We propose a linear optical scheme to implement the nondeterministic two-qubit controlled phase gate with the certain success probability. The proposed setup involves simple linear optical elements, a pair of the two-photon polarization entangled states, and the conventional photon detectors that only distinguish the vacuum and nonvacuum Fock number states. A sophisticated single-photon detector distinguishing one or two photon states is unnecessary. This can greatly simplify the experimental realization of a linear optical quantum computer.

Femtosecond coherent phase control of resonance mediated $(2+1)$ three-photon absorption is studied both theoretically and experimentally. The regime is perturbative of third order. The photoexcitation coherently combines elements of both nonresonant and resonance-mediated multiphoton transitions. By proper simple pulse shaping the three-photon absorption in Na is effectively controlled experimentally, enhanced up to $\sim 300\%$ of the absorption induced by the transform-limited pulse. It is achieved by phase manipulating intra- and intergroup interferences involving two groups of three-photon excitation pathways: (i) on resonance and (ii) near resonance with the intermediate resonance state accessed by nonresonant two-photon transition.

Inspired by the idea of conditional probabilities, we introduce a variant of conditional density operators. But unlike the conditional probabilities which are bounded by 1, the conditional density operators may have eigenvalues exceeding 1 for entangled states. This has the consequence that although any bivariate classical probability distribution has a natural separable decomposition in terms of conditional probabilities, we do not have a quantum analogue of this separable decomposition in general. The “nonclassical” eigenvalues of conditional density operators are indications of entanglement. The resulting separability criterion turns out to be equivalent to the reduction criterion introduced by Horodecki [Phys. Rev. A 59, 4206 (1999)] and Cerf et al. [Phys. Rev. A 60, 898 (1999)]. This supplies an intuitive probabilistic interpretation for the reduction criterion. The conditional density operators are also used to define a form of quantum conditional entropy which provides an alternative mechanism to reveal quantum discord.

In cross-section calculations of electron or photon double ionization processes of two-electron atoms, it is desirable to have a symmetric description of initial and final states. In this contribution, we search the doubly bound analog, for $S$ states, of the well-known and widely used three-body Coulomb (C3) double-continuum wave function. This is performed with two alternative approaches: through an analytic continuation of the C3 continuum and through a “C3 approach” of the Hylleraas equation for $S$ bound states. The double-bound analog consists of the product of two Coulomb bound states (one for each electron-nucleus interaction) multiplied by a Coulomb distortion factor which describes the electron-electron correlation. Our result differs from Pluvinage’s wave function which is commonly misbelieved to be the bound counterpart of the C3 double continuum. With a rigorous treatment of the analytic continuation of the distortion factor, the double-bound equivalent of the C3 model is also found. Though the purpose is not to obtain good two-electron bound states, the found bound wave functions are tested, in the case of helium, through various local and mean quantities which probe different regions of the configuration space, in particular those close to the two-particle coalescence points.

The results of a study of the Auger-electron–photoelectron angular correlations in the case of the C $1s$ ionization of the CO molecule are presented and compared with theoretical calculations in the Hartree-Fock approximation based on the two-step model. The measurements have been performed at two photon energies, 305 and $318\mathrm{eV}$, respectively, and at three angles of photoelectron emission relative to the light polarization vector: namely, 0°, 30°, and 60°. A general agreement is found between theory and experiment for the coincidence angular distributions and the relative magnitudes of the Auger-electron–photoelectron angular correlations. However, both experiment and theory show that the Auger-electron–photoelectron angular correlations are not sufficiently sensitive to the details of the Auger-electron wave function to allow a “complete” Auger experiment in molecules. On the other hand, our calculations demonstrate that the Auger-electron angular distribution measured in the molecular frame is very sensitive to the individual contributions of different partial waves of the Auger electron. Therefore we conclude that the complete experiment for the Auger decay in molecules can be realized only measuring the Auger-electron angular distributions in the molecular frame.

The Fano-equivalent profiles of two negative-ion resonant states have been observed in the integrated Stokes parameters of 636.2 nm photons emitted from the $3d^{10}4s4d^1{D}_2$ state of zinc. These profiles reflect the effects of electron exchange and the spin-orbit interaction via the angular-momentum-dependent properties of the alignment and orientation Stokes parameters. The resonant states have energies of $10.98\pm 0.02\mathrm{eV}$ and $11.33\pm 0.02\mathrm{eV}$ and widths of $0.25\pm 0.03\mathrm{eV}$ and $0.33\pm 0.05\mathrm{eV}$, respectively, and are bound to an excited atomic state with a $3d^9$ core hole. The lower-energy resonance shows a strong spin-orbit interaction whereas the higher-energy resonance shows negligible spin-orbit interaction.

