manipulation
(interferometry) of both internal and external atomic degrees of freedom. The promise of the atom chip platform may, however, be hindered by thermal noise originating from the nearby "classical environment", specifically the motion of finite-temperature electrons present in the surface. The coupling of this noise radiation to the atoms results in atom loss, heating and decoherence. Slight imperfections in the fabrication of this nearby environment also lead to static corrugations of the trapping potential, which can cause a variation of the atoms'
density in traps, as well as lead to localization in guides and perturb quantum phase evolution in interferometry. This paper describes the implications of using novel materials on the atom chip for the micro-structures creating the trapping and guiding potentials and specifically electrically anisotropic materials. Using such materials, decoherence and heating rates can be significantly reduced, even for room temperature atom chips. In addition, the amplitude of the static potential corrugation can be reduced, as typical electron scattering patterns within the imperfect wires can be controlled. Materials, fabrication, and experimental considerations are discussed.
[T. David et al., Eur. Phys. J. D 48, 321-332 (2008)]
March 2008
Quantum Phase Estimation Algorithm in Presence of Static Imperfections
Researchers in the domain of superconducting quantum bits are now beginning to explore the world of multiqubit gates and an ubiquitous question to address is how well can we do with the available resources and how to improve the computation accuracy. Recently Dobsicek et al.
proposed an iterative phase estimation algorithm that allows to compute the eigenphases of an operator with a high precision in presence of random errors produced by noise in quantum gates.
The computation of eigenphases of quantum operators plays an especially important role being at the core of
many important quantum algorithms. Our work shows that the success
probability of the algorithm
is affected significantly more by internal static imperfections and residual couplings between qubits, present even in an isolated quantum computer, than by the external random errors.
In spite of these difficulties we demonstrate that a significant accuracy gain can be reached by the Pauli Random Error Correction (PAREC) which corrects static imperfection errors with no additional qubits and with only a moderate increase of the number of quantum gates.
The considered quantum algorithm can be used as a benchmark circuit for present and forthcoming proposed architectures of quantum computers.
[I. Garcia-Mata and D.L. Shepelyansky, Eur. Phys. J. D 47/1 (2008)]
March 2008
X-Ray Transitions from Antiprotonic Noble Gases
Heavy negatively charged particles like antiprotons are captured by the Coulomb field of nuclei at about the outermost electron shell into highly excited atomic states. The captured particle starts a quantum cascade which, for low-density gaseous targets, is determined by the competition between internal Auger-electron and X-ray emission.
Antiprotons are particularly suitable since, because of their mass the number of de-excitation steps is exceedingly large. As a consequence of it numerous high-lying X-ray transitions become accessible, the yields of which indicate a progressive electron depletion sometime up to complete ionisation. The paper describes how the interplay of antiproton and electrons can be systematically explored for an increasing number of electron shells from the X-ray spectra of antiprotonic argon, krypton, and xenon by using electron multi-configuration calculations, which are able to include both the degree of ionisation and the - state-dependent
- presence of the antiproton. The analysis unravels from the shell-by-shell electron emission how the ionization develops and may serve as a starting point for more dedicated studies at the forthcoming antiproton facilities.
[D. Gotta et al., Eur. Phys. J. D 47/1 (2008)]
January 2008
On the Optimality of Individual Entangling-Probe Attacks Against BB84 Quantum Key Distribution
The study of the security of Quantum Key Distribution (QKD) protocols has undergone rapid evolution during the last ten years. However, some misconceptions are still being published on the topic; in particular, misleading claims have been advanced following the implementation of a particular individual attack, the Slutsky-Brandt attack. This article clarifies why the implementation of such "optimal" attacks is in no contradiction with the established "old-style" theory of BB84 individual attacks, why experiments in general cannot be used to test security, and fills remaining gaps by finally providing tight security bounds for this class of ideal attacks. Extending and systematizing previous ideas, we show that the inference power of the eavesdropper is almost completely determined by the symmetry group of the protocol. Additionally, we emphasize the importance of the specific error reconciliation procedure, which actually determines the protocol. It is shown that leakage of error position is irrelevant (at least for individual attacks). This analysis also brings insight into the relation between the maximal collision probability resulting from an optimal measurement and the fidelity of the final states to be distinguished.
[I.M. Herbauts et al., Eur. Phys. J. D 46, 395-406 (2008)]
January 2008
Thermodynamic and Transport Properties in Equilibrium Air Plasmas in a Wide Pressure and Temperature Range
There is currently renewed interest in the study of thermal plasmas, due to their application in plasma wind tunnels, high pressure discharges, plasma torches, laser sparks and laser induced breakdown spectroscopy.
