We investigate the effects of a nanophotonic boundary on the spontaneous emission properties of an excited two-level atom in cavity quantum electrodynamics (QED) geometry. We show that a boundary provides temporally delayed interference, which can be either constructive or destructive. Consequently, the decay of the atomic excitation can be either increased or greatly inhibited. As a concrete example, we investigate the spontaneous emission process in cavity QED with a terminated line-defect waveguide, and show the rich behavior of the atomic response due to the boundary. We also show that the output photonic wave form is strongly influenced by the boundary.

In a series of articles we studied the quantum properties of a degenerate optical parametric oscillator tuned to the first family of transverse modes at the subharmonic. We found that, for a cavity having rotational symmetry with respect to the optical axis, a TEM${}_{10}$ mode with an arbitrary orientation in the transverse plane is emitted above threshold. We proved then that quantum noise induces a random rotation of this bright TEM${}_{10}$ mode in the transverse plane, while the orthogonal mode, the so-called dark mode, has perfect quadrature squeezing irrespective of the distance to threshold (noncritical squeezing). This result was linked to the spontaneous rotational symmetry breaking which occurs when the bright mode is generated, and here we analyze how the squeezing of the dark mode is degraded when the cavity is not perfectly symmetric. We will show that large levels of squeezing are still attainable for reasonable values of this anisotropy, with the advantage that the orientation of the bright and dark modes is basically fixed, a very attractive situation from the experimental point of view.

We consider Bell experiments with $N$ spatially separated qubits where loss is present and restrict ourselves to two measurement settings per site. We note the Mermin-Ardehali-Belinskii-Klyshko (MABK) Bell inequalities do not present a tight bound for the predictions of local hidden variable (LHV) theories. The Holder-type Bell inequality derived by Cavalcanti, Foster, Reid, and Drummond [E. G. Cavalcanti, C. J. Foster, M. D. Reid, and P. D. Drummond, Phys. Rev. Lett. 99, 210405 (2007)] provides a tighter bound, for high losses. We analyze the actual tight bound for the MABK inequalities, given the measure $W={\prod }_{k=1}^N{\eta }_k$ of overall detection efficiency, where ${\eta }_k$ is the efficiency at site $k$. Using these inequalities, we confirm that the maximally entangled Greenberger-Horne-Zeilinger state enables loophole-free falsification of LHV theories provided ${\prod }_{k=1}^N{\eta }_k>2^{(2-N)}$, which implies a symmetric threshold efficiency of $\eta \to 50\%$, as $N\to \infty$. Furthermore, loophole-free violations remain possible, even when the efficiency at some sites is reduced well below $0.5$, provided $N>3$.

Cyclic boundaries are used in many branches of physics and mathematics, typically to assist the approximation of a large space. We show that when determining the performance of planar, fault-tolerant, topological quantum error correction, using cyclic boundaries leads to a significant underestimate of the logical error rate. We present cyclic and noncyclic surface code simulations exhibiting this discrepancy and analytic formulas precisely reproducing the observed behavior in the limit of low physical error. These asymptotic formulas are then used to prove that the underestimate is exponentially large in the code distance $d$ at any fixed physical error rate $p$ below the threshold error rate $p_{\mathrm{th}}$.

We provide a molecular structure determination method, based on multiple-scattering x-ray photoelectron diffraction (XPD) calculations. This method is applied to our XPD data on several molecules having different equilibrium geometries. Then it is confirmed that, by our method, bond lengths and bond angles can be determined with a resolution of less than 0.1 Å and ${10}^{\circ }$, respectively. Differently from any other scenario of ultrafast structure determination, we measure the two- or three-dimensional XPD of aligned or oriented molecules in the energy range from 100 to 200 eV with a $4\pi$ detection velocity map imaging spectrometer. Thanks to the intense and ultrashort pulse properties of x-ray free-electron lasers, our approach exhibits the most probable method for obtaining ultrafast real-time structural information on small to medium-sized molecules consisting of light elements, i.e., a “molecular movie.”

