We experimentally probe electron collisions with ${\mathrm{HC}}_3\mathrm{N}$ in the energy range from 0 to 10 eV with the focus on vibrational excitation and dissociative electron attachment. The vibrational excitation cross sections show a number of resonances which are mode specific: the two dominant ${\pi }^{*}$ resonances are visible in the excitation of all the vibrational modes; however, broad ${\sigma }^{*}$ resonances are visible only in certain bond-stretching vibrational modes. The lower ${\pi }^{*}$ resonance shows a pronounced boomerang structure. Since it overlaps with the threshold peak originating from a long-range electron-molecule interaction, the interference pattern is rather unusual. Somewhat surprisingly, the boomerang structure is visible also in the elastic scattering cross section. The dissociative electron attachment cross sections agree qualitatively with the data of Gilmore and Field [J. Phys. B 48, 035201 (2015)]; however, approximately a factor of two difference is found in the absolute values.

We describe an ab initio and nonperturbative $R$-matrix with time-dependence theory for ultrafast atomic processes in light fields of arbitrary polarization. The theory is applicable to complex, multielectron atoms and atomic ions subject to ultrashort (particularly few-femtosecond and attosecond) laser pulses with any given ellipticity, and it generalizes previous time-dependent $R$-matrix techniques restricted to linearly polarized fields. We discuss both the fundamental equations, required to propagate the multielectron wave function in time, as well as the computational developments necessary for their efficient numerical solution. To verify the accuracy of our approach, we investigate the two-photon ionization of He, irradiated by a pair of time-delayed, circularly polarized, femtosecond laser pulses, and compare photoelectron momentum distributions, in the polarization plane, with those obtained from recent time-dependent close-coupling calculations. The predictive capabilities of our approach are further demonstrated through a study of single-photon detachment from ${\mathrm{F}}^{-}$ in a circularly polarized, femtosecond laser pulse, where the relative contribution of the co- and counter-rotating $2p$ electrons is quantified.

Optical clocks operated on satellites are expected to open up new opportunities in time transfer, geodesy, fundamental physics, and satellite navigation. Here we demonstrate an important first step towards this goal: a modular, compact, optical lattice clock (OLC) system that achieves $2.0\times {10}^{-17}$ fractional uncertainty. The clock is operated with bosonic strontium and improves the performance of bosonic OLCs by a factor of 30. This has important implications for future use of bosonic OLCs in fundamental physics and metrology. We make use of the clock's metrological performance to measure, with independent clocks, the isotope shift of the ${}^{1}S_0\to {}^{3}P_0$ transitions of ${}^{88}\mathrm{Sr}$ and ${}^{87}\mathrm{Sr}$, with an uncertainty of 12 mHz. The ratio of the transition frequencies is thus determined with $3\times {10}^{-17}$ fractional uncertainty.

We report the measurement of the photoionization cross sections of the $5S_{1/2}$ and $5P_{3/2}$ states of ${}^{87}\mathrm{Rb}$ in a two-species Hg and Rb magneto-optical trap (MOT) by the cooling laser for Hg. The photoionization cross sections of Rb in the $5S_{1/2}$ and $5P_{3/2}$ states at 253.7 nm are determined to be $1_{-1}^{+4.3}\times {10}^{-20}{\text{cm}}^2$ and $4.63\left(30\right)\times {10}^{-18}{\text{cm}}^2$, respectively. To measure the $5S_{1/2}$ and $5P_{3/2}$ state fractions in the MOT we detected the photoionization rate of the $5P_{3/2}$ state by an additional 401.5 nm laser. The photoionization cross section of Rb in the $5P_{3/2}$ state at 401.5 nm is determined to be $\text{1.18(10)}\times {10}^{-17}{\text{cm}}^2$.

We theoretically and experimentally study the effect of the coupled-resonator-induced transparency in the Michelson interferometer with the dual-recycled configuration, which is equivalent to the adjustable coupled resonator. The coupling strength, corresponding to the splitting of the reflection spectrum and associated with the width of the coupled-resonator-induced transparent window, can be controlled by adjusting the arm lengths, i.e., the relative phase of the interference arms on the $50:50$ beam splitter. Thus, this tunability of the coupling strength can be very fast, and the absorptive and dispersive properties can be effectively controlled. This work provides an alternative system for coherent control and storage of an optical field.

