Supersonic beams have numerous applications in research fields ranging from spectroscopy with nanodroplets to surface science and matter-wave microscopy. Thus, measurement and prediction of their properties is of considerable interest. In this paper we present measurements of the virtual-source size and its brightness, as well as the terminal speed and terminal speed ratio of a supersonic deuterium (D${}_2$) beam. The speed distribution data were measured with time-of-flight experiments and Fresnel zone-plate imaging was used to measure virtual source size. The point-spread function of the zone plate was simulated based on the measured wavelength distribution and used to extract the width of the virtual source and its brightness from the focus measurement. The experiments were carried out with a 10-$\mu$m-diameter nozzle and a source temperature of $T_0=310$ K in the pressure range $p_0=3$–171 bars and for $T_0=106$ K in the pressure range $p_0=3$–131 bars. We found that using deuterium as opposed to helium results in a virtual source that is about a factor 2 brighter under similar stagnation conditions. A comparison between the measured data and the predictions from a theoretical model based on the Boltzmann equation, which explicitly include the coupling between translational and rotational degrees of freedom as well as the real-gas properties of D${}_2$, resulted in good correspondence for the two different interaction potentials we tried. A careful comparison with the experimental results shows that the potential by Buck et al. [J. Chem. Phys. 78, 4439 (1983)] is moderately better than the Lennard-Jones potential at describing the expansion dynamics.

We have measured the total fluorescent intensity and circular polarization of light emitted in $3p{}^4{P}^o\to 3s{}^{4}P$ transitions of excited neutral nitrogen atoms created by the photofragmentation of the N${}_2$ molecule with circularly polarized light having energies between 21 and 26 eV. The intensity measurements show the effect of predissociation of the ${\text{N}}_2$ Rydberg series $R$($C$) ${}^1{\Sigma }_u^{+}$ states by non-Rydberg doubly excited resonances (NRDERs), while nonzero values of circular polarization allow us to unambiguously identify the presence of a directly excited NRDER with ${}^1{\Pi }_u$ symmetry in this energy range.

Cross sections for $K$-shell ionization by electron impact were determined from films of Al, Si, and Ti and their oxides deposited on carbon substrates, for incident energies between 2.5 and 25 keV. The spectral processing of the x-ray emission spectra took into account corrections due to the presence of a spontaneous oxide layer formed on the monoelemental films and to the supporting material. Carbon $K$-shell ionization cross sections were determined from the contribution of the substrate to the measured spectra, while for oxygen, data from the three oxide films were taken. The mass thickness of the coatings was characterized by x-ray reflectivity. The results obtained were compared with other experimental data sets, semiempirical approaches, and theoretical models.

We study the nonequilibrium many-body dynamics of a cold gas of ground-state alkali-metal atoms weakly admixed by Rydberg states with laser light. On a time scale shorter than the lifetime of the dressed states, effective dipole-dipole or van der Waals interactions between atoms can lead to the formation of strongly correlated phases, such as atomic crystals. Using a semiclassical approach, we study the long-time dynamics where decoherence and dissipative processes due to spontaneous emission and blackbody radiation dominate, leading to heating and melting of atomic crystals as well as particle losses. These effects can be substantially mitigated by performing active laser cooling in the presence of atomic dressing.

We have used the velocity-map imaging (VMI) technique to measure autoionization dynamics in molecular nitrogen initiated by a train of attosecond pulses. The pump-probe measurements show clear evidence of a crossing between potential energy curves of the highly excited N${}_2{}^{+}$ ion and of the N${}_2{}^{2+}$ ion. It is found that the autoionization becomes energetically allowed when the two nuclei are still very close ($\sim 3$ Å), in contrast with observations in other diatomic molecules, and that it can be coherently manipulated by a strong femtosecond infrared pulse.

We provide a full quantum description of the optomechanical system formed by a Fabry-Pérot cavity with a movable micromechanical mirror whose center-of-mass and internal elastic modes are coupled to the driven cavity mode by both radiation pressure and photothermal force. Adopting a quantum Langevin description, we investigate simultaneous cooling of the micromirror elastic and center-of-mass modes, and also the entanglement properties of the optomechanical multipartite system in its steady state.

