Highlights of 2014

The triatomic hydrogen molecular ion is instrumental as a benchmark towards understanding the strong-field dynamics of polyatomic molecules. Using a crossed-beams coincidence three-dimensional momentum imaging method, we demonstrate clear isotopic effects in the fragmentation of D₂H⁺ induced by 7 fs (40 fs), 790 nm laser pulses at an intensity of 10¹⁶ W cm⁻² (5 × 10¹⁵ W cm⁻²). Our experiment uniquely separates all fragmentation channels and provides kinematically complete information for the nuclear fragments. For example, we show that for dissociative ionization of D₂H⁺ there is a large difference in branching ratios of the two-body channels, namely, H⁺+D$_2^+$ dominates D⁺+HD⁺, whereas there is minimal difference in branching ratios between the dissociation channels H⁺+D₂ and D⁺+HD.

The total cross section for simultaneous ionization–excitation of helium by electron impact has been revisited within the framework of the fully nonperturbative B-spline R-matrix with pseudostates approach. A long-standing discrepancy regarding the absolute normalization of the cross section for ionization with simultaneous excitation of the residual ion into the He⁺(2p) is resolved in favour of the values suggested by Bloemen et al (1981 J. Phys. B: At. Mol. Phys. 14 717) and Forand et al (1985 J. Phys. B: At. Mol. Phys. 18 1409) rather than the renormalization proposed by Merabet et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 3383).

The autoionization lifetime of doubly excited H₂ created by single photon absorption has been measured by means of a kinematically complete study. For dissociative ionization the experimentally observed asymmetry in the electron ejection direction with respect to the emitted proton is used to disentangle the two interfering pathways, direct ionization and autoionization. This allows us to determine the autoionization lifetime of the energetically lowest doubly excited $Q_1\,^1\Sigma _\mathrm{u}^+(1)$ state for a large range of internuclear distances, including the previously inaccessible small values. Excellent agreement with available ab initio calculations is obtained.

As He is exposed to double attosecond pulses with a time delay, double ionization can proceed in several different pathways. Through ab initio simulations by the fully dimensional time-dependent Schrödinger equation, we find that interferences among different pathways in the sequential two-photon double ionization can lead to interesting grid-like patterns in the joint energy spectra of the two electrons. We show that not only these interference patterns, but also the total double ionization probability critically depends on the relative carrier–envelope phase (CEP) between the two attosecond pulses. Our findings are successfully explained by a model based on second-order time-dependent perturbation theory. The present study demonstrates the feasibility of CEP control of double ionization, and provides an alternative way to characterize the relative CEP of attosecond pulses.

We investigate experimentally and theoretically the Faraday effect in an atomic medium in the hyperfine Paschen–Back regime, where the Zeeman interaction is larger than the hyperfine splitting. We use a small permanent magnet and a micro-fabricated vapour cell, giving magnetic fields of the order of a tesla. We show that for low absorption and small rotation angles, the refractive index is well approximated by the Faraday rotation signal, giving a simple way to measure the atomic refractive index. Fitting to the atomic spectra, we achieve magnetic field sensitivity at the 10⁻⁴ level. Finally we note that the Faraday signal shows zero crossings which can be used as temperature insensitive error signals for laser frequency stabilization at large detuning. The theoretical sensitivity for ⁸⁷Rb is found to be ∼40 kHz °C⁻¹.

We present a rather elaborate theoretical model describing the dynamics of neon under radiation of photon energies ∼93 eV and pulse duration in the range of 15 fs, within the framework of lowest non-vanishing order of perturbation theory, cast in terms of rate equations. Our model includes sequential as well as direct multiple ionization channels from the 2s and 2p atomic shells, including aspects of fine structure, whereas the stochastic nature of SASE-FEL light pulses is also taken into account. Our predictions for the ionization yields of the different ionic species are in excellent agreement with the related experimental observations at FLASH.

We present the slow time discretization (STD) method for solving the time-dependent Schrödinger equation. The method is an extension of the slow variable discretization method for solving the stationary Schrödinger equation (Tolstikhin et al 1996 J. Phys. B: At. Mol. Opt. Phys. 29 L389), with time treated as the 'slow' variable. It is based on an expansion of the state vector in a discrete variable representation basis, in time, and an adiabatic basis, in Hilbert space. This approach is much more efficient in implementation than a direct solution of the Born–Fock equations. The versatility of the STD time propagator is illustrated through calculations for one-dimensional models of the ionization of hydrogen by an intense laser pulse and resonance charge transfer in proton–hydrogen collisions. The method is shown to perform well in the broad dynamical range considered, from adiabatic to nonadiabatic regimes.

