We suggest a novel mechanism to induce the motion of a chiral material body towards an optical source with no optical traps. Our solution is based on the interference between a chiral light beam and its reflection on an opaque mirror. Surprisingly, it is theoretically shown that the electromagnetic response of the material may be tailored in such a way that independent of the specific body location with respect to the mirror, it is always pulled upstream against the photon flow associated with the incoming wave. Moreover, it is proven that by controlling the handedness of the incoming light it may be possible to harness the sign of the optical force, switching from a pulling force to a pushing force.

We investigate two-photon entangled states using two important degrees of freedom of the electromagnetic field, namely orbital angular momentum (OAM) and spin angular momentum. For photons propagating in the same direction we apply the idea of entanglement duality and develop schemes to do entanglement sorting based on either OAM or polarization. In each case the entanglement is tested using appropriate witnesses. We finally present generalizations of these ideas to three- and four-photon entangled states.

The method of wavefront shaping to control optical properties of opaque media is a promising technique for authentication applications. One of the main challenges of this technique is the sensitivity of the wavefront-sample coupling to translation and/or rotation. To better understand how translation and rotation affect the wavefront-sample coupling we perform experiments in which we first optimize reflection from an opaque surface—to obtain an optimal wavefront—and then translate or rotate the surface and measure the new reflected intensity pattern. By using the correlation between the optimized and translated or rotated patterns we determine how sensitive the wavefront-sample coupling is. These experiments are performed for different spatial-light-modulator (SLM) bin sizes, beam-spot sizes, and nanoparticle concentrations. We find that all three parameters affect the different positional changes, implying that an optimization scheme can be used to maximize the stability of the wavefront-sample coupling. We also develop a model to simulate sample translation or rotation and its effect on the coupling stability, with the simulation results being qualitatively consistent with experiment.

The setting of Mössbauer nuclei embedded in thin-film cavities has facilitated an aspiring platform for x-ray quantum optics as shown in several recent experiments. Here, we generalize the theoretical model of this platform that we developed earlier [Phys. Rev. A 88, 043828 (2013)]. The theory description is extended to cover multiple nuclear ensembles and multiple modes in the cavity. While the extensions separately do not lead to qualitatively new features, their combination gives rise to cooperative effects between the different nuclear ensembles and distinct spectral signatures in the observables. A related experiment by Röhlsberger et al. [Nature (London) 482, 199 (2012)] is successfully modeled, the scalings derived with semiclassical methods are reproduced, and a microscopic understanding of the setting is obtained with our quantum mechanical description.

We have investigated how the dynamics of a quantum particle initially localized in the left boundary site under periodic driving can be manipulated via control of the right boundary site of a lattice array. Because of the adjustable coupling between the right boundary site and its nearest neighbor, we can realize either coherent destruction of tunneling to coherent tunneling (CDT-CT) transition or coherent tunneling to coherent destruction of tunneling (CT-CDT) transition, by driving the right boundary site while keeping the left boundary site driven by a periodically oscillating field with a fixed driving parameter. In addition, we have also revealed that our proposed CDT-CT transition is robust against the second-order coupling (SOC) between next-nearest-neighbor sites in a three-site system, whereas localization can be significantly enhanced by SOC in a four-site system.

An article by Zhao et al. [Phys. Rev. A 86, 053413 (2012)] tests the local frame transformation (LFT) theory by comparing it with benchmark coupled-channel calculations. The system under consideration is an alkali-metal atom that is two-photon ionized in the presence of a static external electric field. Zhao et al. state that the differential cross sections computed in the LFT theory disagree with their supposedly more accurate coupled-channel calculations. They went on to diagnose the discrepancy and claimed that it originates in an inaccurate correspondence between the irregular functions in spherical and parabolic-cylindrical coordinates, a correspondence that lies at the heart of LFT theory. We have repeated the same tests and find that our calculations rule out the discrepancies that were claimed in Zhao et al. [Phys. Rev. A 86, 053413 (2012)] to exist between the LFT approximation and the exact calculations. This Comment thus helps to clarify the accuracy of the Harmin-Fano theory and demonstrates that it is in fact remarkably accurate not only for the total photoionization cross section in the Stark effect, but also for the differential cross section in photoionization microscopy.

The frame-transformation (FT) theory which describes the dynamics of nonhydrogenic atoms in an external uniform electric field was tested for the process of photoionization in a Stark field by Zhao et al. [Phys. Rev. A 86, 053413 (2012)]. Differential cross sections from the FT theory were found to be inconsistent with those from the fully quantum-mechanical coupled-channel theory. The discrepancy was attributed to the frame transformation of irregular wave functions. In a recent investigation, Giannakeas et al. [Phys. Rev. A 91, 067401 (2015)] draw a different conclusion. They show that the FT theory generates irregular wave functions in good agreement with exact solutions for low angular momenta, although an obvious disagreement is seen for high angular momenta. We performed test calculations for numerous Stark states and found that our original conclusion remains valid, namely, the Fano-Harmin frame transformation for the irregular wave function is inaccurate.