The dynamical theory of x-ray diffraction and the coupled-wave theory for modeling diffraction of light from periodic structures are two equivalent theories but with incompatibilities, as they were developed independently along two parallel directions in history. Here we reformulate the two theories into a universal Fourier coupled-wave diffraction theory (FCWDT), in which the fundamental coupled-wave equations for almost all practical diffraction geometry can always be written as a straightforward eigenvalue equation that is easily solvable by standard mathematical procedures. Since it removes most of the approximations and complexities in the two conventional theories, the FCWDT is almost rigorous yet simple and, in principle, can be used to compute scattering of electromagnetic waves from any kinds of periodic (nonmagnetic) structures, including x-ray diffraction from crystals and soft x-ray and light diffraction from periodic multilayers, gratings, and photonic crystals.

We consider the role of quantum correlations in the efficient use of information by a predictive quantum system, generalizing a recently proposed classical measure of nonpredictive information to the quantum regime. We show that, as a quantum system changes state, the nonpredictive information held by another correlated quantum system is exactly equal to the extractable work that is lost from the second system. We use quantum discord to quantify the quantum contribution, and demonstrate the possibility of improved thermodynamic efficiency due to a negative “quantum part” of the lost work. We also give a thermodynamic interpretation to quantum discord, as the reduction in extractable work under an optimal classical approximation of a quantum memory.

We use a one-dimensional optical lattice to modify the dispersion relation of atomic matter waves. Four-wave mixing in this situation produces atom pairs in two well-defined beams. We show that these beams present a narrow momentum correlation, that their momenta are precisely tunable, and that this pair source can be operated in the regimes of low mode occupancy and of high mode occupancy.

We derive an exact formula of geometric measure of quantum discord (GMQD) for an arbitrary 3-qubit state. The dynamics of GMQD in a quantum spin system coupled to a thermal bath is studied by simulation and analytical approximation. It is found that tripartite quantum discord has distinct time behaviors within two different eigenspaces. Further, we extend the square norm distance to define tripartite geometric quantifiers of total and classical correlations. We obtain their explicit expressions and discuss the possibility of having a closed additive relation among various kinds of correlations during the dynamical evolution.

Based on the quantum Zeno effect [B. Misra and E. C. G. Sudarshan, J. Math. Phys. 18, 756 (1977)], we propose a scheme to achieve three-dimensional (3D) entanglement between two distant five-level atoms. In our scheme, the two atoms are trapped individually in two spatially-separated double-mode cavities connected by an optical fiber. It is found that the effective quantum Zeno dynamics of the composite cavity-fiber-cavity coupled system gives rise to the deterministic creation of the 3D entangled state with high fidelity. Moreover, only one step operation is required to complete the generation of the 3D entangled state. The numerical simulations clearly show that the proposed scheme is robust against the deviation of the system parameters and insensitive to various decoherence factors, including atomic spontaneous emissions, cavity decays and fiber photon leakages. We justify our scheme by considering the experimental feasibility within the currently available technology.

The standard method of quantum state tomography (QST) relies on the measurement of a set of noncommuting observables, realized in a series of independent experiments. Ancilla-assisted QST (AAQST) proposed by Nieuwenhuizen and co-workers [Phys. Rev. Lett. 92, 120402 (2004)] greatly reduces the number of independent measurements by exploiting an ancilla register in a known initial state. In suitable conditions AAQST allows mapping out density matrix of an input register in a single experiment. Here we describe methods for explicit construction of AAQST experiments in multiqubit registers. We also report nuclear magnetic resonance studies on AAQST of (i) a two-qubit input register using a one-qubit ancilla in an isotropic liquid-state system and (ii) a three-qubit input register using a two-qubit ancilla register in a partially oriented system. The experimental results confirm the effectiveness of AAQST in such multiqubit registers.

We design a circuit structure with linear depth to implement an $n$-qubit Toffoli gate. The proposed construction uses a quadratic-size circuit that consists of elementary two-qubit controlled-rotation gates around the $x$ axis and uses no ancilla qubit. Circuit depth remains linear in quantum technologies with finite-distance interactions between qubits. The suggested construction is related to the long-standing construction by A. Barenco et al. [Phys. Rev. A 52, 3457 (1995)], which uses a quadratic-size, quadratic-depth quantum circuit for an $n$-qubit Toffoli gate.

