Resonantly enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PS) has proven to be a well-established tool to study the excited-state structure. However, femtosecond-induced REMPI-PS suffers from poor selectivity between neighboring excited states due to the large spectral bandwidth. In this paper, we present a feasible scheme to realize the selective excitation of (2+1) REMPI-PS in sodium atoms. We show that, by shaping the femtosecond laser pulse with a cubic phase modulation, a high-resolution (2+1) REMPI-PS is obtained, and the (2+1) REMPI-PS signal from one excited state is enhanced while that from the other excited state is effectively suppressed. Furthermore, we explain the physical control mechanism of the selective excitation of (2+1) REMPI-PS by considering the two-photon power spectrum.

Exploratory simulations of Bose-Fermi mixtures on the three-dimensional optical lattice at finite temperature are performed by adopting the lattice quantum chromodynamics technique. We analyze the bosonic superfluid transition and its dependence on the strength of the boson-fermion coupling. The particle densities and the pair occupancies are also studied to understand the effect of the boson-fermion coupling to the microscopic properties of the system. The effect of the induced fermion-fermion interaction by the boson density fluctuation is clearly seen.

The Berezinskii-Kosterlitz-Thouless transition in two-dimensional dipolar systems has been studied recently by path integral Monte Carlo simulations [A. Filinov et al., Phys. Rev. Lett. 105, 070401 (2010)]. Here, we complement this analysis and study temperature-coupling strength dependence of the density (particle-hole) and single-particle (SP) excitation spectra both in superfluid and normal phases. The dynamic structure factor, $S(q,\omega )$, of the longitudinal excitations is rigorously reconstructed with full information on damping. The SP spectral function, $A(q,\omega )$, is worked out from the one-particle Matsubara Green's function. A stochastic optimization method is applied for reconstruction from imaginary times. In the superfluid regime sharp energy resonances are observed in both the density and SP excitations. The involved hybridization of both spectra is discussed. In contrast, in the normal phase, when there is no coupling, the density modes, beyond acoustic phonons, are significantly damped. Our results generalize previous zero-temperature analyses based on variational many-body wave functions [F. Mazzanti et al., Phys. Rev. Lett. 102, 110405 (2009); D. Hufnagl et al., Phys. Rev. Lett. 107, 065303 (2011)], where the underlying physics of the excitation spectrum and the role of the condensate has not been addressed.

Using quantum Monte Carlo simulations, we investigate the finite-temperature phase diagram of hard-core bosons ($XY$ model) in two- and three-dimensional lattices. To determine the phase boundaries, we perform a finite-size-scaling analysis of the condensate fraction and/or the superfluid stiffness. We then discuss how these phase diagrams can be measured in experiments with trapped ultracold gases, where the systems are inhomogeneous. For that, we introduce a method based on the measurement of the zero-momentum occupation, which is adequate for experiments dealing with both homogeneous and trapped systems, and compare it with previously proposed approaches.

We investigate the wavelength scaling of high-order-harmonic-generation (HHG) efficiency by few-cycle laser pulses where the influence of the carrier-envelope phase (CEP) becomes prominent. Based on the numerical solution of time-dependent Schrödinger equation, in the infrared region of $\lambda$ (0.8–1.8 $\mu$m), we observe that the power law of the form ${\lambda }^{-x}$ ($x\approx 5$–6) is still present; but for constant intensity and a fixed energy interval, it differs markedly in the CEP. We find that for a specific value of CEP the wavelength dependence follows a ${\lambda }^{-4.6}$ scaling in high-harmonic yield. We also perform classical trajectory calculations to gain some understanding of the CEP dependence of the wavelength scaling.

We measure the time-energy correlation of broadband, spontaneously seeded four-wave mixing (FWM), and demonstrate time-frequency coupling effects; specifically, we observe a power-dependent splitting of the correlation in both energy and time. By pumping a photonic crystal fiber with narrowband picosecond pulses we generate FWM in a unique regime, where broadband ($>100$ nm) sidebands are generated that are incoherent, yet time-energy correlated. Although the observed time-energy correlation in FWM is conceptually similar to parametric down-conversion, its unique dependence on pump intensity due to self- and cross-phase-modulation effects yields spectral and temporal structure in the correlations. While these effects are minute compared to the time duration and bandwidth of the FWM sidebands, they are well observed using sum-frequency generation as a precise, ultrafast, wide-bandwidth correlation detector.

We study the interference pattern of biphotons passing through a dispersive medium, and devise a method for measuring dispersion in coincidence counting of entangled photons. By measuring the Gouy phase shift accompanying dispersion, we show that it fundamentally differs from the Gouy phase expected from classical models. Finally, we show that the second-order correlation function of the dispersed biphoton can be much wider than the pattern predicted from classical analog.

