We present measurements fully characterizing the whole life cycle of femtosecond pulses undergoing filamentation in water at 400 nm. The complete pulse dynamics is monitored by means of a four-dimensional mapping technique for the intensity distribution $I(x,y,z,t)$ during the nonlinear interaction. Measured events (focusing or defocusing cycles, pulse splitting and replenishment, supercontinuum generation, conical emission, nonlinear absorption peaks) are mutually connected.The filament evolution from laser energy deposition in water, which is of paramount importance for a wide range of technological and medical applications, is interpreted in light of simulation results.

Ponderomotive squeezing of the output light of an optical cavity has been recently observed in the megahertz range in two different cavity optomechanical devices. Quadrature squeezing becomes particularly useful at lower spectral frequencies, for example, in gravitational wave interferometers, despite being more sensitive to excess phase and frequency noise. Here we show a phase and frequency-noise cancellation mechanism due to destructive interference which can facilitate the production of ponderomotive squeezing in the kilohertz range and we demonstrate it experimentally in an optomechanical system formed by a Fabry-Pérot cavity with a micromechanical mirror.

The thermal emission from two guided modes in two-dimensional periodic structures is investigated using first-principles calculations. We use a model that integrates a quantum mechanical analysis of light-matter interactions and a classical connection condition that describes the coupling between light modes inside and outside the structure. We focus on what happens when two guided modes share common radiation modes where their thermal emission overlaps in frequency, solid angle, and polarization in free space. It is shown that we have to take into account the fact that the reaction of the radiation from one guided mode works not only on that guided mode but also on the other guided mode, in order to calculate the emissivity from two overlapping guided modes accurately. Emissivity spectra peculiar to the two-guided mode cases are shown. The condition to maximize the light-matter interaction with respect to thermal emission in the case with two guided modes is also discussed.

We study nonperturbatively the time evolution of a qubit subject to amplitude-damping noise. We show that at strong coupling the qubit decoherence can be quenched owing to large environment feedback, such that the qubit can evolve coherently even in the long-time limit. As an application, we show that for a quantum channel that consists of two independent qubits subject to uncorrelated local amplitude-damping noises, it can maintain at strong coupling finite entanglement and better than classical teleportation fidelity at long times.

The dynamics of any electron-based qubit in quantum condensed-matter systems will inherently involve both spin and charge degrees of freedom however it is defined. The spin-orbit interaction will cause both spin precession and spin-charge entanglement and the resulting dynamics can be very complex. Here we report a scheme for performing quantum logic operations in quantum condensed-matter systems using coherent-state spin qubits that exploit both the electron's charge and spin degrees of freedom. In addition to the conceptual interest in such qubits, our analytical and numerical techniques have general applicability to many-electron dynamics in spin-orbit systems.

Steps towards implementing a collision-based two-qubit gate in optical lattices have previously been realized by the parallel merging of all pairs of atoms in a periodicity two superlattice. In contrast, we propose an architecture which allows for the merger of a selected qubit pair in a long-periodicity superlattice structure consisting of two optical lattices with close-lying periodicity. We numerically optimize the gate time and fidelity, including the effects on neighboring atoms and in the presence of experimental sources of error. Furthermore, the superlattice architecture induces a differential hyperfine shift, allowing for single-qubit gates. The fastest possible single-qubit gate times, given a maximal tolerable rotation error on the remaining atoms at various values of the lattice wavelengths, are identified. We find that robust single- and two-qubit gates with gate times of a few hundred microseconds and with error probabilities $\sim$${10}^{-3}$ are possible.

We present experimental evidence for the dominance of prompt single-atom knockout in fragmenting collisions between large polycyclic aromatic hydrocarbon cations and He atoms at center-of-mass energies close to 100 eV. Such nonstatistical processes are shown to give highly reactive fragments. We argue that nonstatistical fragmentation is dominant for any sufficiently large molecular system under similar conditions.

Using the cold target recoil ion momentum spectroscopy (COLTRIMS) technique, we have measured the simultaneous projectile and target ionization in collisions of ${\mathrm{He}}^{+}$ projectiles with a mixture of gaseous ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ for an incident projectile energy of 650 keV. Motivated by Cooper et al. [Phys. Rev. Lett. 100, 043204 (2008)], we look for differences in the ionization cross section of the two isotopes with the highest resolution and statistical significance. Contributions of the electron-electron and electron-nucleus interactions have been clearly separated kinematically by measuring the longitudinal and transverse momentum of the recoiling ion. We find no significant isotope effect in any of our momentum distributions.