We suggest a physical interpretation of the Uhlmann amplitude of a density operator. Given this interpretation we propose an operational approach to obtain the Uhlmann condition for parallelity. This allows us to realize parallel transport along a sequence of density operators by an iterative preparation procedure. At the final step the resulting Uhlmann holonomy can be determined via interferometric measurements.

We propose an efficient scheme to build an arbitrary multipartite Greenberger-Horne-Zeilinger state and discriminate all the universal Greenberger-Horne-Zeilinger states using parity measurement based on dipole-induced transparency in a cavity-waveguide system. A prominent advantage is that initial entangled states remain after nondetective identification and they can be used for successive tasks. We analyze the performance and possible errors of the required single-qubit rotations and emphasize that the scheme is reliable and can satisfy the current experimental technology.

We investigate the influence of memory errors in the quantum repeater scheme for long-range quantum communication. We show that the communication distance is limited in standard operation mode due to memory errors resulting from unavoidable waiting times for classical signals. We show how to overcome these limitations by (i) improving local memory and (ii) introducing two operational modes of the quantum repeater. In both operational modes, the repeater is run blindly, i.e., without waiting for classical signals to arrive. In the first scheme, entanglement purification protocols based on one-way classical communication are used allowing to communicate over arbitrary distances. However, the error thresholds for noise in local control operations are very stringent. The second scheme makes use of entanglement purification protocols with two-way classical communication and inherits the favorable error thresholds of the repeater run in standard mode. One can increase the possible communication distance by an order of magnitude with reasonable overhead in physical resources. We outline the architecture of a quantum repeater that can possibly ensure intercontinental quantum communication.

We study the sequential generation of entangled photonic and atomic multiqubit states in the realm of cavity QED. We extend the work of C. Schön et al. [Phys. Rev. Lett. 95, 110503 (2005)], where it was shown that all states generated in a sequential manner can be classified efficiently in terms of matrix-product states. In particular, we consider two scenarios: photonic multiqubit states sequentially generated at the cavity output of a single-photon source and atomic multiqubit states generated by their sequential interaction with the same cavity mode.

The bright matter-wave soliton propagation through a barrier with a rapidly oscillating position is investigated. The averaged-over rapid oscillations Gross-Pitaevskii equation is derived, where the effective potential has the form of a finite well. Dynamical trapping and quantum tunneling of the soliton in the effective finite well are investigated. The analytical predictions for the effective soliton dynamics is confirmed by numerical simulations of the full Gross-Pitaevskii equation.

We discuss nonclassical properties of single-photon subtracted squeezed vacuum states in terms of the sub-Poissonian statistics and the negativity of the Wigner function. We derive a compact expression for the Wigner function from which we find the region of phase space where Wigner function is negative. We find an upper bound on the squeezing parameter for the state to exhibit sub-Poissonian statistics. We then study the effect of decoherence on the single-photon subtracted squeezed states. We present results for two different models of decoherence, viz. amplitude decay model and the phase diffusion model. In each case we give analytical results for the time evolution of the state. We discuss the loss of nonclassicality as a result of decoherence. We show through the study of their phase-space properties how these states decay to vacuum due to the decay of photons. We show that phase damping leads to very slow decoherence, slower than the photon-number decay.

We show theoretically how the periodic coupling between an engineered reservoir and a quantum Brownian particle leads to the formation of a dynamical steady-state which is characterized by an effective temperature above the temperature of the environment. The average steady-state energy of the system has a higher value than expected from the environmental properties. The system experiences repeatedly a non-Markovian behavior—as a consequence the corresponding effective decay for long evolution times is always on average stronger than the Markovian one. We also highlight the consequences of the scheme for the Zeno-anti-Zeno crossover which depends, in addition to the periodicity $\tau$, also on the total evolution time of the system.

We present sufficient conditions for states to have positive distillable key rate. Exploiting the conditions, we show that the bound entangled states given by Horodecki et al. [Phys. Rev. Lett. 94, 160502 (2005)] have nonzero distillable key rate and finally exhibit new classes of bound entangled states with positive distillable key rate, but with negative Devetak-Winter lower bound of the distillable key rate for the classical-classical-quantum (ccq) states of their privacy squeezed versions, which represent the states after the local measurement performed in the key part without considering the shield part.

We discuss the cases where local decoherence selectively degrades one type of entanglement more than other types. A typical case is called state ordering change, in which two input states with different amounts of entanglement undergoes a local decoherence and the state with the larger entanglement results in an output state with less entanglement than the other output state. We are also interested in a special case where the state with the larger entanglement evolves to a separable state while the other output state is still entangled, which we call selective entanglement breaking. For three-level or larger systems, it is easy to find examples of the state ordering change and the selective entanglement breaking, but for two-level systems it is not trivial whether such situations exist. We present a general strategy to construct examples of two-qubit states exhibiting the selective entanglement breaking regardless of entanglement measure. We also give a more striking example of the selective entanglement breaking in which the less entangled input state has only an infinitesimal amount of entanglement.