We study the effect of a static magnetic field on the band structure of a two-dimensional metallic photonic crystal (MPC). The band structure of the MPC has been calculated using the transfer matrix method. It is found that the position of the photonic band gap and the band edges of the MPC depends on the polarization of the incident light and intensity of the applied magnetic field. In our calculations we consider linearly polarized light as well as right- and left-circularly polarized light. In the case of right-circularly polarized light it is found that as the intensity of the magnetic field increases the width of the band gap of the crystal decreases. At a certain magnetic field strength the band gap disappears altogether. In other words there is a transition from a metallic photonic band gap material to a transparent dielectric material. This is an interesting effect which is similar to the metal-insulator transition that occurs in semiconductors. On the other hand, for left-circularly and linear polarized light the band edges shift to the higher energy and the band gap increases in the presence of a magnetic field. This implies that the MPC switches from a transparent state to reflecting states due to the application of the magnetic field. This is an interesting effect which might be used to make photonic switches.

We demonstrate the existence of Gaussian multipartite bound information which is a classical analog of Gaussian multipartite bound entanglement. We construct a tripartite Gaussian distribution from which no secret key can be distilled, but which cannot be created by local operations and public communication. Further, we show that the presence of bound information is conditional on the presence of a part of the adversary's information creatable only by private communication. Existence of this part of the adversary's information is found to be a more generic feature of classical analogs of quantum phenomena obtained by mapping of nonclassically correlated separable quantum states.

Since its discovery, the “ghost” diffraction phenomenon has emerged as an unconventional technique for optical imaging with very promising advantages. However, extracting the intensity and phase information of a structured and realistic object remains a challenge. Here we show that a ghost hologram can be recorded with a single-pixel configuration by adapting concepts from standard digital holography. The presented homodyne scheme enables phase imaging with nanometric depth resolution and three-dimensional focusing ability and shows a high signal-to-noise ratio.

We generate time-bin entangled photons and measure the resulting quantum state with a tomography system that uses asymmetric interferometers having $3\times 3$ output couplers. This configuration allows for measurements to be made simultaneously in all bases that are required for tomographic reconstruction. By eliminating the burden of tuning interferometer phases and by measuring all the spatial and temporal modes, substantial improvements in measurement speed are observed. Raw fidelities of 84$\%$ with respect to an ideal entangled state are measured in about 2 s using a 12-MHz entangled state generator, with corresponding accidental count subtracted fidelities exceeding 90$\%$.

In principle, a quantum system could be used to simulate another quantum system. The purpose of such a simulation would be to obtain information about problems which are difficult to simulate on a classical computer due to the exponential increase of the Hilbert space with the size of the system and which cannot be readily measured or controlled in an experiment. The system will interact with the surrounding environment and with the other particles in the system, and be implemented using imperfect controls, making it subject to noise. It has been suggested that noise does not need to be controlled to the same extent as it must be for general quantum computing. However, the effects of noise in quantum simulations are not well understood and how best to treat them in most cases is not known. In this paper we study an existing quantum algorithm for simulating the one-dimensional Fano-Anderson model using a liquid-state NMR device. We examine models of noise in the evolution using different initial states in the original model. We also add interacting spins to simulate a realistic situation where an environment of spins is present. We find that states which are entangled with their environment, and sometimes correlated but not necessarily entangled, have an evolution which is described by maps which are not completely positive. We discuss the conditions for this to occur and also the implications.

The problem of the bound states of a positron with atoms and apolar molecules is studied with a regularized polarization potential $V=V_{r_c}-\frac{\alpha r^{-4}}{2}$, where $V_{r_c}$ is a repulsive hard-core potential. A continuous relation between the core radius and the target polarizability is identified for alkanes, leading to the observed linear relation between their experimental binding energies and polarizabilities. New values for the binding energies for I and Pt atoms are suggested. Some predictions for related molecules are made, suggesting positron binding to GeH${}_4$ and, possibly, to C${}_2$F${}_6$.

We consider above-threshold ionization of polyatomic molecules by a strong laser field within the molecular strong-field approximation (MSFA). A polyatomic molecule is modeled by an $(N+1)$-particle system, which consists of $N$ heavy atomic (ionic) centers and an electron. After the separation of the center-of-mass coordinate, the dynamics of this system is reduced to the relative electronic and nuclear coordinates. Two forms of the MSFA, one with the field-free and the other with the field-dressed initial molecular bound state, are derived. For neutral polyatomic molecules the ionization amplitude takes a simple form which allows interpretation as a multiple-slit-type interference. This is illustrated by a numerical example for the ozone molecule.

