A counterintuitive result in entanglement theory was shown by Cubitt et al. [Phys. Rev. Lett. 91, 037902 (2003)], namely, that entanglement can be distributed by sending a separable state through a quantum channel. In this work, following an analogy between the entanglement and secret-key distillation scenarios, we derive its classical analog: secrecy can be distributed by sending nonsecret correlations through a private channel. This strengthens the close relation between entanglement and secrecy.

We study the Kohn-Sham scheme for the calculation of the steady-state linear response $\lambda n_{\omega }^{\left(1\right)}\left(\mathbf{r}\right)\text{cos}\omega t$ to a harmonic perturbation $\lambda v^{\left(1\right)}\left(\mathbf{r}\right)\text{cos}\omega t$ that is turned on adiabatically. Although in general the exact exchange-correlation potential $v_{\text{xc}}\left(\mathbf{r},t\right)$ cannot be expressed as the functional derivative of a universal functional due to the so-called causality paradox, we show that for a harmonic perturbation the exchange-correlation part of the first-order Kohn-Sham potential $\lambda v_s^{\left(1\right)}\left(\mathbf{r}\right)\text{cos}\omega t$ is given by $v_{\text{xc}}^{\left(1\right)}\left(\mathbf{r}\right)=\delta K_{\text{xc}}^{\left(2\right)}/\delta n_{\omega }^{\left(1\right)}\left(\mathbf{r}\right)$. $K_{\text{xc}}^{\left(2\right)}$ is the exchange-correlation part of the second-order quasienergy $K_v^{\left(2\right)}$. The Frenkel variation principle implies a stationary principle for $K_v^{\left(2\right)}\left[n_{\omega }^{\left(1\right)}\right]$. We also find an analogous stationary principle and Kohn-Sham scheme in the time-dependent extension of one-matrix functional theory, in which the basic variable is the one-matrix (one-body-reduced density matrix).

An accurate determination of the nickel $L_{\text{III}}$ x-ray transition probabilities was performed employing a wavelength-dispersive spectrometer (WDS) at the plane grating monochromator beamline for undulator radiation of the Physikalisch-Technische Bundesanstalt, Germany’s national metrology institute, at the electron storage ring BESSY II. An absolute calibration method developed for this spectrometer improves the reliability of the determination significantly because the calibration method does not require measurements of reference samples and provides a traceable uncertainty budget. The device was calibrated with respect to both its absolute detection efficiency and its response behavior. Therefore, a method to obtain the detection efficiency of a WDS in comparison to an absolutely calibrated energy-dispersive detector was successfully applied. In addition, response functions were developed and experimentally validated. The present results for the $\text{Ni}{L}_{\text{III}}$ transition probabilities differ clearly from theoretical calculations and tabulated data found in the literature. Perspectives of a quantitative study of the interactions causing the satellite transitions are outlined and discussed in view of shake-off and Coster-Kronig processes.

The mapping, exact or approximate, of a many-body problem onto an effective single-body problem is one of the most widely used conceptual and computational tools of physics. Here, we propose and investigate the inverse map of effective approximate single-particle equations onto the corresponding many-particle system. This approach allows us to understand which interacting system a given single-particle approximation is actually describing, and how far this is from the original physical many-body system. We illustrate the resulting reverse engineering process by means of the Kohn-Sham equations of density-functional theory. In this application, our procedure sheds light on the nonlocality of the density-potential mapping of density-functional theory, and on the self-interaction error inherent in approximate density functionals.

We report the results of accurate time-dependent calculations of two-photon ionization of helium by ultrashort pulses. Ionization amplitudes and generalized cross sections are extracted from the wave function using exterior complex scaling. For photon energies above the first ionization threshold, two-photon single ionization is enhanced by core excited resonances, in processes visible with pulses as short as 2 fs, when the photon frequency is equal to a transition energy in ${\text{He}}^{+}$. We explore the dependence of the total cross section in the vicinity of the threshold for sequential double ionization on pulse duration. A signature in the single differential cross section of two-photon sequential ionization with the ground state of the ion as the intermediate state is seen to be suppressed by sufficiently short pulses in favor of the nonsequential process, while the triple differential cross section shows that attosecond pulses can access different electron dynamics than those of longer duration. The peaks in the single differential cross section due to sequential ionization with the excited intermediate states of the ion are observed to occur at energies displaced by about 2 eV from the expected values by interference effects between continuum channels.

We study a gas of repulsively interacting bosons in an optical lattice and explore the physics beyond the lowest-band Hubbard model. Utilizing a generalized Gutzwiller ansatz, we find how the lowest-band physics is modified by the inclusion of the first excited bands. In contrast to the predictions of the lowest-band Bose-Hubbard model based on mean-field theories, a re-entrant behavior of superfluidity is envisaged as well as decreasing width of the Mott lobes at strong coupling.