The effect of vibrational autoionization on the H₂⁺ X ²Σ_g⁺ v⁺ = 3, N⁺ state rotationally resolved photoelectron angular distributions and branching ratios has been investigated with a velocity map imaging spectrometer and synchrotron radiation. In photon excitation regions free from the influence of autoionizing Rydberg states, where direct ionization dominates, the photoelectron anisotropy parameter associated with the X ¹Σ_g⁺ v'' = 0, N'' = 1 → X ²Σ_g⁺ v⁺ = 3, N⁺ = 1 transition has a value close to the theoretical maximum. However, in the vicinity of a Rydberg state, vibrational autoionization leads to a substantial reduction in anisotropy. The value of the anisotropy parameter associated with the S-branch of the photoelectron spectrum is found to be considerably higher than that predicted under the assumption that the outgoing electron can be represented solely as a p-wave. This suggests that the f-wave contribution must be taken into account to obtain a proper description of the photoionization dynamics. The observed variations in the rotationally resolved branching ratios, in the vicinity of an autoionizing resonance, depend upon the rotational level of the Rydberg state. The rotationally averaged photoelectron anisotropy parameters have been compared with the corresponding, previously calculated, theoretical results and reasonable agreement has been found. The influence of vibrational autoionization on the H₂⁺ X ²Σ_g⁺ v⁺ = 0, 1, 2, 3 vibrational branching ratios has also been investigated, and the experimental results show that, in energy regions encompassing Rydberg states, these ratios deviate strongly from the Franck–Condon factors for direct ionization.

We investigate the impact of combined electric and magnetic fields on the structure of ultralong-range polar Rydberg molecules. Our focus is hereby on the parallel as well as the crossed field configuration, taking into account both the s-wave and p-wave interactions of the Rydberg electron and the neutral ground state atom. We show the strong impact of the p-wave interaction on the ultralong-range molecular states for a pure B-field configuration. In the presence of external fields, the angular degrees of freedom acquire vibrational character, and we encounter two- and three-dimensional oscillatory adiabatic potential energy surfaces for the parallel and crossed field configuration, respectively. The equilibrium configurations of local potential wells can be controlled via the external field parameters for both field configurations depending on the specific degree of electronic excitation. This allows us to tune the molecular alignment and orientation. The resulting electric dipole moment is in the order of several kDebye, and the rovibrational level spacings are in the range of 2–$250\;{\rm MHz}$. Both properties are analyzed with varying field strengths.

We report a linear surface-electrode trap that can be used to form parallel ion strings. By adjusting the balance of the radio-frequency (RF) voltages applied to central and RF electrodes, the RF pseudopotential can be varied from single-well to double-well in the radial direction. Ions located on two parallel lines of the RF potential null are in principle free from excess micromotion if appropriate static voltages are applied. Calcium ions were trapped for the evaluation of the designed electrode. An ion string in the single-well potential and two ion strings in the double-well potential were observed by changing the RF voltages. Such traps could be used for quantum simulation of coupled spin systems.

We propose a possible experimental realization of a quantum analogue of Newton's cradle using a configuration which starts from a Bose–Einstein condensate. The system consists of atoms with two internal states trapped in a one-dimensional tube with a longitudinal optical lattice and maintained in a strong Tonks–Girardeau regime at maximal filling. In each site the wave-function is a superposition of the two atomic states and a disturbance of the wave-function propagates along the chain in analogy with the propagation of momentum in the classical Newton cradle. The quantum travelling signal is generally deteriorated by dispersion, which is large for a uniform chain and is known to be zero for a suitably engineered chain, but the latter is hardly realizable in practice. Starting from these opposite situations we show how the coherent behaviour can be enhanced with minimal experimental effort.

