By solving the quantum field theoretical version of the Klein-Gordon equation numerically, we study the creation process for charged boson-antiboson pairs in static electric and magnetic fields. The fields are perpendicular to each other and spatially localized along the same direction, which permits us to study the crucial impact of the magnetic field's spatial extension on dynamics. If its width is comparable to that of the electric field, we find a magnetically induced Lorentz suppression of the pair-creation process. When the width is increased such that the created bosons can revisit the interaction region, we find a region of exponential self-amplification that can be attributed to a spontaneous emissionlike enhancement. If the width is increased further, this trend is reversed and the magnetic field can even shut off the particle production completely.

We examine the dynamics of a dissipative photonic Josephson junction formed by the weak coupling of an optical soliton in a nonlinear dielectric waveguide and a co-propagating surface plasmon along a parallel metal surface with a linear dielectric spacer. We employ a heuristic model with a coupling function that depends on the soliton amplitude and consider two phenomenological dissipation mechanisms separately: angular-velocity dissipation and population imbalance dissipation. In the former dissipation mechanism, the system exhibits a phase-slip phenomenon where the odd-$\pi$ phase modes decay into even-$\pi$ phase modes. The latter damping mechanism sculptures the phase space significantly by introducing complex features, among which, Hopf-type bifurcations are notable. We show that some of the bifurcation points expand to stable limit cycles for certain regimes of the model parameters.

This paper investigates the confinement and quality ($Q)$ factors of the dielectric mode, exterior surface plasmonic mode, and interior surface plasmonic mode in a metal-nanolayer-coated silica microtoroid. It is found that the confinement of these modes relies mainly on the real part of the coating-metal permittivity, and a smaller negative real part produces a lower effective potential barrier which plays distinct roles in these three types of modes. For the exterior plasmonic mode, a lower potential leads to a better confinement and thus lower radiation losses, while for the interior plasmonic mode and the dielectric mode, a higher potential can play a very positive role in suppressing the radiation losses. The metal absorption loss, as expected, is directly related to the imaginary part of the metal permittivity and the energy fraction of the mode in the metal layer, and the latter depends on the confinement or, namely, the effective potential barrier induced by the metal nanolayer. We also compare the plasmonic modes in the coated microcavity and the dielectric modes in the uncoated microcavity at different cavity sizes. An interesting finding is that when the cavity is small enough, which is highly desirable for compact photonic devices, the $Q$ factors of the plasmonic modes even exceed significantly that of the uncoated dielectric modes.

We analyze a nonconventional interferometer that is formed with a parametric amplifier and a beam splitter for beam splitting and recombination. Because the outputs from a parametric amplifier are entangled and their quantum noise is correlated, the employment of the beam splitter will superimpose the two quantum fields and the destructive interference will lead to the subtraction of the quantum noise and to noise reduction in the output of the interferometer and hence an improvement of the signal-to-noise ratio (SNR) beyond the standard quantum limit or the shot noise limit. Furthermore, the injection of a squeezed state into the idler port of the parametric amplifier will lead to further improvement of the SNR. We will discuss the possibility of reaching the Heisenberg limit in such an interferometer. We find that the injection of a coherent state will degrade the performance in reaching the Heisenberg limit, whereas a squeezed state injection can improve it by a factor of 2 at best.

We present a detailed theoretical study of a recent experiment [A. J. Traverso et al., Proc. Natl. Acad. Sci. USA 109, 15185 (2012)] in which a laserlike source is created in air by pumping with a nanosecond pulse. The source generates radiation in the forward and backward directions. The temporal behavior of the emitted pulses is investigated for different pump shapes and durations. Our analysis indicates that the spiky emission is due to quantum coherence via cooperation between atoms of an ensemble, which leads to strong-oscillatory superfluorescence. We show that these cooperative nonadiabatic coherence effects cannot be described by rate equations and instead a full set of the Maxwell-Bloch equations must be used. We consider a range of parameters and study transitions between various regimes. Understanding these coherence-brightened processes in air should lead to improvements in environmental, atmospheric remote sensing and other applications.