For these systems, fluid dynamic simulation is a powerful tool to investigate a given device, and to perform measurement analysis. These simulations require accurate calculation of thermodynamic and transport properties of high temperature mixtures. Currently available databases of relevant quantities are not adequate for the temperature and pressure ranges encountered in current applications. The new proposed database for air plasmas, obtained using accurate molecular and atomic partition functions and higher approximations of the Chapman Enskog method with improved collision integrals, are reported in closed form ready to be easily implemented in fluid dynamics codes.
[A. D'Angola et al., Eur. Phys. J. D Vol 46, 129-150 (2008)]
June 2007
Determination of the Calcium Ground State Scattering Length by Photoassociation Spectroscopy at Large Detunings
The knowledge on the interaction of cold and ultracold atoms and molecules is
essential for the description of the quantum phenomena observed in modern
experiments with ultracold and degenerate gases. For 40Ca, we have determined
the important parameter for ultracold collisions the s-wave scattering length by
the detailed analysis of photoassociation spectra. The non-degenerate ground
state of calcium leads to a simple electronic structure of the molecular
interaction potential and allows for an exemplary quantitative description of
the photoassociation process. Our new data covers an extended range of molecular
binding energies and therefore samples an enlarged interval of the internuclear
distances. We discovered an ambiguity in the interpretation of our previous
photoassociation measurements and could now resolve the discrepancy between
values of the scattering length determined from classical and photoassociation
spectroscopy.
[F. Vogt et al., Eur. Phys. J. D Vol 44, No 1, pp73 (2007)]
June 2007
Coherent Harmonic Generation on UVSOR-II Storage Ring
Future light sources aim at reaching sub nm wavelengths with fs pulse duration,
preserving highest degree of coherence. In particular, accelerator based light
sources provide unique possibilities of tunability in a wide spectral range,
brilliance, coherence, and adjustable polarization. In a Free Electron Laser,
the interaction between an external laser and a relativistic electron beam leads
to an improved longitudinal coherence and shorter sub ps pulses. Coherent
Harmonic Generation, an FEL configuration, is performed at UVSOR-II storage
ring. An external laser is injected inside an undulator located on a straight
section of the ring. Since 2005, intensive investigations are performed to
improve the setup for Harmonic Generation (external seed transport, temporal and
spatial overlap adjustment of the two beam), and to characterize the harmonic
content of the radiation produced. In addition to being of high interest for
users' experimental studies, Coherent Harmonic Generation experiments provide
significant insight for the future Linac based HGHG FEL sources, which should
reach x-ray domain, with short pulse duration and high peak power.
[M. Labat et al., Eur. Phys. J. D Vol 44, No 1 pp187 (2007)]
February 2007
Unconditional Security of Practical Quantum Key Distribution
Quantum Key Distribution (QKD) is the most visible practical achievement of quantum information theory. In this article, we analyze in detail the security of very practical schemes that have
been used to implement QKD since many years: the BB84 protocol using simple weak laser pulses as a source. Our analysis applies also to a quantum channel between Alice and Bob that might show
significant losses and noise. The detectors on Bob's side need not to be characterized in detail, instead, it is only required that the detection devices work with the same efficiency in both polarization
bases. This includes realistic detection set-ups taking into account full optical modes, rather than qubit detectors. We find the experimental parameters that allow accomplishing secure QKD. In contrast to
most other security proofs, we analyze in detail also all statistical effects so that we can obtain a security statement that applies to a finite sequence of signal pulses in the experiments. Although this
research has been finalized in 2001, and therefore lacks references to more recent developments such as decoy states and differential phase-shift QKD, or to topics such as composability, it is still of importance due to its extensive analysis.
[H. Inamori, N. Lütkenhaus and D. Mayers, Eur. Phys. J. D (2007)]
February 2007
Application of a fs frequency comb for highest precision measurements of molecular transitions
An optical frequency comb has been employed for the determination of transition
frequencies in K2 around 366 THz. The narrow natural line width of
intercombination transitions together with a collimated molecular beam allowed a
precision to better than 40 kHz for absolute and better than 12 kHz for
difference frequencies. First studies of frequency shifts of the molecular
transitions are reported, which are caused by slow atom-molecule collisions in
the beam. The shifts are observed by altering the density of the atoms by
optical deflection. An indication of a collisional shift of the K2 frequencies
of few kHz was found. Improvement is expected with a modified experiment
employing a Ramsey-Bordé matter wave interferometer with the molecular beam.