We study the statics and dynamics of a binary dipolar Bose–Einstein condensate soliton for repulsive inter and intraspecies contact interactions with the two components subject to different spatial symmetries—distinct quasi-one-dimensional and quasi-two-dimensional shapes—using numerical solution and variational approximation of a three-dimensional mean-field model. The results are illustrated with realistic values of parameters in the binary ¹⁶⁴Dy-¹⁶⁸Er mixture. The possibility of forming robust dipolar solitons of a very large number of atoms make them of great experimental interest. The existence of the solitons is illustrated in terms of stability phase diagrams. Exotic shapes of these solitons are illustrated in isodensity plots. The variational results for statics (size and chemical potential) and dynamics (small oscillation) of the binary soliton compare well with the numerical results. A way of preparing and studying these solitons in the laboratory is suggested.

We calculate high-order harmonic spectra from graphene based on the strong-field approximation using circularly polarized infrared laser pulses. We allow for the plane of polarization to be tilted with respect to the two-dimensional graphene sheet, demonstrating that the structure of the harmonic spectra strongly depends on the tilt angle.

We present an interferometric method to recover the phase diffraction patterns of a Gaussian optical vortex (OV) beam with different topological charges. The patterns are encoded and converted into polarization information, which can be decoded and measured by an analyser and a camera. Helical diffraction structures due to the spiral phase plates are obtained by a Mach–Zehnder interferometer. A numerical method based on angular spectrum representation is developed to simulate both the polarization and the distorted phase pattern of the OV beam through propagation. Simulation results show good agreement with the experiment.

We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and a certain class of non-Gaussian states. We show that any separation in the peaks of the marginal distributions of one subsystem conditioned on two different outcomes of homodyne measurements performed on the other subsystem indicates correlation between the corresponding quadratures, and hence nonzero discord. We also apply this method to non-Gaussian states that are prepared by overlapping a statistical mixture of coherent and vacuum states on a beam splitter. We experimentally demonstrate this technique by verifying nonzero quantum discord in a bipartite Gaussian and certain non-Gaussian states.

High-dimensional quantum key distribution (QKD) offers the possibility of encoding multiple bits of key on a single entangled photon pair. An experimentally promising approach to realizing this is to use energy–time entanglement. Currently, however, the control of very high-dimensional entangled photons is challenging. We present a simple and experimentally compact approach, which is based on a cavity that allows one to measure two different bases: the time of arrival and another that is approximately mutually unbiased to the arrival time. We quantify the errors in the setup, due both to the approximate nature of the mutually unbiased measurement and as a result of experimental errors. It is shown that the protocol can be adapted using a cut-off so that it is robust against the considered errors, even within the regime of up to 10 bits per photon pair.

For many open quantum systems, a master equation approach employing the Markov approximation cannot reliably describe the dynamical behaviour. This is the case, for example, in a number of solid state or biological systems, and it has motivated a line of research aimed at quantifying the amount of non-Markovian behaviour (NMB) in a given model. Within this framework, we investigate the dynamics of a quantum harmonic oscillator linearly coupled to a bosonic bath. We focus on Gaussian states, which are suitably treated using a covariance matrix approach. Concentrating on an entanglement based NMB quantifier (NMBQ) proposed by Rivas et al (2010 Phys. Rev. Lett. 105 050403), we consider the role that near resonant and off-resonant modes play in affecting the NMBQ. By using a large but finite bath of oscillators for both Ohmic and super Ohmic spectral densities we find, by systematically increasing the coupling strength, initially the near resonant modes provide the most significant non-Markovian effects, while after a certain threshold of coupling strength the off-resonant modes play the dominant role. We also consider the NMBQ for two other models where we add a single strongly coupled oscillator to the model in extra bath mode and 'buffer' configurations, which affects the modes that determine NMB.

We present a study of direct laser driven electron acceleration and scaling of attosecond bunch compression in unbound vacuum. Simple analytical expressions and detailed three-dimensional numerical calculations including space charge and non-paraxial laser fields reveal the conditions for compression to attosecond electron sheets. Intermediate emittance minima suitable for brilliant x-ray generation in optical undulators are predicted. We verify a favourable coherent enhancement of the resulting x-ray fields and demonstrate feasibility for realistic laser parameters.

