We present a simple architecture for deterministic quantum circuits operating on single-photon qubits. Few resources are necessary to implement two elementary gates and can be recycled for computing with large numbers of qubits. The deterministic realization of some key multiqubit gates, such as the Fredkin and Toffoli gate, is greatly simplified in this approach.

Multiphoton states are widely applied in quantum information technology. By the methods presented in this paper, the structure of a multiphoton state in the form of multiple single-photon qubit products can be mapped to a single-photon qudit, which could also be in a separable product with other photons. This makes possible the manipulation of such multiphoton states by processing single-photon states. The optical realization of unknown qubit discrimination [B. He, J. A. Bergou, and Y.-H. Ren, Phys. Rev. A 76, 032301 (2007)] is simplified with the transformation methods. Another application is the construction of quantum logic gates, where the inverse transformations back to the input state spaces are also necessary. We especially show that the modified setups to implement the transformations can realize the deterministic multicontrol gates (including Toffoli gate) operating directly on the products of single-photon qubits.

We have measured double-electron capture cross sections for ${\text{Li}}^{3+}$ projectiles and He targets in the 2.0–5.5 MeV energy range (3.4–5.6 a.u. velocity range). Double-electron-capture cross sections for this collision system were measured before, but only at relatively lower energies. There is a good agreement between the three available sets of experimental data for double capture at the lower end of the present data energy interval, i.e., where the ranges from different measurements overlap. The present measurements for other collision channels that include single and double ionization, single capture, and transfer ionization corroborate the reliability of the data normalization adopted in our work. Our experimental data for double capture are compared with the corresponding theories in order to assess the relative performance of the main two strategies, the semiclassical independent particle model, and the full quantum-mechanical four-body theories. We thoroughly discuss possible reasons for the discrepancies among various theoretical predictions from these two frameworks and point out several potential paths for their improvement.

We calculate the microwave spectra of ultracold ${}^{40}\text{K}{}^{87}\text{R}\text{b}$ alkali-metal dimers, including hyperfine interactions and in the presence of electric and magnetic fields. We show that microwave transitions may be used to transfer molecules between different hyperfine states, but only because of the presence of nuclear quadrupole interactions. Hyperfine splittings may also complicate the use of ultracold molecules for quantum computing. The spectrum of molecules oriented in electric fields may be simplified dramatically by applying a simultaneous magnetic field.

A rotating ultracold $S$-wave superfluid Fermi gas is studied when the mismatch in chemical potentials $\delta \mu$ (and the population imbalance $\delta n$) corresponds to the Larkin-Ovchinnikov-Fulde-Ferrell state in the vicinity of the Lifshitz critical point. It is shown that under these conditions the critical angular velocity ${\Omega }_{c2}$ in two-dimensional systems is an oscillating function of temperature and $\delta n$ giving rise to reentrant superfluid phases. The reason for this behavior is the population by Cooper pairs of the Landau levels $\left(n\ge 1\right)$ above the lowest one $\left(n=0\right)$.

We analyze the entanglement distribution and the two-qubit residual entanglement in multipartite systems. For a composite system consisting of two cavities interacting with independent reservoirs, it is revealed that the entanglement evolution is restricted by an entanglement monogamy relation derived here. Moreover, it is found that the initial cavity-cavity entanglement evolves completely to the genuine four-partite cavities-reservoirs entanglement in the time interval between the sudden death of cavity-cavity entanglement and the birth of reservoir-reservoir entanglement. In addition, we also address the relationship between the genuine block-block entanglement form and qubit-block form in the interval.

Recent work has argued that the concepts of entanglement and nonlocality must be taken seriously even in systems consisting of only a single particle. These treatments, however, are nonrelativistic, and, if single-particle entanglement is fundamental, it should also persist in a relativistic description. Here, we consider a spin-1/2 particle in a superposition of two different velocities as viewed by an observer in a relativistically boosted inertial frame and show that the entanglement between the two velocity modes survives right up to the speed of light. We also discuss how quantum gates could be implemented in this way and apply our results to the case of a superconductor. In particular, we show that an $s$-wave superconductor would have $p$-wave components for a boosted observer.

Recently, the principle of information causality has appeared as a good candidate for an information-theoretic principle that would single out quantum correlations among more general nonsignaling models. Here, we present results going in this direction, namely, we show that part of the boundary of quantum correlations actually emerges from information causality.