[I. Sherstov et al., Eur. Phys. J. D (2007)]
November 2006
Conservation laws of classical mechanics state that the total momentum and kinetic energy of a system is unchanged in an elastic collision. This holds in
all galilean frames. For instance, in one dimension an atom of a given velocity v that undergoes an elastic collision with a macroscopic wall that moves at a velocity v/2, will remain at rest in the laboratory frame after the collision.
In this paper the authors have implemented this simple idea by launching transversally guided packets of ultracold atoms on a moving macroscopic magnetic mirror. They demonstrate that up to 95% of the incident longitudinal kinetic energy
is removed through the collision. They have also shown how this technique can be used to generate a continuous and intense flux of slow atoms.
[G. Reinaudi et al., Eur. Phys D Vol 40, 405 - 410 (2006)]
November 2006
In this paper we present a novel theoretical approach for the description of the protein folding process.
The suggested method treats this process,
i.e. the dynamical transition from a rigid 3D-structure of a complex molecule (protein, polypeptide, DNA etc.) to an unfolded, so-called random coil state, as a phase transition. It is demonstrated that in a complex molecular system one
can identify specific, internal degrees of freedom responsible for the folding dynamics. Often, the potential energy surface of a complex molecule with respect to these specific degrees of freedom can be calculated and thoroughly analysed
on the basis of ab initio methods. It is shown that this knowledge turns out to be sufficient for the development of complete thermodynamic description of such molecular systems, which include calculation of all essential thermodynamic
variables and characteristics, e.g. heat capacity, phase transition temperature, free energy etc. The method has been proved to be working well for the description of the phase transition in polyalanine of different length by the comparison
with the results of several independent experiments and the results of molecular dynamics simulations. Our method is free of any model parameters and is based solely on fundamental physical principles. It can be used as a reliable alternative
to molecular dynamics approach, particularly in those cases when molecular dynamics simulations are hardly possible because of computer power limitations.
[A.V. Yakubovich et al., Eur. Phys D Vol 40, 363 - 367 (2006)]
October 2006
Atom interferometry gravity-gradiometer for the determination of the Newtonian gravitational constant G
Experiments that mix gravity and quantum mechanics are uncommon. In a new experiment conducted by Guglielmo Tino and colleagues in Firenze, laser-cooled rubidium atoms launched in an atomic fountain are used to measure the value of the Newtonian
gravitational constant, G. Newton¢s G is quite different from the other fundamental constants of physics. On the one hand, it is not related to these other constants by any simple relationship. On the other hand, it is the less precisely measured
constant of physics. Indeed, the famous result of Henry Cavendish¢s experiment has been improved in precision by only a factor of hundred in the last two centuries. From the time of Cavendish, the preferred method of measuring G has been the
torsion balance, in which the restoring force of a twisted fibre balances the weak gravitational torque produced by the attraction between macroscopic test masses. In the new Firenze MAGIA experiment, supported by Instituto Nazionale di Fisica
Nucleare, microscopic atomic probes and an atom interferometry detection scheme are used for the first time to measure this elusive fundamental constant. With planned improvements to the apparatus, the researchers will be able to reduce the
present uncertainty and to measure G with an uncertainty of 100 parts per million providing new information to understand systematic errors that limited the experiments performed so far.
[A. Bertoldi et al., Eur. Phys D Vol 40, 271 - 279 (2006)]
May 2006
Radiative corrections to the magnetic-dipole transition amplitude in B-like ions
The investigations of forbidden transitions in few-electron, highly-charged ions,
provide a unique opportunity for probing the relativistic electron-correlation and quantum electrodynamics (QED) corrections to the transition rates. With the help of new devices like ion trap and Electron-Beam Ion TRAP (EBIT), new experimental
results of unprecedented accuracy have become available. Up to now it was usually sufficient to include radiative corrections in the transition energy to obtain meaningful comparisons. Now calculation of radiative corrections to the transition matrix
element itself is required. Moreover since one is dealing with highly charged ions, the electron nucleus interaction can no longer be treated perturbatively in the QED part of the calculation. In this paper, the one-electron radiative corrections to the
magnetic-dipole transition amplitude in B-like ions are calculated to all orders in the parameter αZ, where α is the fine structure constant and Z is the nuclear charge number. The results obtained improve the theoretical prediction for the
lifetime of the (1s2 2s2 2p)2P3/2 level in Ar13+, which is now 9.5378(3) ms. This value is in disagreement with the most accurate experimental data, 9.573(4)(5) ms, recently obtained at the Heidelberg EBIT (A. Lapierre et al.,
Phys. Rev. Lett. 95 (2005) 183001). At present there is no explanation for this discrepancy.