Quantum-mechanical interference between transitions from the metastable $2{}^3{S}_1{m}_J=0$ state to $2{}^3{P}_1{m}_J=\pm 1$ and to $2{}^3{P}_2{m}_J=\pm 1$ is shown to cause shifts in these resonances, despite the fact that the resonances are separated by more than 1000 natural widths. The $2{}^3{P}_1$-to-$2{}^3{P}_2$ fine-structure interval can be determined from the difference of these laser transitions, and a comparison between experiment and theory for this interval allows for precise tests of the quantum-electrodynamic (QED) theory used to calculate the interval. The shifts described here are large enough to be important for this test of QED and therefore to affect the continuing program of determining the fine-structure constant from comparison between accurate experimental measurements and theoretical calculations of the helium $2{}^{3}P$ energy intervals.

We consider the efficiency of classically simulating measurement-based quantum computation on surface-code states. We devise a method for calculating the elements of the probability distribution for the classical output of the quantum computation. The operational cost of this method is polynomial in the size of the surface-code state, but in the worst case scales as $2^{2g}$ in the genus $g$ of the surface embedding the code. However, there are states in the code space for which the simulation becomes efficient. In general, the simulation cost is exponential in the entanglement contained in a certain effective state, capturing the encoded state, the encoding, and the local postmeasurement states. The same efficiencies hold, with additional assumptions on the temporal order of measurements and on the tessellations of the code surfaces, for the harder task of sampling from the distribution of the computational output.

Characterization and quantification of multipartite entanglement is one of the challenges in state-of-the-art experiments in quantum-information processing. According to theory, this is achieved via entanglement monotones, that is, functions that do not increase under stochastic local operations and classical communication (SLOCC). Typically such monotones include the wave function and its time reversal (antilinear-operator formalism) or they are based on not completely positive maps (e.g., partial transpose). Therefore, they are not directly accessible to experimental observations. We show how entanglement monotones derived from polynomial local SL$(2,\mathrm{width}.04\mathrm{em}\mathrm{height}1.46\mathrm{ex}\mathrm{depth}-.07\mathrm{ex}\mathrm{C})$ invariants can be re-written in terms of expectation values of observables. Consequently, the amount of entanglement—of specific SLOCC classes—in a given state can be extracted from the measurement of correlation functions of local operators.

In a recent work, Markham and Sanders proposed a framework to study quantum secret-sharing (QSS) schemes using graph states. This framework unified three classes of QSS protocols, namely, sharing classical secrets over private and public channels, and sharing quantum secrets. However, previous work on graph-state secret sharing mostly focused on threshold schemes. In this paper, we focus on general access structures. We show how to realize a large class of arbitrary access structures using the graph-state formalism. We show an equivalence between $\left[\right[n,1\left]\right]$ binary quantum codes and graph-state secret-sharing schemes sharing one bit. We also establish a similar (but restricted) equivalence between a class of $\left[\right[n,1\left]\right]$ Calderbank-Shor-Steane codes and graph-state QSS schemes sharing one qubit. With these results we are able to construct a large class of graph-state quantum secret-sharing schemes with arbitrary access structures.

Nuclear mean-square charge radii of radioactive isotopes are obtained from isotope-shift measurements using laser spectroscopy. To extract the values of $\delta \langle r^2\rangle$ the atomic field and mass shift factors need to be evaluated. We here summarize the currently available experimental and theoretical techniques for calculating the atomic field and mass shift factors for complex, open-shell atoms, including the $5s^2{}^1{S}_0-4d5p{}^1{P}_1$ and $4d5s{}^3{D}_2-4d5p{}^3{P}_1$ transitions for singly charged yttrium ($Z=39$).

We consider the influence of vibrational motion of nuclei on the tunneling ionization of a molecule in a strong light field. Unlike the molecular Ammosov-Delone-Krainov (MO-ADK) model [X. M. Tong et al., Phys. Rev. A 66, 033402 (2002)], the Franck-Condon factors that take into consideration the quantization of nuclear vibrations are introduced in the theory. Modification of the vibrational motion of nuclei by the external field leads to the time-dependent constant in the asymptotic form of the electronic wave function. This in turn leads to a more significant modification of the Keldysh theory for the tunnel effect in molecules as compared to the MO-ADK model. We take into account the possibility of vibrational quantum numbers changing in a neutral molecule and in a molecular ion in the process of the tunneling ionization, which in principle enables one to get molecular ions of a certain isotopic composition if the vibrational states of the neutral molecules with this isotopic composition are pumped before ionization. We present numerical results illustrating this possibility for the isotopes of a hydrogen molecule.