A combination of femtosecond time-resolved photoelectron imaging technique and time-resolved mass spectroscopy technique are implemented to investigate the electronic excited-state dynamics in N,N-dimethylformamide (NNDMF). The ultrafast internal conversion (IC) of $S_1$ ($n$π${}^{*}$) and $S_2$ (ππ${}^{*}$) excited states of NNDMF are observed in this experiment. The molecule is excited to the lowest-lying ππ${}^{*}$ state ($S_2$ state) following absorption of two 400-nm photons. It is found that the population of the $S_2$ (ππ${}^{*}$) state undergoes ultrafast IC to the highly vibrationally excited $S_1$ ($n$π${}^{*}$) state within 99 fs by very fast C-N stretching, while the nonradiative deactivation of the $S_1$ ($n$π${}^{*}$) state occurs in 2.4 ps, and it is to a large extent due to the C-N bond cleavage from the $S_1$ potential energy surface, which would be able to efficiently compete with the IC of $S_1$ → $S_0$ through $S_1$/$S_0$ conical intersections.

We address the problem of calculating finite-temperature response functions of an experimentally relevant low-dimensional, strongly correlated system: the integrable one-dimensional Bose gas with a repulsive $\delta$-function interaction (the Lieb-Liniger model). Focusing on the dynamical density-density function, we present a Bethe ansatz-based method allowing for its accurate evaluation in finite but large systems, over broad ranges of momenta, frequencies, temperatures, and interaction parameters which are difficult to access using other methods. This allows us to quantify the reshaping of the zero-temperature critical behavior by thermal fluctuations, in experimentally accessible regimes.

The coupling between the spin degrees of freedom and the orbital angular momentum has a profound effect on the properties of nuclei, atoms, and condensed-matter systems. Recently, synthetic gauge fields have been realized experimentally in neutral cold-atom systems, giving rise to a spin-orbit coupling term with “strength” $k_{\text{so}}$. This paper investigates the interplay between the single-particle spin-orbit coupling term of Rashba type and the short-range two-body $s$-wave interaction for cold atoms under external confinement. Specifically, we consider two different harmonically trapped two-atom systems. The first system consists of an atom with spin-orbit coupling that interacts with a structureless particle through a short-range two-body potential. The second system consists of two atoms that both feel the spin-orbit coupling term and that interact through a short-range two-body potential. Treating the spin-orbit term perturbatively, we determine the correction to the ground-state energy for various generic parameter combinations. Selected excited states are also treated. An important aspect of our study is that the perturbative treatment is not limited to small $s$-wave scattering lengths but provides insights into the system behavior over a wide range of scattering lengths, including the strongly interacting unitary regime. We find that the interplay between the spin-orbit coupling term and the $s$-wave interaction generically enters, depending on the exact parameter combinations of the $s$-wave scattering lengths, at order $k_{\text{so}}^2$ or $k_{\text{so}}^4$ for the ground state and leads to a shift of the energy of either sign. While the absence of a term proportional to $k_{\text{so}}$ follows straightforwardly from the functional form of the spin-orbit coupling term, the absence of a term proportional to $k_{\text{so}}^2$ for certain parameter combinations is unexpected. The well-known fact that the spin-orbit coupling term couples the relative and center-of-mass degrees of freedom has interesting consequences for the trapped two-particle systems. For example, we find that, for certain parameter combinations, the spin-orbit coupling term turns sharp crossings into avoided crossings with an energy splitting proportional to $k_{\text{so}}$. Our perturbative results are confirmed by numerical calculations that expand the eigenfunctions of the two-particle Hamiltonian in terms of basis functions that contain explicitly correlated Gaussians.

We analyze the eigenstates of a two-dimensional lattice with additional harmonic confinement in the presence of an artificial magnetic field. While the softness of the confinement makes a distinction between bulk and edge states difficult, the interplay of harmonic potential and lattice leads to a different classification of states in three energy regions: In the low-energy regime, where lattice effects are small, all states are transporting topologically nontrivial states. For large energies above a certain critical value, the periodic lattice causes localization of all states through a mechanism similar to Wannier-Stark localization. In the intermediate energy regime transporting, topologically nontrivial states coexist with topologically trivial countertransporting chaotic states. The character of the eigenstates, in particular their transport properties, are studied numerically and are explained using a semiclassical analysis.