We study the transmission properties of a probe field passing through a sample of $N$-type cold atoms possessing a high Rydberg state. Due to the conditioned dipole blockade of Rydberg excitation, this atomic sample might exhibit a nonlinear optical response in terms of electromagnetically induced transparency (EIT) as manifested by the sensitivity of the transmitted probe intensity and correlation to input probe intensity and correlation. It is of particular interest that the nonlinear EIT response can be opposite (either normal or abnormal) in two different regions of probe detuning, which is a striking feature of the four-level N system of interacting atoms. The realization of both normal and abnormal nonlinear EIT effects in one atomic sample is expected to have wider applications in manipulation of photonic statistics of weak light signals, e.g., to attain prominent photonic bunching and/or antibunching in a dynamic quantum delay or memory process.

Many situations in quantum theory and other areas of physics lead to quasiprobabilities which seem to be physically useful but can be negative. The interpretation of such objects is not at all clear. In this paper, we show that quasiprobabilities naturally fall into two qualitatively different types, according to whether their non-negative marginals can or cannot be matched to a non-negative probability. The former type of quasiprobabilities, which we call viable, are qualitatively similar to true probabilities, but the latter type, which we call nonviable, may not have a sensible interpretation. Determining the existence of a probability matching given marginals is a nontrivial question in general. In simple examples, Fine's theorem indicates that inequalities of the Bell and Clauser-Horne-Shimony-Holt (CHSH) types provide criteria for its existence, and these examples are considered in detail. Our results have consequences for the linear positivity condition of Goldstein and Page in the context of the histories approach to quantum theory. Although it is a very weak condition for the assignment of probabilities, it fails in some important cases where our results indicate that probabilities clearly exist. We speculate that our method, of matching probabilities to a given set of marginals, provides a general method of assigning probabilities to histories and we show that it passes the Diósi test for the statistical independence of subsystems.

Decoherence of qubits under the influence of random phase fluctuations is studied by means of the stochastic Liouville equation, where the phase fluctuations are characterized by means of the Gauss-Markov process and the two-state-jump Markov process. We investigate how an observation of a stochastic variable during the time evolution influences the decoherence of the qubit system. We examine the decays of distinguishability and entanglement of qubit states. It is shown that the decoherence of qubits can be suppressed by the observation of the stochastic variable. It is also found that the suppression of the decoherence does not depend on the observed value.

Deterministic quantum computing with one qubit (DQC1) is a mixed-state quantum computation algorithm that evaluates the normalized trace of a unitary matrix and is more powerful than the classical counterpart. We find that the normalized trace of the unitary matrix can be directly described by the entangling power of the quantum circuit of DQC1, so the nontrivial DQC1 is always accompanied by nonvanishing entangling power. In addition, it is shown that the entangling power also determines the intrinsic complexity of this quantum computation algorithm, i.e., larger entangling power corresponds to higher complexity. In addition, it is also shown that nonvanishing entangling power always exists in other similar tasks of DQC1.

We construct a minimal robust controlled-not gate with an Ising-type interaction by which elementary two-qubit gates are implemented. It is robust against inaccuracy of the coupling strength and the obtained quantum circuits are constructed with the minimal number ($N=3$) of elementary two-qubit gates and several one-qubit gates. It is noteworthy that all the robust circuits can be mapped to one-qubit circuits robust against a pulse length error. We also prove that a minimal robust swap gate cannot be constructed with $N=3$ but requires $N=6$ elementary two-qubit gates.