The ionization of Ne and Kr at 400 nm has been measured from 10${}^{13}$ to 10${}^{17}$ W/cm${}^2$ for charge states up to Kr${}^{8+}$. Nonsequential ionization is seen in the higher charge states Ne${}^{3+}$, Ne${}^{4+}$, Ne${}^{5+}$, Kr${}^{5+}$, Kr${}^{6+}$, Kr${}^{7+}$, and Kr${}^{8+}$ when the atomic response to the increasing external field strength moves from multiphoton to tunneling. A similar development in nonsequential ionization is seen in a semiclassical model and an empirical model of rescattering. The models show that nonsequential ionization not apparent in lower charge states can become prominent for higher charge states. It is possible to generalize these results and predict optimal wavelengths for multielectron excitation via nonsequential ionization. The findings indicate that ultraviolet to vacuum ultraviolet wavelengths can give the largest recollision for higher charge states.

Ground-state properties of a two-dimensional fluid of bosons with repulsive dipole-dipole interactions are studied by means of the Euler-Lagrange hypernetted-chain approximation. We present a self-consistent semianalytical theory of the pair distribution function $g\left(r\right)$ and ground-state energy of this system. Our approach is based on the solution of a zero-energy scattering Schrödinger equation for the “pair amplitude” $\sqrt{g\left(r\right)}$ with an effective potential from Jastrow-Feenberg correlations. We find excellent agreement with quantum Monte Carlo results over a wide range of coupling strength, nearly up to the critical coupling for the liquid-to-crystal quantum phase transition. We also calculate the one-body density matrix and related quantities, such as the momentum distribution function and the condensate fraction.

Thermodynamic phase fluctuations in optical fiber cavities are analyzed in detail, with an emphasis on asymptotic behavior in the low-frequency regime. Both exact series solutions and closed-form approximations are derived for thermal noise under a range of different boundary conditions. These results clarify theoretical limits to strain resolution in optical fiber sensors at low frequency, enabling correct interpretation of recent experiments.

We derive a frequency-domain-based approach for radiation from double-heterostructure cavity (DHC) modes. We use this to compute the quality factors and radiation patterns of DHC modes. The semianalytic nature of our method enables us to provide a general relationship between the radiation pattern of the cavity and its geometry. We use this to provide general designs for ultrahigh-quality-factor DHCs with radiation patterns that are engineered to emit vertically.

Two classes of conservative, linear, optical rotary effects (optical activity and Faraday rotation) are distinguished by their behavior under time reversal. Faraday rotation, but not optical activity, is capable of coherent perfect rotation, by which we mean the complete transfer of counterpropagating coherent light fields into their orthogonal polarization. Unlike coherent perfect absorption, however, this process is explicitly energy conserving and reversible. Our study highlights the necessity of time-reversal-odd processes (not just absorption) and coherence in perfect mode conversion and thus informs the optimization of active multiport optical devices.

We analyze in detail the analytical solutions of the Dirac equation with scalar $S$ and vector $V$ Coulomb radial potentials near the limit of spin and pseudospin symmetries (i.e., when those potentials have the same magnitude and either the same sign or opposite signs, respectively). By performing an expansion of the relevant coefficients we also assess the perturbative nature of both symmetries and their relations to the (pseudo)spin-orbit coupling. The former analysis is made for both positive and negative energy solutions and we reproduce the relations between spin and pseudospin symmetries found before for nuclear mean-field potentials. We discuss the node structure of the radial functions and the quantum numbers of the solutions when there is spin or pseudospin symmetry, which we find to be similar to the well-known solutions of hydrogenic atoms.

We consider the dynamical properties of dissipative continuous-time quantum walks on directed graphs. Using a large-deviation approach we construct a thermodynamic formalism allowing us to define a dynamical order parameter, and to identify transitions between dynamical regimes. For a particular class of dissipative quantum walks we propose a quantum generalization of the the classical PageRank vector, used to rank the importance of nodes in a directed graph. We also provide an example where one can characterize the dynamical transition from an effective classical random walk to a dissipative quantum walk as a thermodynamic crossover between distinct dynamical regimes.