Spin dynamics and induced spin effects in above-threshold ionization of hydrogenlike highly charged ions in super-strong laser fields are investigated. Spin-resolved ionization rates in the tunnelling regime are calculated by employing two versions of a relativistic Coulomb-corrected strong-field approximation (SFA). An intuitive simpleman model is developed which explains the derived scaling laws for spin flip and spin asymmetry effects. The intuitive model as well as our ab initio numerical simulations support the analytical results for the spin effects obtained in the dressed SFA where the impact of the laser field on the electron spin evolution in the bound state is taken into account. In contrast, the standard SFA is shown to fail in reproducing spin effects in ionization even at a qualitative level. The anticipated spin-effects are expected to be measurable with modern laser techniques combined with an ion storage facility.

The photon-ion merged-beams technique has been employed at the new Photon-Ion spectrometer at PETRA III for measuring multiple photoionization of Xe^{q +} (q = 1–5) ions. Total ionization cross sections have been obtained on an absolute scale for the dominant ionization reactions of the type hν + Xe^{q +} → Xe^{r +} + (q − r)e⁻ with product charge states q + 2 ⩽ r ⩽ q + 5. Prominent ionization features are observed in the photon-energy range 650–750 eV, which are associated with excitation or ionization of an inner-shell 3d electron. Single-configuration Dirac–Fock calculations agree quantitatively with the experimental cross sections for non-resonant photoabsorption, but fail to reproduce all details of the measured ionization resonance structures.

We study the electron momentum correlation of an aligned diatomic molecule using a semiclassical model that includes tunnelling ionization and consequent depletion of excited states in the non-sequential double ionization (NSDI) process. The electron correlation distribution shows an alignment dependence. Our analysis shows that this alignment effect can be attributed to alignment-dependent ionization of the excited states involved in the NSDI process.

Time-resolved profiles of the Balmer series Hα line emitted by a laser-produced hydrogen plasma have been measured to determine the shift and width for electron densities from below 10¹⁸ to above 10²⁰ cm⁻³ at an average temperature of 28000 K. Fits of the profiles that allow for self-absorption in the plasma yield shifts and widths that are consistent with experiments on lower density and cooler gas-liner pinch plasmas. The width scales as $N_{e}^{0.70\pm 0.03}$ and the shift as $N_{e}^{0.92\pm 0.03}$ between $8.7\times {{10}^{17}}$ and $1.4\times {{10}^{20}}$ cm⁻³. Hα shifts monotonically and nearly linearly to the red with increasing density under the reported conditions. A comparison to theory calculations using exact potentials for ${\rm H}_{2}^{+}$ shows that an intrinsic asymmetry becomes significant only in the upper limit of this range when a satellite develops in the far red wing.

X-ray imaging crystal spectroscopy (XICS) has been implemented on magnetic confinement fusion devices as a novel means of measuring local plasma temperature, impurity density, and flow profiles. At Alcator C-Mod, XICS allows for spatially-resolved, high spectral resolution measurements between 0.3 and 0.4 nm, enabling detailed analysis of He-like argon x-ray emission. Electron temperatures in the range of 0.5 keV ⩽T_e ⩽3.0 keV are determined from He-like argon emissivity ratios of the n = 3 dielectronic satellites to the w-line and its surrounding n ⩾ 3 satellites, specifically the wavelength range of 3.9440 Å ⩽ λ ⩽ 3.9607 Å. These data are validated against measurements of T_e from existing electron cyclotron emission and Thomson scattering diagnostics. Line ratio data are analysed via a tomographic inversion procedure, overcoming the traditional issue of spectra being averaged over the plasma cross-section. The implications of utilizing x-ray line ratios as a valid local temperature diagnostic are not limited to Alcator C-Mod; properties of plasma in future experiments as well as in astrophysical settings can also be investigated. The results of this experiment confirm that x-ray line ratios can be used as an accurate electron temperature diagnostic. The electron temperature can be determined from the relation to the line ratio, x, as T_e[keV] = 0.1552x^−0.7781 with 0.0223 < x < 0.2449.

Molecular-type systems, (m₁)^±(m₂)^±(m₃)^∓, consisting of three particles of masses m_i and of unit electric charges with the Coulomb interactions weakened by the Debye screening are considered. Existence and range of Borromean binding in symmetric systems (of masses m₁ = m₂) is investigated with respect to the masses of their constituents. It is shown that such systems can exist in Borromean ground states if the mass ratio q = m₃/m₁ is less or equal to 1.668. This improves considerably the lower bound to the limit of existence of the Borromean binding of symmetric systems suggested by Pont and Serra (2009 Phys. Rev. A 79 032508) as q ⩽ 1. Qualitative meaning of the improvement is that the Borromean binding occurs not only for systems where two identical particles are heavier than the third one but it is also possible for systems of the opposite mass relation. The range of screening parameter in which a system is Borromean known as a Borromean window is also determined for μ⁺μ⁺e⁻, π⁺π⁺μ⁻, π⁺μ⁻μ⁻ and e⁺e⁻e⁻.