Results on heat current, entropy production rate, and entanglement are reported for a quantum system coupled to two different temperature heat reservoirs. By applying a temperature gradient, different quantum states can be found with exactly the same amount of entanglement but different purity degrees and heat currents. Furthermore, a nonequilibrium enhancement-suppression transition behavior of the entanglement is identified.

Apparent generalized oscillator strengths (apparent GOS’s) have been measured for three types of optically forbidden transitions in rare-gas atoms as functions of the squared momentum transfer $K^2$ at small $K^2$ range $(⩽0.4\mathrm{a.u.})$. The apparent GOS’s were deduced from the differential cross sections for excitation, which were measured by means of the electron energy-loss spectroscopy. Electron impact energies were 100, 300, and $500\mathrm{eV}$, and the scattering angles were from 0.8° to 10°. In the case where the first Born approximation does not hold, the apparent GOS as a function of $K^2$ (the apparent GOS function) shows characteristic dependence on the electron collision energy according to the character of the transition. In the present observation, for the $n{p}^6$ ${}^{1}S_0\to n{p}^5(n+1)p^{\prime }{[1∕2]}_0$ transitions, the specific behavior has been observed in the apparent GOS functions characteristic of that for the ${}^{1}S_0\to {}^{1}S_0$ type transition, in which the term symbols of the initial and the final states do not change. For the $n{p}^6$ ${}^{1}S_0\to n{p}^5(n+1)p{[5∕2]}_{2,3}$; ${[3∕2]}_{1,2}$ transitions, a certain new type of deviations from the first Born approximation, which is interpreted to be characteristic of the ${}^{1}S_0\to {}^{1}D_2$ type transition, have been observed in the apparent GOS functions with some modifications depending on respective atomic species. For the $5p^6$ ${}^{1}S_0\to 5p^{5}5d$ ${[7∕2]}_3$; ${[5∕2]}_3$ transitions in Xe, it is observed that the apparent GOS curves have no impact energy dependence for impact energies from $100\mathrm{eV}\text{to}500\mathrm{eV}$, which suggests that the first Born approximation is valid for such low impact energies and the curves agree with the Bethe-GOS. It is found that the GOS’s varies in proportional to $K^4$ at small $K^2$ region $(⩽0.1\mathrm{a.u.})$, which suggests that the octupole moment is dominant in these transitions. We interpret that these features of the behavior are specific for the ${}^{1}S_0\to {}^{1}F_3$ type transition. On the whole, it is found that the behavior of the apparent GOS function can be well understood by considering the main contribution from a certain $LS$-coupling state when the excited state is represented by the intermediate coupling scheme which is expressed as a linear combination of the $LS$-coupled states.

We present experimental studies of high-order harmonic generation in silver plasma ablation performed with the Ti:sapphire laser beams of the Advanced Laser Light Source ($800\mathrm{nm}$ wavelength, $360\mathrm{mJ}$ total energy). We have independently varied the intensity of the prepulse (which creates the plasma ablation) and the intensity of the main pulse (which generates the harmonics), and studied their influence on the harmonic spectrum. We show here that the presence of doubly ionized atoms in the ablation, created either by a strong prepulse intensity or with the irradiation of the main pulse, is ineffective for the generation of harmonics.

We use the x-ray extended-range technique (XERT) [C. T. Chantler et al., Phys. Rev. A 64, 062506 (2001)] to measure the mass attenuation coefficients of tin in the x-ray energy range of $29–60\mathrm{keV}$ to $0.04–3\%$ accuracy, and typically in the range $0.1–0.2\%$. Measurements made over an extended range of the measurement parameter space are critically examined to identify, quantify, and correct a number of potential experimental systematic errors. These results represent the most extensive experimental data set for tin and include absolute mass attenuation coefficients in the regions of x-ray absorption fine structure, extended x-ray absorption fine structure, and x-ray absorption near-edge structure. The imaginary component of the atomic form factor $f_2$ is derived from the photoelectric absorption after subtracting calculated Rayleigh and Compton scattering cross sections from the total attenuation. Comparison of the result with tabulations of calculated photoelectric absorption coefficients indicates that differences of $1–2\%$ persist between calculated and observed values.

We present ab initio calculations of cross sections for vibrational excitation and electron capture in collisions of ${\mathrm{H}}^{+}$ with ${\mathrm{H}}_2$ and its isotopical variants at impact energies between $10\mathrm{eV}$ and $10\mathrm{keV}$. Calculations have been carried out by means of a vibronic close-coupling expansion in both quantal and semiclassical treatments to evaluate vibrationally resolved total cross sections. We also report total cross sections and spectra for dissociative capture and ${\mathrm{H}}_2$ dissociation.