We present an analytically exact scheme of unraveling a multitude of flat, dispersionless photonic bands in a kagome waveguide strip where each elementary plaquette hosts a deterministic fractal geometry of arbitrary generation. The number of nondispersive eigenmodes grows as higher and higher order fractal geometry is embedded in the kagome motif. Such eigenmodes are found to be localized with finite support in the kagome strip and exhibit a hierarchy of localization areas. The onset of localization can, in principle, be delayed in space by an appropriate choice of frequency of the incident wave. The length scale at which the onset of localization for each mode occurs can be tuned at will as prescribed here using a real-space renormalization method. Conventional methods of extracting the nondispersive modes in such geometrically frustrated lattices fail as a non-translationally-invariant fractal decorates the unit cells in the transverse direction. The scheme presented here circumvents this difficulty, and thus may inspire experimentalists to design similar fractal-incorporated kagome or Lieb classes of lattices to observe a multifractal distribution of flat photonic bands.

We study energy and particle transport for one-dimensional strongly interacting bosons through a ballistic single channel connecting two atomic reservoirs. We show the emergence of particle- and energy-current separation, leading to the violation of the Wiedemann-Franz law. As a consequence, we predict different time scales for the equilibration of temperature and particle imbalances between the reservoirs. Going beyond the linear spectrum approximation, we show the emergence of thermoelectric effects, which could be controlled by either tuning interactions or the temperature. Our results describe, in a unified picture, fermions in condensed-matter devices and bosons in ultracold atom setups. We conclude by discussing the effects of a controllable disorder.

A combined highly differential momentum imaging experiment and ab initio potential-energy curve calculation using density-functional theory (DFT) have been performed to understand the broad resonant peak around 5.7 eV due to dissociative electron attachment (DEA) to the chlorine molecule. Both the kinetic energy and angular distribution over the entire $2\pi$ angle of the fragment negative ions have been measured. Two heavily overlapping resonances are identified for the observed broad resonant peak that could settle the long-standing debate. The symmetry of the involved two temporary negative ion (TNI) states are determined from the angular distribution data. Experimental observations are strongly supported with the computed potential-energy curve using DFT.

In recent experiments, spin-orbit-coupled (SOC) bosonic gases in an optical lattice have been successfully prepared into any Bloch band [Hamner et al., Phys. Rev. Lett. 114, 070401 (2015)], which promises a viable contender in the competitive field of simulating gauge-related phenomena. However, the ground-state phase diagram of such systems in the superfluid regime is still lacking. Here we present a detailed study of the phase diagram in an optically trapped Bose gas with equal-weight Rashba and Dresselhaus SO coupling. We identify four different quantum phases, which include three normal phases and a mixed phase, by considering the wave vector $k_1$, the longitudinal $\langle {\sigma }_z\rangle$, and the transverse $\langle {\sigma }_x\rangle$ spin polarizations as three order parameters. The ground state of normal phases is a Bloch wave with a single wave vector $k_1$, which can position in arbitrary regions in the Brillouin zone. By contrast, the ground state of the mixed phase is a superposition of two Bloch waves with opposite $k_1$, which, remarkably, may lack periodicity even though the system's Hamiltonian is periodic. This mixed phase in the lattice setting can be seen as the counterpart of the stripe phase associated with the uniform SOC gas. Furthermore, due to the lattice-renormalized SOC, the phase diagram of the model system becomes significantly different from the uniform case when the lattice strength grows. Finally, a scheme for experimentally probing the mixed phase using Bragg spectroscopy is proposed.

We report robust polarimetric measurements of mixed-state geometric phases. An all-optical setup was used to generate geometric phases with great versatility, thereby extending the scope of analogous methods used in neutron polarimetry. Our method allows us to explore geometric phases generated by unitarily evolving mixed states. These are realized as partially polarized single-photon states whose degree of polarization and normalized Stokes vector can be fixed independently of one another. The geometric phases correspond to both open and closed trajectories in Poincaré space. By exploiting the gauge invariance of geometric phases we nullify their dynamical part, thereby making the geometric phase coincide with the total (Pancharatnam) phase, which can be directly addressed by employing standard techniques.