Coherent scattering of an electron beam by the Kapitza-Dirac effect from a standing laser wave which comprises two frequency components is studied. To this end, the Schrödinger equation is solved numerically with a suitable ponderomotive potential. Besides, an analytical solution for electron diffraction in the asymptotic domain of large field amplitudes is obtained and a mathematical model in reduced dimensionality for the scattering amplitude in the Bragg regime is presented. We demonstrate distinct interference signatures and relative phase effects when the standing wave contains a fundamental frequency and its second harmonic. The influence of the relative field intensities on the Rabi oscillation dynamics is also discussed.

In free-space quantum key distribution (QKD), the sensitivity of the receiver's detector channels may depend differently on the spatial mode of incoming photons. Consequently, an attacker can control the spatial mode to break security. We experimentally investigate a standard polarization QKD receiver and identify sources of efficiency mismatch in its optical scheme. We model a practical intercept-and-resend attack and show that it would break security in most situations. We show experimentally that adding an appropriately chosen spatial filter at the receiver's entrance may be an effective countermeasure.

We present two hyperentanglement concentration schemes for two-photon states that are partially entangled in the polarization and time-bin degrees of freedom. The first scheme distills a maximally hyperentangled state from two identical less-entangled states with unknown parameters via the Schmidt projection method. The other scheme can be used to concentrate an initial state with known parameters and requires only one copy of the initial state for the concentration process. Both these two protocols can be generalized to concentrate $N$-photon hyperentangled Greenberger-Horne-Zeilinger states that are simultaneously entangled in the polarization and time-bin degrees of freedom. Our schemes require only linear optics and are feasible with current technology. Using the time-bin degree of freedom rather than the spatial-mode degree of freedom can provide savings in quantum resources, which makes our schemes practical and useful for long-distance quantum communication.

We have recorded extreme ultraviolet spectra from ${\mathrm{W}}^{11+}$ to ${\mathrm{W}}^{15+}$ ions using a flat-field spectrometer installed at the Shanghai high-temperature superconducting electron-beam ion trap. The spectra were recorded at beam energies ranging between 200 and 400 eV and showed spectral lines and transition arrays in the 170–260 Å region. The charge states and spectra transitions were identified by comparison with calculations using a detailed relativistic configuration interaction method and collisional-radiative model, both incorporated in the flexible atomic code. Atomic structure calculations showed that the dominant emission arises from $5d$ $\to$ $5p$ and $5p$ $\to$ $5s$ transitions. The work also identified the ground-state configuration of $W^{13+}$ as $4f^{13}5s^2$ both theoretically and experimentally.

We explore the formation of Ar hollow states with two or three inner-shell holes by free-electron-laser radiation. We find that even-charged Ar ion states can be more populated than odd-charged Ar ion states. This depends on the pulse intensity and the number of energetically accessible inner-shell holes. Fully accounting for fine structure, we demonstrate that one-electron spectra bear the imprints of Ar hollow states with two inner-shell holes. Moreover, we show how the Auger spectra of these hollow states can be extracted from two-electron coincidence spectra.

We investigate the nonequilibrium dynamics of the bosonic Hubbard model starting from inhomogeneous superfluid or Mott insulator initial states using the truncated Wigner approximation (TWA). We find that the relaxation of the system develops in two steps for sufficiently large interaction strengths: after a fast relaxation the system gets caught in metastable prethermalized states that precede the true equilibrium state. We find that the lifetime of these prethermalized states increases by several orders of magnitude as we increase the on-site interaction strength beyond a threshold value. We show that the emergence of long-lived metastable states in the quantum dynamics is associated with an ergodic (active) to nonergodic (inactive) dynamical phase transition in the ensemble of classical trajectories that contribute to the semiclassical limit. This dynamical phase transition, which is very similar to that found in different classical models of glasses, is closely related to the dynamic heterogeneity of the classical relaxation.

This paper examines chains of $N$ coupled harmonic oscillators. In isolation, the $j\mathrm{th}$ oscillator $\left(1\le j\le N\right)$ has the natural frequency ${\omega }_j$ and is described by the Hamiltonian $\frac{1}{2}{p}_j^2+\frac{1}{2}{\omega }_j^2{x}_j^2$. The oscillators are coupled adjacently with coupling constants that are purely imaginary; the coupling of the $j\mathrm{th}$ oscillator to the $(j+1)\mathrm{th}$ oscillator has the bilinear form $i\gamma x_j{x}_{j+1}$ ($\gamma$ real). The complex Hamiltonians for these systems exhibit partial $\mathcal{PT}$ symmetry; that is, they are invariant under $i\to -i$ (time reversal), $x_j\to -x_j$ ($j$ odd), and $x_j\to x_j$ ($j$ even). [They are also invariant under $i\to -i,x_j\to x_j$ ($j$ odd), and $x_j\to -x_j$ ($j$ even).] For all $N$ the quantum energy levels of these systems are calculated exactly and it is shown that the ground-state energy is real. When ${\omega }_j=1$ for all $j$, the full spectrum consists of a real energy spectrum embedded in a complex one; the eigenfunctions corresponding to real energy levels exhibit partial $\mathcal{PT}$ symmetry. However, if the ${\omega }_j$ are allowed to vary away from unity, one can induce a phase transition at which all energies become real. For the special case $N=2$, when the spectrum is real, the associated classical system has localized, almost-periodic orbits in phase space and the classical particle is confined in the complex-coordinate plane. However, when the spectrum of the quantum system is partially real, the corresponding classical system displays only open trajectories for which the classical particle spirals off to infinity. Similar behavior is observed when $N>2$.