The notion of nanomachines has recently emerged to engage and use collective action of ensembles of nanoscale components or systems. Here we present a heat-gradient driven nanomachine concept which through appropriate coupling between quantum nanosystems is capable of realizing and maintaining an inversion. Based on a Lindblad form of the quantum master equation with a semiclassical coupling to the lattice displacement phonon field we show that this positive inversion can be harnessed to generate coherent optomechanical oscillations and phonon lasing.

Two quantum states of two half-charge optical vortexes with relative azimuthal angle $\pi$ are orthogonal, by which two half-charge spiral phase plates with intersection angle $\pi$ can be used to demonstrate a parabolic coincident fringe in the Bell inequality experiment with high-dimensional two-photon orbital angular momentum entanglement and to thereby obtain a strong Bell parameter of $3\frac{1}{5}$. I theoretically demonstrate various orthogonal relations between two quantum states, each of which is a state with a rotational symmetry superposition made up of $n$ fractional orbital angular momentum states, where $n\in N$. I propose a Bell inequality experiment with two $n$-section spiral phase plates that have these two quantum states, whereby various angle periods with parabolic coincident fringes $2\pi /n$ and Bell's parameter $3\frac{1}{5}$ in each period can be obtained.

Numerical methods for the description of nonequilibrium many-particle quantum systems such as equation of motion techniques often cannot compute the full statistics of observables but only moments of it, such as mean, variance, and higher order moments. We employ here the maximum-entropy method to numerically construct unbiased statistics based on the knowledge of moments. We verify the feasibility of the proposed method by numerical simulation of a simple birth-death model for quantum-dot-microcavity lasers, where the full photon and carrier statistics are available for comparison. We show that not only the constructed statistics but also the computed entropy and the Lagrange multipliers, which appear here as a by-product, provide valuable insight into the physics of the considered system. For example, the entropy reveals that, in contrast to common wisdom, the photon statistics of the microcavity laser above threshold is better described by a Gaussian distribution than by a Poisson distribution. Our approach is general and can be applied to many other systems emerging in physics and related fields.

We study electron motion in electromagnetic (EM) fields in the radiation-dominated regime. It is shown that the electron trajectories become close to some asymptotic trajectories in the strong-field limit. The description of the electron dynamics by these asymptotic trajectories significantly differs from the ponderomotive description that is barely applicable in the radiation-dominated regime. The particle velocity of the asymptotic trajectory is completely determined by the local and instant EM field. The general properties of the asymptotic trajectories are discussed. In most standing EM waves (including identical tightly focused counterpropagating beams) the asymptotic trajectories are periodic with the period of the wave field. Furthermore, for a certain model of the laser beam we show that the asymptotic trajectories are periodic in the reference frame moving along the beam with its group velocity, which may explain the effect of the radiation-reaction trapping.

We present a circuit-QED scheme which makes it possible to reach the ultrastrong-coupling regime of a nondipolar interaction between a single qubit and a quantum resonator. We show that the system Hamiltonian is well approximated by a two-photon quantum Rabi model and propose a simple scattering experiment to probe its fundamental properties. In particular, we identify a driving scheme that reveals the change in selection rules characterizing the breakdown of the rotating-wave approximation and the transition from strong to ultrastrong two-photon interactions. Finally, we show that a frequency crowding in a narrow spectral region is observable in the output fluorescence spectrum as the coupling strength approaches the collapse point, paving the way to the direct observation of the onset of the spectral collapse in a solid-state device.

We introduce a framework for the construction of completely positive maps for subsystems of indistinguishable fermionic particles. In this scenario, the initial global state is always correlated, and it is not possible to tell system and environment apart. Nonetheless, a reduced map in the operator sum representation is possible for some sets of states where the only nonclassical correlation present is exchange.

We show that two-particle interferences can be used to probe the nuclear motion in a doubly excited hydrogen molecule. The dissociation of molecular hydrogen by electron impact involves several decay channels, associated with different molecular rotational states, which produce quantum interferences in the detection of the atomic fragments. Thanks to the correlations between the angular momentum and the vibrational states of the molecule, the fragments arising from each dissociation channel carry out a phase shift which is a signature of the molecule rotation. These phase shifts, which cannot be observed in a single-atom detection scheme, may be witnessed under realistic experimental conditions in a time-of-flight coincidence measurement. We analyze the interferences arising from the two lowest-energy rotational states of a parahydrogen molecule. Our result shows the relevance of two-fragment correlations to track the molecular rotation.