We investigate the superfluid-insulator transition of one-dimensional interacting bosons in both deep and shallow periodic potentials. We compare a theoretical analysis based on quantum Monte Carlo simulations in continuum space and Luttinger liquid approach with experiments on ultracold atoms with tunable interactions and optical lattice depth. Experiments and theory are in excellent agreement. Our study provides a quantitative determination of the critical parameters for the Mott transition and defines the regimes of validity of widely used approximate models, namely, the Bose-Hubbard and sine-Gordon models.

We demonstrate a velocity selection scheme that mitigates suppression of electromagnetically induced transparency (EIT) by Doppler shifts for coupling wavelengths larger than the probe wavelength. An optical pumping beam counterpropagating with the EIT probe beam transfers atoms between hyperfine states in a velocity-selective fashion. Measurement of the transmitted probe beam synchronous with chopping of the optical pumping beam enables a Doppler-free EIT signal to be detected. Transition frequencies between $5P_{1/2}$ and $n{S}_{1/2}$ states for $n=26$, 27, and 28 in ${}^{39}\mathrm{K}$ are obtained via EIT spectroscopy in a heated vapor cell with a probe beam stabilized to the $4S_{1/2}\to 5P_{1/2}$ transition. Using previous high-resolution measurements of the $4S_{1/2}\to n{S}_{1/2}$ transitions, we make a determination of the absolute frequency of the $4S_{1/2}\to 5P_{1/2}$ transition. Our measurement is shifted by 560 MHz from the currently accepted value with a twofold improvement in uncertainty. These measurements will enable novel experiments with Rydberg-dressed ultracold Fermi gases composed of ${}^{40}\mathrm{K}$ atoms.

The crucial feature of a memoryless stochastic process is that any information about its state can only decrease as the system evolves. Here we show that such a decrease of information is equivalent to the underlying stochastic evolution being divisible. The main result, which holds independently of the model of the microscopic interaction and is valid for both classical and quantum stochastic processes, relies on a quantum version of the so-called Blackwell-Sherman-Stein theorem in classical statistics.

With improved gate calibrations reducing unitary errors, we achieve a benchmarked single-qubit gate fidelity of $0.9995\pm 0.0002$ with superconducting qubits in a circuit quantum electrodynamics system. We present a method for distinguishing between unitary and nonunitary errors in quantum gates by interleaving repetitions of a target gate within a randomized benchmarking sequence. The benchmarking fidelity decays quadratically with the number of interleaved gates for unitary errors but linearly for nonunitary errors, allowing us to separate systematic coherent errors from decoherent effects. With this protocol, we show that the fidelity of the gates is not limited by unitary errors.

We propose a heralded quantum repeater protocol based on the general interface between the circularly polarized photon and the quantum dot embedded in a double-sided optical microcavity. Our effective time-bin encoding on photons results in the deterministic faithful entanglement distribution with one optical fiber for the transmission of each photon in our protocol, not two or more. Our efficient parity-check detector implemented with only one input-output process of a single photon as a result of cavity quantum electrodynamics makes the entanglement channel extension and entanglement purification in quantum repeater far more efficient than others, and it has the potential application in fault-tolerant quantum computation as well. Meanwhile, the deviation from a collective-noise channel leads to some phase-flip errors on the nonlocal electron spins shared by the parties and these errors can be depressed by our simplified entanglement purification process. Finally, we discuss the performance of our proposal, concluding that it is feasible with current technology.