Electron emission spectra obtained by thermal collisions of ${\text{He}}^{\ast }(2{}^{1}S$ and $2{}^{3}S$) atoms with Au(111) and Pt(111) surfaces were measured to clarify the electronically excited atom-metal interactions. It has been recognized that the metastable atoms de-excite on ordinary noble- and transition-metal surfaces via resonance ionization (RI) followed by Auger neutralization (AN) without no indication of Penning ionization (PI). Our data show that this traditional criterion partially breaks down in the ${\text{He}}^{\ast }\text{-Au}\left(111\right)$ collision system. The local electronic states near the surface were examined by first-principles calculations using density functional theory. It reveals that the itinerant $sp$ states are significantly spilled out toward the vacuum compared to the localized $5d$ states, and their asymptotic features play a crucial role in determining the branching ratio between PI and $\text{RI}+\text{AN}$.

We investigate the finite-temperature properties of attractive three-component (colors) fermionic atoms in optical lattices using a self-energy functional approach. As the strength of the attractive interaction increases in the low-temperature region, we observe a second-order transition from a Fermi liquid to a color superfluid (CSF), where atoms from two of the three colors form Cooper pairs. In the strong attractive region, we observe a first-order transition from a CSF to a trionic state, where three atoms with different colors form singlet bound states. A crossover between a Fermi liquid and a trionic state is observed in the high-temperature region. We present a phase diagram covering zero to finite temperatures. We demonstrate that the CSF transition temperature is enhanced by the anisotropy of the attractive interaction.

The theory of quantum feedback networks has recently been developed with the aim of showing how quantum input-output components may be connected together so as to control, stabilize, or enhance the performance of one of the subcomponents. In this paper, we show how the degree to which an idealized component (a degenerate parametric amplifier in the strong-coupling regime) can squeeze input fields may be enhanced by placing the component in loop in a simple feedback mechanism involving a beam splitter. We study the spectral properties of output fields, placing particular emphasis on the elastic and inelastic components of the power density.

Relativistic oscillator strengths are computed for $n{s}_{1/2}\text{−}n{p}_{1/2,3/2}$ transitions in silver and gold isoelectronic sequences employing configuration-interaction method with numerical Dirac-Fock wave functions generated with noninteger outermost core shell occupation number. This method is optimized to represent efficiently the core-valence electron correlation that is known to be important for transitions under consideration. The presented results are compared with our earlier data computed with core-valence correlation represented in the semiclassical core polarization picture within single configuration Dirac-Fock scheme as well as available experimental values.

Precise numerical calculation of radiofrequency intervals between hyperfine sublevels of the (37,35) state of the antiprotonic helium-4 atom is presented. Theoretical consideration includes the QED corrections of order $m{\alpha }^6$ to the electron spin-orbit interaction. Only the leading order contribution in the electron-to-antiproton and electron-to-$\alpha$ particle mass ratios is considered, so the $\alpha$ particle and antiproton are treated nonrelativistically. The effective Hamiltonian is derived using the formalism of the nonrelativistic quantum electrodynamics.

The nuclear spin conversion (NSC) rate for a gas sample of ${\text{NH}}_3$ is calculated in the framework of the quantum relaxation model. It is based on the knowledge of the vibration-inversion levels of the lowest vibrational states, the relaxation rates, and the intramolecular magnetic interactions. The magnitudes of the spin-spin and spin-rotation interaction terms have been obtained with the help of ab initio calculations. The value of the NSC at 296 K for a pressure of 1 Torr is $3.23\times {10}^{-6}{\text{s}}^{-1}$ for ${{}^{14}\text{N}\text{H}}_3$ and $9.15\times {10}^{-6}{\text{s}}^{-1}$ for ${{}^{15}\text{N}\text{H}}_3$. A linear dependence of the NSC rates on the pressure is valid up to 1 atm for ${{}^{14}\text{N}\text{H}}_3$ and up to 30 Torr for ${{}^{15}\text{N}\text{H}}_3$, where quasidegenerate ortho/para pairs are present in the excited ${\nu }_2=1$ state.

We study transmission through a system with $N=10$ states coupled to $K=2$ continua of scattering wave functions in the framework of the $S$ matrix theory by using the Feshbach projection operator formalism for open quantum systems. Due to the coupling of the system (being localized in space) to the (extended) continuum of scattering wave functions, the Hamilton operator $H_{\text{eff}}$ of the system is non-Hermitian. The numerical calculations are performed for different distributions of both the positions $E_i^0$ $\left(i=1,\dots ,N\right)$ of the states of the isolated (closed) system and the elements of the coupling vectors $V^c$ between system and continua $\left(c=1,\dots ,K\right)$. The overall coupling strength $\alpha$ simulating the degree of resonance overlapping, is used as a parameter. In all cases, the complex eigenvalues and eigenfunctions of $H_{\text{eff}}$ are controlled by $\alpha$. In the regime of overlapping resonances, the well-known spectroscopic reordering processes (resonance trapping) take place because the phases of the eigenfunctions of $H_{\text{eff}}$ are not rigid in the neighborhood of singular points (being crossing points of eigenvalue trajectories). Finally, width bifurcation generates $K=2$ short-lived and $N-K$ trapped long-lived states. Thus, narrow (Fano-like) resonances may appear in the transmission at high level density. They are similar to, but different from the Fano resonances in the scattering theory with $K=1$. Phase lapses are related to zeros in the transmission probability. Their number and position (in energy) are determined by the $V^c$ and $E_i^0$, but not by $\alpha$. However, number and position of the resonance states depend on $\alpha$ due to resonance trapping occurring in the regime of overlapping resonances. As a consequence, universal phase lapses between every two resonances may appear at high level density while the system will show mesoscopic features at low level density. The phase lapses are not a single phenomenon. Due to their link to singularities in the continuum, they are related also to other “puzzling” experimental results such as dephasing at low temperature.