We review the status of strong-field and attosecond processes in bulk transparent solids near the Keldysh tunneling limit. For high enough fields and low-frequency excitations, the optical and electronic properties of dielectrics can be transiently and reversibly modified within the applied pulse. In Ghimire et al (2011 Phys. Rev. Lett. 107 167407) non-parabolic band effects were seen in photon-assisted tunneling experiments in ZnO crystals in a strong mid-infrared field. Using the same ZnO crystals, Ghimire et al (2011 Nat. Phys. 7 138–41) reported the first observation of non-pertubative high harmonics, extending well above the bandgap into the vacuum ultraviolet. Recent experiments by Schubert et al (2014 Nat. Photonics 8 119–23) showed a carrier envelope phase dependence in the harmonic spectrum in strong-field 30 THz driven GaSe crystals which is the most direct evidence yet of the role of sub-cycle electron dynamics in solid-state harmonic generation. The harmonic generation mechanism is different from the gas phase owing to the high density and periodicity of the crystal. For example, this results in a linear dependence of the high-energy cutoff with the applied field in contrast to the quadratic dependence in the gas phase. Sub-100 attosecond pulses could become possible if the harmonic spectrum can be extended into the extreme ultraviolet (XUV). Here we report harmonics generated in bulk MgO crystals, extending to $\sim 26$ eV when driven by ∼35 fs, 800 nm pulses focused to a ∼1 VÅ$^{-1}$ peak field. The fundamental strong-field and attosecond response also leads to Wannier–Stark localization and reversible semimetallization as seen in the sub-optical cycle behavior of XUV absorption and photocurrent experiments on fused silica by Schiffrin et al (2013 Nature 493 70–4) and Schultze et al (2013 Nature 493 75–8). These studies are advancing our understanding of fundamental strong-field and attosecond physics in solids with potential applications for compact coherent short-wavelength sources and ultra-high speed optoelectronics.

Interaction of a laser pulse with a centrally symmetric medium, such as an isotropic gas of atoms, leads to the generation of harmonic emission which contains exclusively odd harmonics of the incident field. This result is the consequence of both the central symmetry of the medium and the temporal symmetry of the oscillating electric field, $E(t+\pi /{{\omega }_{l}})=-E(t)$, where ω_l is the laser frequency. In the case of oriented heteronuclear molecules, the spatial symmetry no longer holds and both odd and even harmonics become allowed. Here we show, by solving the time-dependent Schrödinger equation for H$_{2}^{+}$, D$_{2}^{+}$, and T$_{2}^{+}$, that even-order harmonic generation is also possible for sufficiently long infrared (IR) laser pulses in homonuclear molecules. The appearance of even harmonics is a signature of the coupled electron-nuclear dynamics and reflects field-induced electron localization initiated by the strong laser field, which breaks the spatial symmetry in the system. The analysis of even harmonics generated by pulses of different durations might therefore provide information on correlated electron-nuclear dynamics and charge migration in more complex un-oriented molecular ensembles.

We present the detailed analysis of a new two-pulse orientation scheme that achieves macroscopic field-free orientation at the high particle densities required for attosecond and high-harmonic spectroscopies (Kraus et al 2013 arXiv:1311.3923). Carbon monoxide molecules are oriented by combining one-colour and delayed two-colour non-resonant femtosecond laser pulses. High-harmonic generation is used to probe the oriented wave-packet dynamics and reveals that a very high degree of orientation (N_up/N_total = 0.73–0.82) is achieved. We further extend this approach to orienting carbonyl sulphide molecules. We show that the present two-pulse scheme selectively enhances orientation created by the hyperpolarizability interaction whereas the ionization-depletion mechanism plays no role. We further control and optimize orientation through the delay between the one- and two-colour pump pulses. Finally, we demonstrate a complementary encoding of electronic-structure features, such as shape resonances, in the even- and odd-harmonic spectrum. The achieved progress makes two-pulse field-free orientation an attractive tool for a broad class of time-resolved measurements.

