We study the impact of dephasing on the excitation dynamics of a cloud of ultracold two-level Rydberg atoms for both resonant and off-resonant laser excitation, using the wave-function Monte Carlo technique. We find that while for resonant laser driving, dephasing mainly leads to an increase of the Rydberg population and a decrease of the Mandel $Q$ parameter, at off-resonant driving strong dephasing toggles between direct excitation of pairs of atoms and subsequent excitation of single atoms, respectively. These two excitation mechanisms can be directly quantified via the pair-correlation function, which shows strong suppression of the two-photon resonance peak for strong dephasing. Consequently, qualitatively different dynamics arise in the excitation statistics for weak and strong dephasing in off-resonant excitation. Our findings show that time-resolved excitation number measurements can serve as a powerful tool to identify the dominating process in the system's excitation dynamics.

We report a study of the optical dipole force on a beam of neutral three-level, $\Lambda$-like atomic system induced by a femtosecond super-Gaussian pulse. We show that maximum coherence between the ground state $|1\rangle$ and the excited state $|2\rangle$ could be achieved using a train of femtosecond pulses. In addition, it is possible to control the trajectory of the atoms in an atomic beam by using the same scheme. The robustness of the scheme against the variation of the pulse parameters is also investigated.

Vibronically resolved squared form factors of $a^{\prime \prime }{}^1{{\Sigma }_g}^{+}$, $b{}^1{\Pi }_u$, $b^{\prime }{}^1{{\Sigma }_u}^{+}$, $c_3{}^1{\Pi }_u$, $c_4^{\prime }{}^1{{\Sigma }_u}^{+}$, and $o_3{}^1{\Pi }_u$ of molecular nitrogen are determined by inelastic x-ray scattering with a high resolution of 70 meV. Through a comparison with the previous results measured by electron energy loss spectroscopy, it is found that the first Born approximation is satisfied at an incident electron energy of 300 eV in the small momentum transfer region for a dipole-allowed transition of $b{}^1{\Pi }_u$, but the first Born approximation is not reached at an impact energy of 500 eV for the dipole-forbidden transition of $a^{\prime \prime }{}^1{{\Sigma }_g}^{+}$. It is also observed from the present experimental results that the momentum transfer dependence behavior of the excitations to $b{}^1{\Pi }_u({\nu }^{\prime }=3,4)$ is different from that of the excitations to $b{}^1{\Pi }_u({\nu }^{\prime }=1,2)$, which can be attributed to the perturbation of $c_3{}^1{\Pi }_u$ to $b{}^1{\Pi }_u$. The present results provide experimental benchmark data of inelastic squared form factors of molecular nitrogen.

We inspect the first-order electron-electron-capture scenario for transfer ionization that has been recently formulated by Voitkiv and Ma [Phys. Rev. A 86, 012709 (2012) and references therein]. Using the multichannel scattering theory for many-body systems with Coulomb interactions, we show that this scenario is just a part of the well-studied Oppenheimer-Brinkman-Kramers approximation. Accurate numerical calculations in this approximation for the proton-helium transfer ionization reaction exhibit no appreciable manifestation of the claimed mechanism.

In the Comment, it is claimed that the so-called electron-electron-Auger mechanism of transfer ionization, proposed in Voitkiv et al. [Phys. Rev. Lett. 101, 223201 (2008)] and Voitkiv [J. Phys. B 41, 195201 (2008)], does not exist. In the Reply, we argue against this claim.

In systems with “balanced loss and gain,” the $\mathcal{PT}$ symmetry is broken by increasing the non-Hermiticity or the loss-gain strength. We show that finite lattices with oscillatory, $\mathcal{PT}$-symmetric potentials exhibit unexpected $\mathcal{PT}$-symmetry breaking and restoration. We obtain the $\mathcal{PT}$ phase diagram as a function of potential periodicity, which also controls the location complex eigenvalues in the lattice spectrum. We show that the sum of $\mathcal{PT}$ potentials with nearby periodicities leads to $\mathcal{PT}$-symmetry restoration, where the system goes from a $\mathcal{PT}$-broken state to a $\mathcal{PT}$-symmetric state as the average loss-gain strength is increased. We discuss the implications of this transition for the propagation of a light in an array of coupled waveguides.