The thermodynamic properties and finite-size scaling of weakly interacting Bose gases with a finite number of particles confined in a cubic box of volume $L^3$ with periodic boundary conditions are investigated theoretically based on an exact recursion relation for the canonical-ensemble partition function. In a similar manner to the case of an ideal Bose gas, the recursive scheme of the interacting finite system at any temperatures is established in the framework of the Bogoliubov theory. The temperature dependence of the condensate fraction and specific heat with different particles and interactions is obtained numerically. By defining two different transition temperatures of the finite systems, the effect of interactions on the transition temperatures yields the different and nonmonotonous behaviors. The finite-size scaling condensate fraction at the transition temperature for the systems is also discussed. It is shown that the calculated finite-size scaling is universally independent on the various system sizes and transition temperatures.

We theoretically study the superradiant gain and the direction of this coherent radiant for an array of Bose-Einstein condensates in an optical lattice. We find that the density grating is formed to amplify the scattering light with the phase-matching condition. The scattering spectroscopy in the momentum space can provide a method to measure the overlap of wave functions of neighboring sites, which is related to the inner-site and intersite coherences.

A scheme of generating controllable $\left(2+1\right)$ photons in a double-$\Lambda$ atomic system based on active Raman gain is presented in this paper. Such $\left(2+1\right)$ photons can be a potential candidate to generate a correlated photon pair as one photon of “2” photons acts as a trigger. Our proposal is an alternative approach to generate photon pairs where the frequencies are tunable for subsequent atomic system experiments compared to spontaneous parametric down-conversion case. Compared to other schemes of generating correlated photon pairs, our scheme exhibits several features due to the exploit of the stimulated Raman process and injection-seeding mechanism.

We analyze the coherence singularities that occur in the far field that is generated by a broad class of partially coherent sources. It is shown that for rotationally symmetric planar quasihomogeneous sources the coherence singularities form a two-dimensional surface in a reduced three-dimensional space. We illustrate our results by studying the topology of the coherence singularity of a partially coherent vortex beam. We find that the geometry of the phase singularity can be associated with conic sections such as ellipses, lines, and hyperbolas.

Quantum phase transitions that take place between two distinct topological phases remain an unexplored area for the applicability of the fidelity approach. Here, we apply this method to spin systems in two and three dimensions and show that the fidelity susceptibility can be used to determine the boundary between different topological phases particular to these models, while at the same time offer information about the critical exponent of the correlation length. The success of this approach relies on its independence on local-order parameters or breaking symmetry mechanisms, with which nontopological phases are usually characterized. We also consider a topological insulator-superconducting phase transition in three dimensions and point out the relevant features of fidelity susceptibility at the boundary between these phases.

Both cavity quantum electrodynamics and photons are promising candidates for quantum information processing. We consider a combination of both candidates with a single photon going through spatially separate cavities to entangle the atomic qubits, based on the input-output process of the cavities. We present a general expression for the input-output process regarding the low-$Q$ cavity confining a single atom, which works in a wide range of parameters. Focusing on low-$Q$ cavity case, we propose some schemes for quantum information processing with Faraday rotation using single photons, which is much different from the high-$Q$ cavity and strong-coupling cases.

We study photon- and ion-induced electron emission from one-electron diatomic molecular ions. For photon impact we find an oscillatory behavior of the asymmetry parameter $\beta$ and of the total cross section which can be related to the presence of Cooper minima in the partial cross sections. This behavior can also be related to coherent electron emission from the two centers of the molecule. By comparing the results for homonuclear and heteronuclear molecular ions, we show that the periodic structures predicted by both pictures appear clearly only for the former targets. This conclusion is supported by study of the electron emission spectra induced by swift ion impact.

The propagation of transverse spatial correlations of photon pairs through arbitrary first-order linear optical systems is studied experimentally and theoretically using the fractional Fourier transform. Highly correlated photon pairs in an Einstein-Podolsky-Rosen-like state are produced by spontaneous parametric down-conversion and subject to optical fractional Fourier transform systems. It is shown that the joint detection probability can display either correlation, anticorrelation, or no correlation, depending on the sum of the orders $\alpha$ and $\beta$ of the transforms of the down-converted photons. We present analytical results for the propagation of the perfectly correlated EPR state and numerical results for the propagation of the two-photon state produced from parametric down-conversion. We find good agreement between the theory and experiment.

We can encode an image from an orthogonal basis set onto a single photon from a downconverted pair via the use of an amplitude mask. We can then discriminate the image imprinted on the photon from other images in the set using holographic-matched filtering techniques. We demonstrate this procedure experimentally for an image space of two objects, and we discuss the possibility of applying this method to a much larger image space. This process could have important implications for the manipulation of images at the quantum level.