Experiments performed in recent years on positron scattering from molecular hydrogen indicated a rise of the total cross section in the limit of zero energy, but essentially disagree on the amplitude of this rise. Mitroy and collaborators [J.-Y. Zhang et al., Phys. Rev. Lett. 103, 223202 (2009)] predicted a scattering length somewhat different from values deduced experimentally. Using a Markov chain Monte Carlo modified effective range theory (MCMC-MERT) we show that the prediction of Mitroy and collaborators allows one to validate the recent experimental results and determine possible uncertainties. By comparing the MERT analysis with the fixed-nuclei density functional calculations we also deduce that probably the effect of virtual positronium formation (or coupling to the virtual positronium state) determines an almost constant value of the total cross section from 3 eV up to the positronium-formation threshold.

The long-range, anisotropic nature of the interaction among atoms in an ultracold dipolar gas leads to a rich array of possibilities for studying many-body physics. In this work, an ultracold gas of highly excited atoms is used to study energy transport due to the long-range dipole-dipole interaction. A technique is developed to measure both the internal energy states of the interacting Rydberg atoms and their positions in space. This technique is demonstrated by observing energy exchange between two spatially separated groups of Rydberg atoms excited to two different internal states. Simulations confirm the general features of the energy transport in this system and highlight subtleties associated with the homogeneity of the electric field used in this experiment.

We study the nonequilibrium steady state arising from the interplay between coherent and dissipative dynamics in strongly interacting Rydberg gases using a recently introduced variational method [H. Weimer, Phys. Rev. Lett. 114, 040402 (2015)]. We give a detailed discussion of the properties of this approach and we provide a comparison with methods related to the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy. We find that the variational approach offers some intrinsic advantages and we also show that it is able to explain the experimental results obtained in an ultracold Rydberg gas on a quantitative level.

We theoretically and experimentally examine thermal oscillations in a calcium fluoride whispering-gallery-mode resonator that lead to strong mode-frequency oscillations. We show that these oscillations arise from interplay among thermal expansion, the thermo-optic effect, and Kerr effects. In certain regimes we observe chaotic behavior and demonstrate that the threshold for this behavior can be predicted theoretically. We then demonstrate a self-stabilization technique that suppresses the oscillations and delivers high temperature and frequency stability without reference to external standards.

Based on studies of four specific networks, we conjecture a general relation between the walk dimensions $d_w$ of discrete-time random walks and quantum walks with the (self-inverse) Grover coin. In each case, we find that $d_w$ of the quantum walk takes on exactly half the value found for the classical random walk on the same geometry. Since walks on homogeneous lattices satisfy this relation trivially, our results for heterogeneous networks suggest that such a relation holds irrespective of whether translational invariance is maintained or not. To develop our results, we extend the renormalization-group analysis (RG) of the stochastic master equation to one with a unitary propagator. As in the classical case, the solution $\rho (x,t)$ in space and time of this quantum-walk equation exhibits a scaling collapse for a variable $x^{d_w}/t$ in the weak limit, which defines $d_w$ and illuminates fundamental aspects of the walk dynamics, e.g., its mean-square displacement. We confirm the collapse for $\rho (x,t)$ in each case with extensive numerical simulation. The exact values for $d_w$ themselves demonstrate that RG is a powerful complementary approach to study the asymptotics of quantum walks that weak-limit theorems have not been able to access, such as for systems lacking translational symmetries beyond simple trees.

We investigate projectile-charge-state-differential electron removal from neon atoms by impact of ${\mathrm{He}}^{\mathrm{2+}}$, ${\mathrm{Li}}^{\mathrm{3+}}$, ${\mathrm{B}}^{\mathrm{2+}}$, and ${\mathrm{C}}^{\mathrm{3+}}$ ions at intermediate projectile energies $(25\text{keV}/\text{u}$ to $1\text{MeV}/\text{u})$. The many-electron problem is described with an independent electron model in which active electrons at both collision centers are propagated in a common mean-field potential. Response to electron removal is taken into account in terms of a time-dependent screening potential, and a Slater-determinant-based method is used for the final-state analysis. Total cross sections for net recoil ion production, multiple ionization, and capture channels are mostly in good agreement with published experimental data. Results from equicharged bare and dressed ions are compared and the net recoil ion production cross section is broken down into contributions associated with different final projectile charge states in order to shed light on the role of the projectile electrons.