We review the derivation of the effective Dirac equation for ultracold atoms in one-dimensional bichromatic optical lattices, following the proposal by Witthaut et al. [Phys. Rev. A 84, 033601 (2011)]. We discuss how such a derivation—based on a suitable rotation of the Bloch basis and on a coarse-graining approximation—is affected by the choice of the Wannier functions entering the coarsening procedure. We show that in general the Wannier functions obtained by rotating the maximally localized Wannier functions for the original Bloch bands can be sufficiently localized for justifying the coarse-graining approximation. We also comment on the relation between the rotation needed to achieve the Dirac form and the standard Foldy-Wouthuysen transformation. Our results provide a solid ground for the interpretation of the experimental results by Salger et al. [Phys. Rev. Lett. 107, 240401 (2011)] in terms of an effective Dirac dynamics.

We investigate the possibility to produce a Bose-Einstein condensate made of positronium atoms in a porous silica material containing isolated nanometric cavities. The evolution equation of a weakly interacting positronium system is presented. The model includes the interactions among the atoms in the condensate, the surrounding gas of noncondensed atoms, and the pore surface. The final system is expressed by the Boltzmann evolution equation for noncondensed particles coupled with the Gross-Pitaevskii equation for the condensate. In particular, we focus on the estimation of the time necessary to form a condensate containing a macroscopic fraction of the positronium atoms initially injected in the material. The numerical simulations reveal that the condensation process is compatible with the lifetime of ortho-positronium.

Extreme and rare events are nowadays the object of intensive research. Rogue waves are extreme waves that appear suddenly in many natural systems, even in apparently calm situations. Here we study numerically the rogue wave dynamics in an optically injected semiconductor laser with external periodic forcing that is implemented via direct modulation of the laser pump current. In the region of optical injection parameters where the laser intensity is chaotic and occasional ultrahigh pulses occur, our aim is to control the system by applying a weak modulation. We find that for an adequate range of frequency and amplitude parameters, the modulation can completely suppress the extreme pulses. We also show that the interplay between modulation and an external source of noise can significantly modify their probability of occurrence. These results can motivate a range of experimental and theoretical investigations in other natural systems.

It is shown that Mie's solution to Maxwell's equations no longer holds for the analysis of resonance of a plasmonic metal nanosphere. The conventional Mie's solution is based on the spherical Bessel and the spherical Hankel functions of an outgoing wave, whereas the permittivity of metals of a negative real part leads to a phase velocity that directs inward to the sphere, which is opposite from the direction of the energy flow as often discussed for negative-index metamaterials. This is a fundamental problem overlooked for a long time; a correction can be found from the viewpoint of a time-reversal problem involving negative permittivity media. The continuity of the field solution at the sphere surface is shown to be corrected by replacing the spherical Hankel function of an outgoing wave with that of an incoming wave, i.e., by adopting the complex conjugate of the conventional solutions. The corrected theory has been verified by the analyses of various metal nanospheres. In addition, the derivation of the scattering cross sections based on the corrected theory has elucidated that the conservation law of energy holds and that, more importantly, the conventional Mie's solution gives the same amplitude of the cross sections when they are obtained for real, not complex, frequency.

The electromagnetically induced transparency (EIT) spectrum of atoms diffusing in and out of a narrow beam is measured and shown to manifest the two-dimensional $\delta$-function anomaly in a classical setting. In the limit of small-area beams, the EIT line shape is independent of power, and equal to the renormalized local density of states of a free particle Hamiltonian. The measured spectra for different powers and beam sizes collapses to a single universal curve with a characteristic logarithmic Van Hove singularity close to resonance.

We discuss phase transitions in $\mathcal{PT}$-symmetric optical systems. We show that, due to frequency dispersion of the dielectric permittivity, an optical system can have $\mathcal{PT}$ symmetry at isolated frequency points only. An assumption of the existence of a $\mathcal{PT}$-symmetric system in a continuous frequency interval violates the causality principle. Therefore, the ideal symmetry-breaking transition cannot be observed by simply varying the frequency.

We show that with adiabatic passage, one can reliably drive two-photon optical transitions between the ground states and interacting Rydberg states in a pair of atoms. For finite Rydberg-interaction strengths an adiabatic pathway towards the doubly Rydberg excited state is identified when a constant detuning is applied with respect to an intermediate optically excited level. The Rydberg interaction among the excited atoms provides a phase that may be used to implement quantum gate operations on atomic ground-state qubits.