In this work we study the properties of a purification-based entropic metric for measuring the distance between both quantum states and quantum processes. This metric is defined as the square root of the entropy of the average of two purifications of mixed quantum states which maximize the overlap between the purified states. We analyze this metric and show that it satisfies many appealing properties, which suggest this metric is an interesting proposal for theoretical and experimental applications of quantum information.

We present a variational approach to evaluate relativistic corrections of order ${\alpha }^2$ to the Bethe logarithm for the ground electronic state of the Coulomb two-center problem. That allows us to estimate the radiative contribution at $m{\alpha }^7$ order in molecular-like three-body systems such as hydrogen molecular ions ${\text{H}}_2$${}^{+}$ and ${\text{HD}}^{+}$ or antiprotonic helium atoms. While we get ten significant digits for the nonrelativistic Bethe logarithm, calculation of the relativistic corrections is much more involved, especially for small values of bond length $R$. We were able to achieve a level of three to four significant digits starting from $R=0.2$ bohr, which will allow us to reach ${10}^{-10}$ relative uncertainty on transition frequencies.

Correlation effects are examined in the context of time-dependent density-functional-theory calculations of antiproton-helium collisions. An approximation for the correlation potential as well as two models for the correlation integral are explored. The first of these makes use of frozen correlation, while the second is appropriated from the world of laser-induced ionization. Total cross sections for both single and double ionization in the impact energy range 5–2000 keV are presented. While the results of the first model provide little improvement over an independent electron model description the second model agrees quite well with experimental results for both single and double ionization. Our results also lend credence to the belief that an appropriate approximation of the correlation integral is more important in reproducing correlation effects than the correlation potential.

The dynamics of an atom in a strong infrared laser field (${10}^{13}$ W/cm${}^2$) result in substantial changes to the field-free electronic energy levels, which can be probed on time scales shorter than the laser cycle using isolated attosecond pulses. Here, we measure the transient absorption of an isolated attosecond pulse by laser-dressed bound states of neon near the first ionization threshold. The observed subcycle changes in the absorption spectrum result from both laser-induced ac Stark shifts and from quantum interferences between different multiphoton excitation pathways. We further demonstrate the ability to experimentally turn off the quantum interference mechanism by eliminating one of the interfering pathways.

For the ionization of diatomic molecules the conditions and mechanisms giving rise to large positive and negative time delays of the ejected electrons are studied. It is shown that for H${}_2$ and H${}_2^{+}$ the singularities in the angular distribution of time delay occur when at a certain energy the amplitudes of all partial spheroidal waves, except one, turn into zero, i.e., they coincide with the so-called Cooper minimum. By analytical consideration of the emitted electron wave-packet evolution it is demonstrated that large negative values of the phase derivative with respect to energy do not violate the causality principle. We also analyze the dependence of time delay upon the electron energy for Coulomb spheroidal waves with different spheroidal quantum numbers.

The change of the probability of proton emission in the nuclear photoeffect due to an intense coherent (laser) field is discussed near the threshold, where the hindering effect of the Coulomb field of the remainder nucleus is essential. The ratio of the laser-assisted and laser-free differential cross section is deduced and found to be independent of the polarization state of the $\gamma$ field and the two types of initial nuclear state considered. The numerical values of this ratio are given at some characteristic parameters of the intense field.

We show theoretically that pump-probe spectroscopy with strong pulses can be employed to effectively amplify weak electronic transitions and to learn about the associated material system dynamics. As an illustration, we discuss the possibility of the detection of weak transitions to discrete vibrational levels in the excited electronic state of the weakly bound He($2{}^{1}S$)-Ne dimer.

We study the properties of the Bose-Hubbard model for square and cubic superlattices. To this end we generalize a recently established effective potential Landau theory for a single component to the case of multiple components and not only find the characteristic incompressible solid phases with fractional filling, but also obtain the underlying quantum phase diagram in the whole parameter region at zero temperature. A comparison of our analytic results with corresponding ones from quantum Monte Carlo simulations demonstrates the high accuracy of the generalized effective-potential Landau theory (GEPLT). Finally, we comment on the advantages and disadvantages of the GEPLT in view of a direct comparison with a corresponding decoupled mean-field theory.