Using a simplistic model in one spatial dimension, we estimate the impact of the electron-electron, positron-positron, and electron-positron forces on the yield of the pair-creation process triggered by an external superstrong force. We separate the scalar and vector potentials in the Dirac equation into external and internal parts, where the dynamics of the internal potentials are governed by the Maxwell equations with the Dirac charge and current densities as source terms. In order to explore the impact of the interfermionic forces, we compute the number of created pairs, the time evolution of the charge, and current density as well as the electronic and positronic spatial probability density. We find that once the particle pair is created, the electron-positron attraction keeps the particles from being accelerated out of the interaction zone and the resulting Pauli blocking suppresses the pair-creation process. On a longer time scale, however, the forces lead to an enhancement of pair creation.

Entanglement in fermion many-body systems is studied using a generalized definition of separability based on partitions of the set of observables, rather than on particle tensor products. In this way, the characterizing properties of nonseparable fermion states can be explicitly analyzed, allowing a precise description of the geometric structure of the corresponding state space. These results have direct applications in fermion quantum metrology: Sub-shot-noise accuracy in parameter estimation can be obtained without the need of a preliminary state entangling operation.

The coupled pseudostate approximation (CP) introduced by McGovern et al. [Phys. Rev. A 79, 042707 (2009)] to study differential ionization in heavy particle collisions is extended to the case of a Li target. Symmetry properties dependent upon the initial state of the target, and the role of the interaction between the projectile and target nuclei, are discussed. Comparison is made with the recent double differential cross-section measurements ($d^2\sigma /dEd{q}_t$) of LaForge et al. [J. Phys. B 46, 031001 (2013)] for proton impact at 6 MeV and O${}^{8+}$ impact at 1.5 MeV/amu on Li($2s$) and Li($2p$). It is shown that the proton measurements lie very much in the first Born regime and that the comparison is therefore more of a test of the experiment than of the theory which, at the first Born level, is considered to be quite accurate. By contrast, the O${}^{8+}$ measurements present a nonperturbative scenario which provides a substantive challenge to the full CP theory. Although there are exceptions, the CP approximation generally agrees well with the shape of the O${}^{8+}$ data but there is a drift in normalization between the calculated and measured cross sections with increasing electron ejection energy. This same drift is seen in comparison between CP and measurements of $d\sigma /dE$ from Fischer et al. [Phys. Rev. Lett. 109, 113202 (2012)]. The convergence of the CP approximation with respect to the angular momenta of the pseudostates and their number is investigated. It is concluded that the 164-state set employed here should be satisfactory. Comparison is also made with the even more recent fully differential (triple differential) measurements of Hubele et al. [Phys. Rev. Lett. 110, 133201 (2013)] for O${}^{8+}$ impact on Li($2s$) and Li($2p$) at 1.5 MeV/amu. The CP approximation reproduces the “wings” seen in the Li(2s) measurements, which are due to the interaction between the projectile and target nuclei and the node in the $2s$ wave function, and the asymmetric shape of the $2p$ data. The orientational dichroism between the $2p_{+1}$ and $2p_{-1}$ states is illustrated. Overall, and while not perfect, the agreement between the pseudstate theory and the experiments is very encouraging.

Nixon and Murray [K. L. Nixon and A. J. Murray, Phys. Rev. Lett. 112, 023202 (2014)] have measured electron ionization of the $3 {}^1{P}_1$ state of ${}^{24}\mathrm{Mg}$. which was excited from the ground state by linearly polarized laser radiation. The direction of laser polarization was parallel to the scattering plane and based on their measurements, they proposed a functional form for the differential cross section as a function of the angle that the polarization direction makes with the incident electron beam. We show that this proposed functional form is exact and express the parameters of this form in terms of the $T$-matrix elements for ionization calculated in the scattering frame. We also show that this form occurs for radiation linearly polarized in an arbitrary direction. Furthermore, we similarly treat the case of excitation by circularly polarized light where a more complex shape is obtained, though this reduces to the form for linearly polarized radiation for ionization in the coplanar geometry. Using the measured results from Nixon and Murray, we calculate the ratio of the magnitudes of the $T$-matrix elements as well as the difference in their phases.

When a gauge potential is constructed from the Berry phase, there is an associated scalar potential coming with the gauge potential. In this paper, we investigate the impact of this scalar potential on cold atoms by constructing an artificial gauge potential in a cold-atomic system. We interpret the scalar potential as the coupling between the atom and the mass of the non-Abelian part of the gauge potential. We demonstrate that the gauge potential can produce spin-orbit coupling and that the scalar potential will suppress the spin Hall currents which are generated by spin-orbit coupling. We also discuss the observation of these phenomena in a real experiment.