[Volotka;Glazov;Plunien;Shabaev;Tupitsyn, EurPhys D Vol 38, 293 - 298 (May 2006)]
May 2006
Simplified approach to double jumps for fluorescing dipole-dipole interacting atoms
The famous macroscopic quantum jumps can occur for a single atom with a
metastable state when illuminated by light. Then periods of fluorescence of fixed intensity alternate stochastically with dark periods, i.e. the atom is "blinking". The periods can be seconds or even minutes long. If one has two or three such
atoms in a trap, there are dark periods and periods of single and multiple intensity, from overlap. Now, the atoms also experience the weak dipole-dipole interaction which might, in principle, affect the atomic fluorescence, in particular the
rate of double or triple jumps, i.e. almost simultaneous jumps by double and threefold intensity steps. In some experiments, e.g. for Ba+, a huge number of such double and triple jumps was reported and attributed to this interaction, while in
other experiments no effect of this kind was seen. For many years it has been an open question whether the dipole-dipole interaction could have such dramatic effects in some cases but not in others. Hannstein and Hegerfeldt show conclusively
that although the dipole-dipole interaction can influence the jump rates up to a factor of two, for the parameters of the experiments its effect is not only small but also negligible. Thus the reported huge double jump rate cannot be due to
the dipole-dipole interaction and requires a different explanation.
[Hannstein; Hegerfeldt, Eur. Phys. D Vol 38, 415 - 422 (May 2006)]
March 2006
Ultra-long-range states in excited 3He2
Advances in techniques for cooling atoms have led to many new possibilities. Among these is
that of performing precise spectroscopy of the diatomic molecules formed when two cold atoms absorb a photon. While most cold atom work has involved the alkalis, some groups have cooled metastable Helium atoms. These have a lifetime of
about 8000 s. and, because of their high excitation energy (20 eV), single-atom detection is possible. Bose-Einstein condensates were prepared in 2001. Leonard et al. excited these atoms to weakly bound, ultra-long-range, molecules with
internuclear separations up to 1150 bohr. The theoretical description of these molecules is relatively simple. Helium has a stable isotope, 3He, which has also been cooled to mK temperatures. An additional feature of 3He is that its nuclear
magnetic moment gives rise to hyperfine structure. Dickinson has now explored theoretically whether cold metastable 3He atoms can also form ultra-long range molecules and has concluded that they should exist.
[Dickinson, Eur. Phys. D Vol 37, 439 - 439 (2006)]
February 2006
Radiative coreections to the magnetic-dipole transition amplitude in B-like ions
The investigations of forbidden transitions in few-electron,
highly-charged ions, provide a unique opportunity for probing the relativistic electron-correlation and quantum electrodynamics (QED) corrections to the transition rates. With the help of new devices like ion trap and Electron-Beam
Ion TRAP (EBIT), new experimental results of unprecedented accuracy have become available. Up to now it was usually sufficient to include radiative corrections in the transition energy to obtain meaningful comparisons. Now calculation
of radiative corrections to the transition matrix element itself is required. Moreover since one is dealing with highly charged ions, the electron nucleus interaction can no longer be treated perturbatively in the QED part of the calculation.
In this paper, the one-electron radiative corrections to the magnetic-dipole transition amplitude in B-like ions are calculated to all orders in the parameter αZ, where α is the fine structure constant and Z is the nuclear charge
number. The results obtained improve the theoretical prediction for the lifetime of the (1s2 2s2 2p)2P3/2 level in Ar13+, which is now 9.5378(3) ms. This value is in disagreement with the most accurate experimental data, 9.573(4)(5) ms,
recently obtained at the Heidelberg EBIT (A. Lapierre et al., Phys. Rev. Lett. 95 (2005) 183001). At present there is no explanation for this discrepancy.