We present a general approach for calculating the magic-angle condition for time-resolved spectroscopy of isotropic samples. Allowing for arbitrary static polarizations and propagation directions of each pulse enables us to retrieve not only the known magic-angle conditions for linear and circular polarization, but also analogous conditions for anisotropy-free spectroscopy using elliptically polarized laser pulses. The results are exemplified on transient absorption and transferred to coherent two-dimensional spectroscopy. Furthermore we derive for transient absorption spectroscopy the relation between the measurable anisotropy and the molecular structure, i.e., the angle between the pumped and probed transition dipole moments (TDMs). The impact of multiple spectrally overlapping signals is considered and discussed on the example of a molecule with a two-fold degenerate TDM.

We present measurements of photoelectron angular distributions (PADs) for the ionization of aligned CO₂ molecules by an extreme ultraviolet laser pulse (17–45 eV) that is produced via high-harmonic generation, and that is polarized perpendicularly to the alignment laser axis. Differential PADs are recorded by taking the difference between two measurements for aligned and anti-aligned molecules. The results are compared with ab initio multi-channel R-matrix calculations, which are extended to include the computation of PADs. The calculations agree very well with the experiment, and are resolved with respect to the internal state of the ion and the polarization of the ionizing laser field relative to the molecular frame. For a sufficiently high, but experimentally achievable degree of alignment, the nodal structure of the molecular orbital becomes visible in the PAD.

Laser-driven plasma accelerators can generate accelerating gradients three orders of magnitude larger than radio-frequency accelerators and have achieved beam energies above 1 GeV in centimetre long stages. However, the pulse repetition rate and wall-plug efficiency of laser plasma accelerators is limited by the driving laser to less than approximately 1 Hz and 0.1% respectively. Here we investigate the prospects for exciting the plasma wave with trains of low-energy laser pulses rather than a single high-energy pulse. Resonantly exciting the wakefield in this way would enable the use of different technologies, such as fibre or thin-disc lasers, which are able to operate at multi-kilohertz pulse repetition rates and with wall-plug efficiencies two orders of magnitude higher than current laser systems. We outline the parameters of efficient, GeV-scale, 10 kHz plasma accelerators and show that they could drive compact x-ray sources with average photon fluxes comparable to those of third-generation light source but with significantly improved temporal resolution. Likewise free-electron laser (FEL) operation could be driven with comparable peak power but with significantly larger repetition rates than extant FELs.

Ultracold electron sources based on near-threshold photoionization of laser-cooled atomic gases can produce ultrashort electron pulses with a brightness potentially exceeding conventional pulsed electron sources. They are presently being developed for single shot ultrafast electron diffraction, where a bunch charge of 100 fC is sufficient. For application as an injector for x-ray free electron lasers (FEL) a larger bunch charge is generally required. Here we present preliminary calculations of an ultracold electron source operating at bunch charges up to 1 pC. We discuss the relevant bunch degradation processes that occur when the charge is increased. Using general particle tracer tracking simulations we show that bunches can be produced of sufficient quality for driving a 1 Å self amplified spontaneous emission free electron laser (SASE-FEL) at 1.3 GeV electron energy. In addition we speculate on the possibility of using the ultracold source for driving a 15 MeV SASE-FEL in Compton backscatter configuration into the quantum FEL regime.

This tutorial presents the theory necessary to model the propagation of light through an atomic vapour. The history of atom–light interaction theories is reviewed, and examples of resulting applications are provided. A numerical model is developed and results presented. Analytic solutions to the theory are found, based on approximations to the numerical work. These solutions are found to be in excellent agreement with experimental measurements.

Progress in the reliable preparation, coherent propagation and efficient detection of many-body states has recently brought collective quantum phenomena of many identical particles into the spotlight. This tutorial introduces the physics of many-boson and many-fermion interference required for the description of current experiments and for the understanding of novel approaches to quantum computing.

The field is motivated via the two-particle case, for which the uncorrelated, classical dynamics of distinguishable particles is compared to the quantum behaviour of identical bosons and fermions. Bunching of bosons is opposed to anti-bunching of fermions, while both species constitute equivalent sources of bipartite two-level entanglement. The realms of indistinguishable and distinguishable particles are connected by a monotonic transition, on a scale defined by the coherence length of the interfering particles.