Ultrafast dynamics of a molecular wave packet created by a strong 120-fs near-infrared (800 nm) laser pulse in iodine has been probed by synchronized 13.4-nm, 35-fs extreme-ultraviolet pulses delivered by the free-electron laser facility in Hamburg, FLASH. The kinetic energy release of the multiply charged ionic fragments reveals three essential steps of strong-field-induced molecular fragmentation dynamics: (i) The creation of I${}_2$${}^{2+}$ and (I${}_2$${}^{2+}$)${}^{*}$ molecular ions proceeds within (75 $\pm$ 15) fs full-width-at-half-maximum. (ii) With the onset of the I${}_2$${}^{2+}$ fragmentation the probability to lose a further electron within the same optical laser pulse rises with increasing I${}^{+}$—I${}^{+}$ internuclear separation and reaches its maximum after $\sim$30 fs with respect to the pulse maximum. (iii) Charge separation into the I${}_2$${}^{2+}\to {\mathrm{I}}^{2+}+\mathrm{I}$ dissociative channel with an asymmetric charge distribution is completed after (121 $\pm$ 22) fs.

We investigate the macroscopic quantum spin coherence of a spinor condensate confined in an anisotropic potential. Under the single-mode approximation, we show that a spin-1 Rb condensate can be modeled as a biaxial quantum magnet. A direct consequence of the biaxial anisotropy is that the tunneling splitting oscillates as a function of the external magnetic field applied along the hard axis. We also propose an experimental scheme to detect the oscillatory behavior of the tunneling splitting by employing the Landau-Zener tunneling.

We investigate the momentum distribution of weakly interacting one-dimensional (1D) Bose gases at thermal equilibrium both experimentally and theoretically. Momentum distribution of 1D Bose gases is measured using a focusing technique, whose resolution we improve via a guiding scheme. The momentum distribution compares very well with quantum Monte Carlo calculations for the Lieb-Liniger model at finite temperature, allowing for an accurate thermometry of the gas that agrees with (and improves upon) the thermometry based on in situ density fluctuation measurements. The quasicondensation crossover is investigated via two different experimental parameter sets, corresponding to the two different sides of the crossover. Classical field theory is expected to correctly describe the quasicondensation crossover of weakly interacting gases. We derive the condition of validity of the classical field theory, and find that, in typical experiments, interactions are too strong for this theory to be accurate. This is confirmed by a comparison between the classical field predictions and the numerically exact quantum Monte Carlo calculations.

Spatial interferometry, based on the measurement of total absorbed power, can be used to determine the state of coherence of the electromagnetic field to which any energy-absorbing structure is sensitive. The measured coherence tensor can be diagonalized to give the amplitude, phase, polarization patterns, and responsivities of the individual electromagnetic modes through which the structure can absorb energy. Because the electromagnetic modes are intimately related to dynamical modes of the system, information about collective excitations can be found. We present simulations, based on the discrete dipole approximation (DDA), showing how the dynamical modes of systems of surface dipoles can be recovered. Interactions are taken into consideration, leading to long-range coherent phenomena, which are revealed by the method. The use of DDA enables the interferometric response of a wide variety of objects to be modeled, from patterned photonic films to biological macromolecules.

We have observed Feshbach resonances for ${}^{133}\mathrm{Cs}$ atoms in two different hyperfine states at static magnetic fields of a few milligauss. These resonances are unusual for two main reasons. First, they are the lowest static-field resonances investigated up to now, and we explain their multipeak structure in these ultralow fields. Second, they are robust with respect to temperature effects. We have measured them using an atomic fountain clock and reproduced them using coupled-channels calculations, which are in excellent agreement with our measurements. We show that these are $s$-wave resonances due to a very weakly bound state of the triplet molecular Hamiltonian. We also describe a model explaining their asymmetric shape in the regime where the kinetic energy dominates over the coupling strength.

We propose relative entropy-based quantifiers for genuine multipartite total, quantum, and classical correlations. These correlation measures are applied to investigate the generation of genuine multiparticle correlations in decoherent dynamics induced by the interaction of two qubits with local-independent environments. We consider amplitude- and phase-damping channels and compare their capabilities to spread information through the creation of many-body correlations. We identify changes in behavior for the genuine four- and three-partite total correlations and show that, contrary to amplitude environments, phase-noise channels transform the bipartite correlation initially shared between the qubits into genuine multiparticle system-environment correlations.