We investigate the possibility of bistable lasing in microcavity lasers as opposed to bulk lasers. To that end, the dynamic behavior of a microlaser featuring two coupled interacting modes is analytically investigated within the framework of a semiclassical laser model, suitable for a wide range of cavity shapes and mode geometries. Closed-form coupled mode equations are obtained for all classes of laser dynamics. We show that bistable operation is possible in all of these classes. In the simplest case (class A lasers) bistability is shown to result from an interplay between coherent (population-pulsation) and incoherent (hole-burning) processes of mode interaction. We find that microcavities offer better conditions to achieve bistable lasing than bulk cavities, especially if the modes are not degenerate in frequency. This results from better matching of the spatial intensity distribution of microcavity modes. In more complicated laser models (classes B and C) bistability is shown to persist for modes even further apart in frequency than in the class A case. The predictions of the coupled mode theory have been confirmed using numerical finite-difference time-domain calculations.

We present a Floquet treatment of multiphoton quantum interference in a strongly driven superconducting flux qubit. The periodically time-dependent Schrödinger equation can be reduced to an equivalent time-independent infinite-dimensional Floquet matrix eigenvalue problem. For resonant or nearly resonant multiphoton transitions, we extend the generalized Van Vleck (GVV) nearly degenerate high-order perturbation theory for the treatment of the Floquet Hamiltonian, allowing the reduction of the infinite-dimensional Floquet matrix to an $N\times N$ effective Hamiltonian, where $N$ is the number of eigenstates under consideration. The GVV approach allows accurate treatment of ac Stark shift, power broadening, time-dependent and time-averaged transition probability, etc., well beyond the rotating wave approximation. We extend the Floquet and GVV approaches for numerical and analytical studies of the multiphoton resonance processes and quantum interference phenomena for the superconducting flux qubit system $(N=2)$ driven by intense ac fields.

Theoretical investigations of hyperfine quenching of the two metastable levels $1s^{2}2s2p{}^{3}P_0$ and ${}^{3}P_2$ have been performed along the Be-like isoelectronic sequence for ions between $Z=6–22$. It is shown that the lifetime of the latter level is sensitive to hyperfine quenching in the beginning of the isoelectronic sequence, but the sensitivity decreases towards the high-$Z$ end. It is also shown that the hyperfine dependent branching fraction between the $2s^2{}^{1}S_0–2s2p{}^{3}P_2$ and the $2s2p{}^{3}P_1–2s2p{}^{3}P_2$ transitions can lead to some rather drastic changes in the spectra. Our predicted value of $0.677{\mathrm{s}}^{-1}$ for the hyperfine-induced $2s^2{}^{1}S_0–2s2p{}^{3}P_0$ transition rate for ${}^{47}\mathrm{Ti}^{18+}$ are in disagreement with a recent experimental value of $0.56\left(3\right){\mathrm{s}}^{-1}$. Our calculated values of this quenching are in agreement with other recent calculations all along the sequence.

We investigate the presence of Feshbach resonances in ultracold alkali-dialkali reactive collisions. A potential-energy surface of ${\text{Na}}_3$ in the lowest quartet state is constructed and used in quantum scattering calculations. An analysis of scattering features is performed through a systematic variation in the nonadditive three-body interaction potential. Our results should provide useful information for interpreting future atom-molecule collision experiments.

Alkali-metal vapor cells with antirelaxation coating (especially paraffin-coated cells) have been a central tool in optical pumping and atomic spectroscopy experiments for 50 years. We have discovered a dramatic change of the alkali-metal vapor density in a paraffin-coated cell upon application of an electric field to the cell. A systematic experimental characterization of the phenomenon is carried out for electric fields ranging in strength from 0–8 kV/cm for paraffin-coated cells containing rubidium and cells containing cesium. The typical response of the vapor density to a rapid (duration $\lesssim 100\text{ms}$) change in electric field of sufficient magnitude includes (a) a rapid (duration of $\lesssim 100\text{ms}$) and significant increase in alkali-metal vapor density followed by (b) a less rapid (duration of $\sim 1\text{s}$) and significant decrease in vapor density (below the equilibrium vapor density), and then (c) a slow (duration of $\sim 100\text{s}$) recovery of the vapor density to its equilibrium value. Measurements conducted after the alkali-metal vapor density has returned to its equilibrium value indicate minimal change (at the level of $\lesssim 10\%$) in the relaxation rate of atomic polarization. Experiments suggest that the phenomenon is related to an electric-field-induced modification of the paraffin coating.