[Volotka, et al., Eur. Phys. D (2006)]
December 2005
A High-Resolution Ramsey-Bord spectrometer for Optical Clocks Based on Cold Mg Atoms
Optical clocks combined with the revolutionary technique of mode-locked
femto lasers break new grounds for improved tests in fundamental physics and generation of ultrastable frequencies in the optical domain. Fermionic and bosonic magnesium is one of the few atomic species which offers handles for laser cooling and
trapping as well as narrow clock transitions. This paper reports on the status and the prospects of a magnesium optical clock. In a Ramsey-Borde interferometer resolutions as high as 290 Hz are obtained, which corresponds to a quality factor of
Q= 2.3 x 10{12}. This is the highest quality factor ever obtained for atoms in free fall. An upper limit for the laser linewidth of 170 Hz is deduced from the atom interferometric signal.
[Keupp et al., Eur. Phys. D Vol 36, 289-294 (2005)]
December 2005
Transient Dynamics of Linear Quantum Amplifiers
This paper generalizes the quantum theory of linear amplification of light to include the transient dynamics
describing a smooth onset of amplification or attenuation. Since an amplifier does not immediately turn on or off there exists a transient period, and the physics of this regime has not been studied until now. In this paper the dynamics of the
field in the linear amplifier are analysed by solving the corresponding master equation with time-dependent coefficients. The persistence of nonclassical properties is studied and compared with the previous time-independent theory. The results
of the paper can be directly applied to describe transients in quantum technological applications based on linear amplifiers such as quantum optical networks.
[Maniscalco et al., Eur. Phys. D (2005)]
August 2005
The D1 П state of the NaRb molecule
High resolution Fourier-transform fluorescence spectroscopy is proven to be an outstanding method for
obtaining very accurate potentials for molecular ground states. The method is however much less applied for the excited electronic states since a very limited number of excited state levels are usually involved. In this paper the authors
exploit the possibility to considerably enhance the number of excited levels by means of collisional population. This method is applied to the D1П state of NaRb molecule which is currently of interest for ultra-cold photoassociation.
More than 1400 highly accurate (0.01 cm-1) term values are obtained using just a few standard Ar+ lines as well as a frequency doubled cw YAG laser line and from them an accurate potential curve, a large set of Λ-doubling constants
(q-factors) are derived and regions of local perturbations are identified.
[Knoeckel et al., Eur. Phys. D Vol 36, 49-55 (2005)]
August 2005
Electron impact ionization of atomic clusters in ultraintense laser fields
With the advent of femtosecond, ultraintense lasers (peak intensities
1015- 1020 Wcm-2), the exploration of energy acquisition and disposal in molecular clusters (whose density is comparable to that of the condensed phase) encompasses attosecond-femtosecond electron dynamics. Of considerable interest
is the dynamics of the energetic nanoplasma (electron energies 50eV-10keV) produced by inner ionization of clusters. A computational study of the reactive nanoplasma dynamics in Xen clusters (n = 55-2171, cluster radii 8-35Å)
elucidated the inelastic processes of electron impact ionization, which are efficient at lower laser intensities (1015-1016 Wcm-2) and for large clusters (n > 2000). 'Laser free' electron impact ionization prevails after the termination
of the laser pulse. Interesting implications involve ionization control in ultraintense laser fields and studies of electron dynamics on the time scale of a few optical laser cycles.
[Heidenreich et al., Eur. Phys. D Vol 35, 567-577 (2005)]
July 2005
Single-photon wavefront-splitting interference. An illustration of the light quantum in action
Wave particle-duality is a crucial concept in quantum
mechanics, and most textbooks of quantum physics describe a thought experiment based on Young's double slit interference setup. For a single quantum particle sent one at a time through this interferometer, it is not possible to record
at the same time the interference pattern associated to its wave like behavior, and to measure by which slit each particle went through. In contrast to the widespread belief that this GedankenExperiment had been turned into real experiments
by use of very attenuated light from a classical source, it is only with the development of single photon sources that such a scheme can be realized with light.
We describe a new realization of this textbook experiment using a clock-triggered
single-photon source and a wavefront-splitting interferometer based on a Fresnel's biprism. The experimental evidence for single-photon behaviour is given by the lack of coincidences between two detectors in the two paths of the interferometer.
The complementary experiment consists of the observation of interference fringes in the overlapping region of the two wavefronts. An intensified CCD camera allows us to record the photon-per-photon interference build-up in the single photon regime,
and the corresponding movie can be downloaded for pedagogical purposes.