As we move to larger systems, any attempt to understand many particles via the two-particle paradigm fails: in contrast to two-particle bunching and anti-bunching, the very same signatures can be exhibited by bosons and fermions, and coherent effects dominate over statistical behaviour. The simulation of many-boson interference, termed boson-sampling, entails a qualitatively superior computational complexity when compared to fermions. The problem can be tamed by an artificially designed symmetric instance, which allows a systematic understanding of coherent bosonic and fermionic signatures for arbitrarily large particle numbers, and a means to stringently assess many-particle interference. The hierarchy between bosons and fermions also characterizes multipartite entanglement generation, for which bosons again clearly outmatch fermions. Finally, the quantum-to-classical transition between many indistinguishable and many distinguishable particles features non-monotonic structures, which dismisses the single-particle coherence length as unique indicator for interference capability. While the same physical principles govern small and large systems, the deployment of the intrinsic complexity of collective many-body interference makes more particles behave differently.

Time-resolved velocity map photoelectron imaging is performed using sub-20 fs deep ultraviolet and vacuum ultraviolet pulses to study electronic dynamics of isolated polyatomic molecules. The non-adiabatic dynamics of pyrazine, furan and carbon disulfide (CS₂) are described as examples. Also described is sub-picosecond time-resolved x-ray direct absorption spectroscopy using a hard x-ray free electron laser (SACLA) and a synchronous near ultraviolet laser to study ultrafast electronic dynamics in solutions.

This paper provides an overview of ultrafast wavefront rotation of femtosecond laser pulses and its various applications in highly nonlinear optics, focusing on processes that lead to the generation of high-order harmonics and attosecond pulses. In this context, wavefront rotation can be exploited in different ways, to obtain new light sources for time-resolved studies, called 'attosecond lighthouses', to perform time-resolved measurements of nonlinear optical processes, using 'photonic streaking', or to track changes in the carrier–envelope relative phase of femtosecond laser pulses. The basic principles are explained qualitatively from different points of view, the experimental evidence obtained so far is summarized, and the perspectives opened by these effects are discussed.

This article addresses topics regarding time measurements performed on quantum systems. The motivation is linked to the advent of 'attophysics' which makes feasible to follow the motion of electrons in atoms and molecules, with time resolution at the attosecond (1 as = 10⁻¹⁸ s) level, i.e. at the natural scale for electronic processes in these systems. In this context, attosecond 'time-delays' have been recently measured in experiments on photoionization and the question arises if such advances could cast a new light on the still active discussion on the status of the time variable in quantum mechanics. One issue still debatable is how to decide whether one can define a quantum time operator with eigenvalues associated to measurable 'time-delays', or time is a parameter, as it is implicit in the Newtonian classical mechanics. One objective of this paper is to investigate if the recent attophysics-based measurements could shed light on this parameter–operator conundrum. To this end, we present here the main features of the theory background, followed by an analysis of the experimental schemes that have been used to evidence attosecond 'time-delays' in photoionization. Our conclusion is that these results reinforce the view that time is a parameter which cannot be defined without reference to classical mechanics.

Molecular dynamics is an active area of research, focusing on revealing fundamental information on molecular structures and photon–molecule interaction and with broad impacts in chemical and biological sciences. Experimental investigation of molecular dynamics has been advanced by the development of new light sources and techniques, deepening our understanding of natural processes and enabling possible control and modification of chemical and biomolecular processes. Free-electron lasers (FELs) deliver unprecedented intense and short photon pulses in the vacuum ultraviolet and x-ray spectral ranges, opening a new era for the study of electronic and nuclear dynamics in molecules. This review focuses on recent molecular dynamics investigations using FELs. We present recent work concerning dynamics of molecular interaction with FELs using an intrinsic clock within a single x-ray pulse as well as using an external clock in a pump–probe scheme. We review the latest developments on correlated and coincident spectroscopy in FEL-based research and recent results revealing photo-induced interaction dynamics using these techniques. We also describe new instrumentations to conduct x-ray pump–x-ray probe experiments with spectroscopy and imaging detectors.

The recent advances in developing compact laser plasma accelerators that deliver high quality electron beams in a more reliable way offer the possibility to consider their use in designing a compact free electron laser (FEL). Because of the particularity of these beams (especially concerning the divergence and the energy spread), specific electron beam handling is proposed in order to achieve FEL amplification.