An experimental study is reported about the recombination of $\text{D}_3{}^{+}$ ions with electrons in a low-temperature plasma (200–300 K) consisting of He with a small admixture of ${\text{D}}_2$. At several temperatures, the pressure dependence of the apparent binary recombination rate coefficient $\left({\alpha }_{\text{eff}}\right)$ was measured over a broad range of helium pressures (200–2000 Pa). The binary and ternary recombination rate coefficients were obtained from measured pressure dependences of ${\alpha }_{\text{eff}}$. The binary recombination rate coefficient obtained ${\alpha }_{\text{bin}}\left(300\text{K}\right)=\left(2.7\pm 0.9\right)\times {10}^{-8}{\text{cm}}^3{\text{s}}^{-1}$ is in agreement with recent theory. The ternary recombination rate coefficient obtained is $K_{\text{He}}\left(300\text{K}\right)=\left(1.8\pm 0.6\right)\times {10}^{-25}{\text{cm}}^6{\text{s}}^{-1}$. In analogy with the recently described process of helium-assisted ternary recombination of $\text{H}_3{}^{+}$ ions, it is suggested that the ternary helium-assisted recombination of $\text{D}_3{}^{+}$ ions proceeds through the formation of a neutral long-lived highly excited Rydberg molecule ${\text{D}}_3$ followed by a collision with a He atom.

We investigate the ground-state properties of two-component Bose gases confined in one-dimensional harmonic traps in the scheme of density-functional theory. The density-functional calculations employ a Bethe-ansatz-based local-density approximation for the correlation energy, which accounts for the correlation effect properly from the weakly interacting regime to the strongly interacting regime. For the binary Bose mixture with spin-independent interaction, the homogeneous reference system is exactly solvable by the Bethe-ansatz method. Within the local-density approximation, we determine the density distribution of each component and study its evolution from Bose distributions to Fermi-like distribution with the increase in interaction. For the binary mixture of Tonks-Girardeau gases with a tunable interspecies repulsion, with a generalized Bose-Fermi transformation we show that the Bose mixture can be mapped into a two-component Fermi gas, which corresponds to exact soluble Yang-Gaudin model for the homogeneous system. Based on the ground-state energy function of the Yang-Gaudin model, the ground-state density distributions are calculated for various interspecies interactions. It is shown that with the increase in interspecies interaction, the system exhibits composite-fermionization crossover.

We experimentally investigate diffraction of a ${}^{87}\text{R}\text{b}$ Bose-Einstein condensate from a one-dimensional optical lattice. We use a range of lattice periods and timescales, including those beyond the Raman-Nath limit. We compare the results to numerical solutions of the Gross-Pitaevskii equation and classical calculations, with quantitative and qualitative agreement, respectively. The classical calculations predict that the envelope of the time-evolving diffraction pattern is shaped by caustics: singularities in the phase-space density of classical trajectories. This behavior becomes increasingly clear as the lattice period grows.

We present a study of the dynamics and decay pattern of phase singularities due to the action of a system with a discrete rotational symmetry of finite order. A topological charge conservation rule is identified. The role played by the underlying symmetry is emphasized. An effective model describing the short range dynamics of the vortex clusters has been designed. A method to engineer any desired configuration of clusters of phase singularities is proposed. Its flexibility to create and control clusters of vortices is discussed.

We present a derivation of the main contrast mechanisms accessible in scanning transmission x-ray microscopy with a pixel-array detector. We consider the effect of the probe defocus and show that it can produce strong differential absorption contrast. The effect of noise is derived and used to compare the relative merits of absorption and differential contrast imaging in various experimental conditions. We illustrate the main results with an experiment that combines a through-focus series with the near-edge signal of a cerium oxide sample. The measurements are seen to follow closely the derived contrast expressions, including the defocus-dependent differential absorption contrast. The analysis includes additional considerations about the application of principal component analysis on a through-focus image series.