We show that the metastable, symmetry-breaking ground states of quantum many-body Hamiltonians have vanishing quantum mutual information between macroscopically separated regions and are thus the most classical ones among all possible quantum ground states. This statement is obvious only when the symmetry-breaking ground states are simple product states, e.g., at the factorization point. On the other hand, symmetry-breaking states are in general entangled along the entire ordered phase, and to show that they actually feature the least macroscopic correlations compared to their symmetric superpositions is highly nontrivial. We prove this result in general, by considering the quantum mutual information based on the two-Rényi entanglement entropy and using a locality result stemming from quasiadiabatic continuation. Moreover, in the paradigmatic case of the exactly solvable one-dimensional quantum $XY$ model, we further verify the general result by considering also the quantum mutual information based on the von Neumann entanglement entropy.

We have developed a significantly more flexible variant of the relativistic atomic method of calculation that combines configuration-interaction and coupled-cluster approaches. The new version is no longer restricted to a specific choice of the initial approximation corresponding to the self-consistent field of the atomic core. We have applied this approach to calculation of different properties of atomic lead, including the energy levels, hyperfine structure constants, electric-dipole transition amplitudes, and $E1$ parity nonconserving (PNC) amplitude for the $6p^2{}^{3}P_0\text{−}6p^2{}^{3}P_1$ transition. The uncertainty of the $E1$ PNC amplitude was reduced by a factor of two in comparison with the previous most accurate calculation [V. A. Dzuba et al., Europhys. Lett. 7, 413 (1988)]. Our value for the weak charge $Q_W=-117\left(5\right)$ is in agreement with the standard-model prediction.

A recent model to describe electron correlations in time-dependent density-functional-theory (TDDFT) studies of antiproton-helium collisions is extended to deal with positively charged projectiles. The main complication is that a positively charged projectile can capture electrons in addition to ionizing them to the continuum. As a consequence, within the TDDFT framework one needs to consider three, instead of just one, correlation integrals ($I_c$'s) when formally expressing the probabilities for the one- and two-electron processes in terms of the density. We discuss an extension of an adiabatic model for $I_c$ to a two-centered system. Total cross sections for single ionization, double ionization, single capture, transfer ionization, and double capture are presented for both proton-helium and ${\text{He}}^{\mathrm{2+}}$-He collisions for impact energies in the approximate range 10–1000 keV/amu. One- and two-electron removal cross sections are also presented for the $p$-He system, with a comparison to updated antiproton-helium results. Our results, while mixed, demonstrate the relative importance of dynamic and functional correlations in a TDDFT description of collision processes.

We study the behavior of nonadiabatic population transfer between resonances at an exceptional point in the spectrum of the hydrogen atom. It is known that, when the exceptional point is encircled, the system always ends up in the same state, independent of the initial occupation within the two-dimensional subspace spanned by the states coalescing at the exceptional point. We verify this behavior for a realistic quantum system, viz., the hydrogen atom in crossed electric and magnetic fields. It is also shown that the nonadiabatic hypothesis can be violated when resonances in the vicinity are taken into account. In addition, we study nonadiabatic population transfer in the case of a third-order exceptional point, in which three resonances are involved.

We present a subcycle nonadiabatic strong-field tunneling theory and derive the position of tunnel exit, the transverse and longitudinal momentum distributions at the tunnel exit, and the ionization rate in an instantaneous laser field. These tunneling coordinates are shown to nonadiabatically couple with each other in an instantaneous laser field when the electron tunnels through the barrier. We have further incorporated the nonadiabatic tunneling theory with the quantum-trajectory Monte Carlo approach to investigate the nonadiabatic effect on the photoelectron angular distributions. The simulated photoelectron angular distributions with the nonadiabatic corrections have been validated by comparison with the ab initio results through numerically solving the time-dependent Schrödinger equation. The nonadiabatic coordinates at the tunnel exit play important roles in both the direct ionization and rescattering process. The nonadiabatic tunneling theory provides an intuitive understanding on subcycle dynamics of tunneling ionization.