We show how quantum coherence effects can be used to improve the resolution and the contrast of diffraction-limited images imprinted onto a probe field. The narrow and sharp spectral features generated by double dark resonances (DDR) are exploited to control absorption, dispersion, and diffraction properties of the medium. The spatially modulated control field can produce inhomogeneous susceptibility of the medium that encodes the spatial feature of the control image to probe field in the presence of DDR. The transmission of a cloned image can be enhanced by the use of an incoherent pump field. We find that the feature size of the cloned image is four times smaller than the initial characteristic size of the control image even though the control image is completely distorted after propagation through a 3-cm-long Rb vapor cell. We further discuss how spatial optical switching is possible by using induced transparency and absorption of the medium.

The quantization of the electromagnetic field in lossy and dispersive dielectric media has been widely studied during the last few decades. However, several aspects of energy transfer and its relation to consistently defining position-dependent ladder operators for the electromagnetic field in nonequilibrium conditions have partly escaped the attention. In this work we define the position-dependent ladder operators and an effective local photon-number operator that are consistent with the canonical commutation relations and use these concepts to describe the energy transfer and thermal balance in layered geometries. This approach results in a position-dependent photon-number concept that is simple and consistent with classical energy conservation arguments. The operators are formed by first calculating the vector potential operator using Green's function formalism and Langevin noise source operators related to the medium and its temperature, and then defining the corresponding position-dependent annihilation operator that is required to satisfy the canonical commutation relations in arbitrary geometry. Our results suggest that the effective photon number associated with the electric field is generally position dependent and enables a straightforward method to calculate the energy transfer rate between the field and the local medium. In particular, our results predict that the effective photon number in a vacuum cavity formed between two lossy material layers can oscillate as a function of the position suggesting that also the local field temperature oscillates. These oscillations are expected to be directly observable using relatively straightforward experimental setups in which the field-matter interaction is dominated by the coupling to the electric field. The approach also gives further insight on separating the photon ladder operators into the conventional right and left propagating parts and on the anomalies reported for the commutation relations of the corresponding operators within optical cavities.

We report on terahertz radiation induced by an optical rectification process in a noncentrosymmetric and insulating oxide, eulytite ${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$, at room temperature. The radiated terahertz pulse consists mainly of a temporal oscillation component with a frequency of 2.01 THz. The generation of this oscillation is ascribed to an increase in the effective generation length for the terahertz radiation up to about 2 mm at 2.01 THz. By using a pair of femtosecond laser pulses with an appropriate interval, polarization, and power, we succeeded in controlling the trajectory of the radiated terahertz wave. This method also enabled us to generate circularly polarized narrowband terahertz radiation with 2.01 THz.

We reviewed the conservation of momentum and energy in partially coherent electromagnetic wave fields in a unified perspective. We find that there is interference between the radiation from polarization and magnetization sources in the momentum flow unlike the result proposed in the previous study. The interference effect can be probed in an example by observing the angular distribution.

Anonymous ranking is a kind of privacy-preserving ranking whereby each of the involved participants can correctly and anonymously get the rankings of his data. It can be utilized to solve many practical problems, such as anonymously ranking the students' exam scores. We investigate the issue of how quantum mechanics can be of use in maintaining the anonymity of the participants in multiparty ranking and present a series of quantum anonymous multiparty, multidata ranking protocols. In each of these protocols, a participant can get the correct rankings of his data and nobody else can match the identity to his data. Furthermore, the security of these protocols with respect to different kinds of attacks is proved.

Electron emissions are studied for the double-electron capture with simultaneous single ionization process in 30 keV/u He${}^{2+}$-Ar atom collision using the reaction microscope technique. Double-differential cross sections have been obtained for emission angles of 0°, 20°, 45°, 90°, 128°, and 175° and electron energies ranging from 0 to 80 eV. No cusp-shaped electrons centered at a speed equal to that of the incident projectile in the forward direction are observed, which is contrary to the earlier results [D. Fregenal, J. Fiol, G. Bernardi, S. Suarez, P. Focke, A. D. Gonzalez, A. Muthig, T. Jalowy, K. O. Groeneveld, and H. Luna, Phys. Rev. A 62, 012703 (2000)]. An explanation has been provided to clarify the observed results in the current reaction channel.