[Jacques et al., Eur. Phys. D Vol 35, 561-565 (2005)]
June 2005
Coherent driving of Tm3+:YAG ions using a complex hyperbolic secant optical field
Since 1917 it is known that absorption of light always competes with stimulated
emission. As a consequence it is practically difficult to efficiently drive atoms between excitation levels by optical means, although such flexible driving is highly desired in the context of quantum information. This paper examines frequency
selective population transfer in an inhomogeneously broadened sample by means of "magic" complex hyperbolic secant optical pulses. This technique has been used successfully in NMR but the required shaping of the driving field has long hampered
its application to the optical domain. Today, fast digital arbitrary waveform generators together with long coherence time lasers make the technique tractable in long coherence lifetime systems such as rare earth ion embedded in a crystal matrix.
Using an original adiabatic passage picture as a guideline, we first discuss the limitations imposed by the finite duration of the pulse and the finite atomic coherence lifetime. Quite general analytical expressions are easily derived within the
frame of the adiabatic picture. Then we experimentally demonstrate a transfer efficiency of more than 95% in Tm3+:YAG two-level ions.
[de Seze et al., Eur. Phys. D Vol 33, 345-355, (2005)]
December 2004
Storage ring free electron laser dynamics in presence of an auxiliary harmonic radio frequency cavity
A Free Electron Laser (FEL) based on the electron
beam of a synchrotron radiation (SR) facility is a complex device because of the complexity of either system, storage ring or FEL. Incorporating an FEL can significantly enhance the range of applications of an SR facility, as e.g. pump-probe
experiments can be anticipated. However, experimental difficulties are substantial and considerable effort has been made at various laboratories for improving the performance of such FEL. In this paper we address the problem of the optimisation of
this instrument. Especially the effect of a harmonic cavity in the ring for shortening electron bunch and increasing the gain of the laser is described. It is shown both by experiments and by simulations, that the combined system behaves as a dynamical
system of coupled oscillators. Typical electron beam instabilities are counteracted by the FEL, which can be considered as an additional electron beam instability itself. The FEL acts as a beam stabilizer under certain conditions, leading to overall
reliable FEL performance.
[C.A. Thomas et al., Eur. Phys. J. D Vol 32, 83-93 (2005)]
August 2004
An atom interferometer for measuring loss of coherence from an atom mirror
As every optical physicist knows, the roughness of an optical
surface such as a mirror results in diffuse scattering of the incoming waves and a loss of their coherence. Loss of coherence is equivalent to reduction of contrast in interference fringes. The same principle applies to any wave
phenomenon, including the reflection of atomic matter waves. In this experiment an atom mirror, based on an evanescent wave, is placed within an atom interferometer and the interference fringe contrast induced by the mirror is
observed. Thus the coherence function of the mirror is directly observed.
[J. Est ve et al., Eur. Phys. J. D Vol 31, 487-491 (2004)]
March 2004
Quantum key distribution over 30 km of standard fiber using energy-time entangled photon pairs: a comparison of two chromatic dispersion reduction methods
Real-world entangled photons quantum cryptography Quantum cryptography is definitely stepping out the quantum lab. We present a full implementation of long range quantum key distribution (QKD) based on energy-time entangled photon pairs. This
system works with standard telecom fibers and solves the problem related to chromatic dispersion. Although system based on faint laser pulses can offer the same unconditional security, our system features some potential advantages:
First,
the signal to noise ratio is improved since there are less empty pulses, which allows to cover longer distances. Second, if there are two photon pairs in the same pulse, a potential eavesdropper cannot gain any information as the pairs are
independant. Finally, although the outcomes at Alice and Bob are perefectly correlated, they are inherently random and therefore no random number generator is needed.
[S. Fasel et al., Eur. Phys. J. D Vol 30, 143-148 (2004)]
December 2003
Quantum chaos and random matrix theory for fidelity decay in quantum computations with static imperfections
Quantum computers, relying on the weird logics
of the quantum world, can be much faster than any classical machine, since they perform, so to speak, all the calculations at once. However, their construction is rather challenging, since they are prone to errors and decoherence. This paper
addresses an important question: what is the influence of static imperfections in computer hardware (parasitic coupling between qubits, imperfections in the quantum gates transformations...). It presents a universal law, based on random matrix
theory, for the computation fidelity decay induced by internal static imperfections. The theoretical predictions are confirmed by extensive numerical simulations of a polynomial quantum algorithm for quantum chaos in a dynamical map with up to
18 qubits. The decay law provides a transition between exponential and gaussian behaviours and is characterized by two time-scales analogous to the Thouless and Heisenberg times for probability decay in mesoscopic systems. These studies establish
the universal accuracy bounds for quantum computation in presence of residual static couplings between qubits. They open a link between random matrix theory and realistic quantum computations. They are essential to estimate the individual gates
fidelity required for a large scale quantum computation.