We extensively test a recent protocol to demonstrate quantum fault tolerance on three systems: (1) a real-time simulation of five spin qubits coupled to an environment with two-level defects, (2) a real-time simulation of transmon quantum computers, and (3) the 16-qubit processor of the IBM Q Experience. In the simulations, the dynamics of the full system is obtained by numerically solving the time-dependent Schrödinger equation. We find that the fault-tolerant scheme provides a systematic way to improve the results when the errors are dominated by the inherent control and measurement errors present in transmon systems. However, the scheme fails to satisfy the criterion for fault tolerance when decoherence effects are dominant.

We develop a resource theory for continuous-variable systems grounded on operations routinely available within current quantum technologies. In particular, the set of free operations is convex and includes quadratic transformations and conditional coarse-grained measurements. The present theory lends itself to quantify both quantum non-Gaussianity and Wigner negativity as resources, depending on the choice of the free-state set—i.e., the convex hull of Gaussian states or the states with positive Wigner function, respectively. After showing that the theory admits no maximally resourceful state, we define a computable resource monotone—the Wigner logarithmic negativity. We use the latter to assess the resource content of experimentally relevant states—e.g., photon-added, photon-subtracted, cubic-phase, and cat states—and to find optimal working points of some resource concentration protocols. We envisage applications of this framework to subuniversal and universal quantum information processing over continuous variables.

Quantum entanglement and coherence are two fundamental features of nature, arising from the superposition principle of quantum mechanics. While considered as puzzling phenomena in the early days of quantum theory, it is only very recently that entanglement and coherence have been recognized as resources for the emerging quantum technologies, including quantum metrology, quantum communication, and quantum computing. In this work we study the limitations for the interconversion between coherence and entanglement. We prove a fundamental no-go theorem, stating that a general resource theory of superposition does not allow for entanglement activation. By constructing a quantum controlled-not gate as a free operation, we experimentally show that such activation is possible within the more constrained framework of quantum coherence. By using recent results from coherence theory, we further show that the trace norm entanglement is not a strong entanglement monotone.

We studied the excitation dynamics of a finite quantum system with an intense optical near field (ONF) from a perspective of light-dressed states. A simple model consisting of a single electron and a nanosized short dipole source were employed. By calculating the time-dependent wave function subjected to the ONF, we demonstrated that the optical responses involved not only the first- and third-order but also the second- and fourth-order harmonic generations that were not obtained from conventional spatially homogeneous laser fields. In order to elucidate the origins of the exotic even-order harmonic generations, the dressed states altered by the ONF were explored. The result showed that the spatial distribution of the dressed states were significantly influenced by the ONF, making forbidden optical transitions allowed by parity breaking. This was caused by the spatial inhomogeneousness of the ONF, specifically its asymmetry, that was inherited to the dressed states.

Single-particle localization of an ultracold atom is studied in one dimension when the atom is confined by an optical lattice and by the incommensurate potential of a high-finesse optical cavity. In the strong-coupling regime the atom is a dynamical refractive medium, the cavity resonance depends on the atomic position within the standing-wave mode, and nonlinearly determines the depth and form of the incommensurate potential. We show that the particular form of the quasirandom cavity potential leads to the appearance of mobility edges, even in presence of nearest-neighbor hopping. We provide a detailed characterization of the system as a function of its parameters and, in particular, of the strength of the atom-cavity coupling, which controls the functional form of the cavity potential. For strong atom-photon coupling the properties of the mobility edges significantly depend on the ratio between the periodicities of the confining optical lattice and of the cavity field.

We have studied resonances in a dynamic slab whose permittivity and/or permeability are periodic functions of time, namely, a temporal photonic crystal. We find strong and narrow resonances for frequencies that are equal to $1/2$ or $3/2$, etc. of the modulation frequency provided that a certain geometric parameter (proportional to the slab thickness and to the modulation frequency) assumes values such that the electric field in the slab is either symmetric or antisymmetric with respect to the slab center. These resonances turn out to be absent whenever the electric and magnetic modulations are in phase and have equal strengths, that is, when there are no band gaps between $k$ bands. The resonance peaks appear for all the modulation harmonics and are superimposed on Fabry-Pérot–like background oscillations. For not very strong modulations, the resonances can be described in terms of the relative impedance of the slab and a parameter that expresses the modulation strength.