We report measurements of one-, two-, and three-photon processes during the elastic scattering of electrons through ${90}^{\circ }$ by helium, argon, and molecular-nitrogen targets, in the presence of 1.17-eV photons from a Nd:YAG laser. The incident energy of the electrons was 200 and 350 eV, and the linear polarization direction of the laser was parallel to the momentum transfer direction. Our measured free-free count rates for the three processes are target independent within the experimental uncertainties, perfectly consistent with the Kroll-Watson approximation, which assumes no interaction of the laser radiation with the target.

We present a scheme for engineering the joint spectrum of photon pairs created via spontaneous parametric down-conversion. Our method relies on customizing the poling configuration of a quasi-phase-matched crystal. We use simulated annealing to find an optimized poling configuration which allows almost arbitrary shaping of the crystal's phase-matching function. This has direct application in the creation of pure single photons—currently one of the most important goals of single-photon quantum optics. We describe the general algorithm and provide code, written in C++, that outputs an optimized poling configuration given specific experimental parameters.

This paper has been motivated by a recent paper by Dey [Phys. Rev. D 91, 044024 (2015)] on the known Arik-Coon $q$ oscillator. We construct $q$ coherent, even and odd $q$-cat states in Fock representation for the Biedenharn-Macfarlane $q$ oscillator with $q>1$ and study their nonclassical properties. The $q$-coherent states minimize the Heisenberg uncertainty relation between the generalized position and momentum operators as well as the $x$ and $y$ components of a $q$-deformed $\text{su}(1,1)$ algebra in the Schwinger boson representation. The latter is also minimized by the even and odd $q$-cat states. We show that, contrary to the undeformed harmonic oscillator, the squeezing effect in both position and momentum operators can be exhibited by odd $q$-cat states. It is also violated by even $q$-cat states. Furthermore, it is shown that the antibunching effect and sub-Poissonian or super-Poissonian statistics can simultaneously appear by each of the even or odd $q$-cat states. Finally, a unitary Fock representation of the $q$-deformed $\text{su}(1,1)$ algebra is obtained by the $q$-deformed Bargmann-Fock realization.

We introduce a family of non-Hermitian optical potentials that are given in terms of double-exponential periodic functions. The center of $\mathcal{PT}$ symmetry is not around zero and the potential satisfies a shifted $\mathcal{PT}\text{-symmetry}$ relation at two distinct locations. Motivated by wave transmission through thin phase screens and gratings, we examine these refractive index modulations from the perspective of optical lattices that are homogeneous along the propagation direction. The diffraction dynamics, abrupt phase transitions in the eigenvalue spectrum, and exceptional points in the band structure are examined in detail. In addition, the nonlinear properties of wave propagation in Kerr nonlinearity media are studied. In particular, coherent structures such as lattice solitons are numerically identified by applying the spectral renormalization method. The spatial symmetries of such lattice solitons follow the shifted $\mathcal{PT}\text{-symmetric}$ relations. Furthermore, such lattice solitons have a power threshold and their linear and nonlinear stabilities are critically dependent on their spatial symmetry point.

We study the dynamics of a system consisting of a two-level atom and a quantum oscillator coupled by the vacuum radiation field. The oscillator and the atom are assumed to have the same resonance frequency. In contrast to the two-atom or two-oscillator case, the atom-oscillator system leads to an unclosed hierarchy of equations for the operators that determine the radiation pattern emitted by the atom-oscillator system. Instead of using the operator equations, we formulate the problem in terms of density-matrix equations which can be solved once the initial conditions are specified. As an example, we looked at the radiation pattern, the decay dynamics, and the spectrum for an initial condition in which the oscillator is in its $n=1$ state and the atom is in its excited state. Dressed states (dressed by the static interaction between the atom and oscillator) prove to be especially useful for interpreting the results and comparing them with the two-atom and two-oscillator systems. All calculations are carried out using the resonance or rotating-wave approximation. The relationship of our work to damped Jaynes-Cummings models is noted.