In quantum information processing, knowledge of the noise in the system is crucial for high-precision manipulation and tomography of coherent quantum operations. Existing strategies for identifying this noise require the use of additional quantum devices or control pulses. We present a noise-identification method directly based on the system's non-Markovian response of an ensemble measurement to the noise. The noise spectrum is identified by reversing the response relationship in the frequency domain. For illustration, the method is applied to superconducting charge qubits, but it is equally applicable to any type of qubits. We find that the identification strategy recovers the well-known Fermi's golden rule under the lowest-order perturbation approximation, which corresponds to the Markovian limit when the measurement time is much longer than the noise correlation time. Beyond such approximation, it is possible to further improve the precision at the so-called optimal point by incorporating the transient response data in the non-Markovian regime. This method is verified with experimental data from coherent oscillations in a superconducting charge qubit.

The operator solution of a completely quantum mechanical Hamiltonian of the Raman processes is used here to investigate the possibility of obtaining intermodal entanglement between different modes involved in the Raman processes [e.g., pump mode, Stokes mode, vibration (phonon) mode and anti-Stokes mode]. Intermodal entanglement is reported between (a) pump mode and anti-Stokes mode, (b) pump mode and vibration (phonon) mode, (c) Stokes mode and vibration phonon mode, and (d) Stokes mode and anti-Stokes mode in the stimulated Raman processes for variation of the phase angle of complex eigenvalue ${\alpha }_1$ of pump mode $a$. Some incidents of intermodal entanglement in the spontaneous and the partially spontaneous Raman processes are also reported. Further, it is shown that the specific choice of coupling constants may produce genuine entanglement among Stokes mode, anti-Stokes mode, and vibration-phonon mode. It is also shown that the two-mode entanglement not identified by Duan's criterion may be identified by Hillery-Zubairy criteria. It is further shown that intermodal entanglement, intermodal antibunching, and intermodal squeezing are independent phenomena.

We propose a scheme to realize deterministic quantum teleportation using linear optics and hybrid qubits. It enables one to efficiently perform teleportation and universal linear-optical gate operations in a simple and near-deterministic manner using all-optical hybrid entanglement as off-line resources. Our analysis shows that our approach outperforms previous ones when considering both the resource requirements and fault-tolerance limits.

We discuss particle entanglement in systems of indistinguishable bosons and fermions, in finite Hilbert spaces, with focus on operational measures of the entanglement of particles. We show how to use von Neumann entropy, negativity, and entanglement witnesses in these cases, proving interesting relations. We obtain analytic expressions to quantify the entanglement of particles in homogeneous $D$-dimensional Hamiltonians with certain symmetries.

We study quantum reflection of antihydrogen atoms from nanoporous media due to the Casimir-Polder potential. Using a simple effective medium model, we show a dramatic increase of the probability of quantum reflection of antihydrogen atoms if the porosity of the medium increases. We discuss the limiting case of reflections at small energies, which have interesting applications for trapping and guiding antihydrogen using material walls.

Direct frequency-comb spectroscopy of a single-photon transition ($5p^6-5p^{5}6s$) in ${}^{132}\mathrm{Xe}$ is performed with a vacuum ultraviolet (vuv) frequency comb at 147 nm. The vuv comb is generated from an Yb-doped fiber laser system at a repetition rate of $\sim$79 MHz via high harmonic generation in a Kr gas jet placed at the focus point of an external enhancement cavity. For Doppler-reduced spectroscopy, a collimated atomic beam of isotope-selected ${}^{132}\mathrm{Xe}$ is used as a target. Repeated measurements with different repetition frequencies and highly sensitive time-resolved detection allow us to determine the absolute transition frequency with an uncertainty of 140 MHz in the vuv region.

We have studied the intrinsic electric dipole moment (EDM) of atomic ${\text{Xe}}^{129}$, ${\text{Yb}}^{171}$, and ${\text{Ra}}^{225}$ arising from the $P$ and $T$ violating tensor-pseudotensor electron-nuclear interactions. The calculations were performed in the framework of the coupled-perturbed Hartree-Fock theory, which may serve as a reference for more accurate methods such as coupled-cluster theory. We present the individual dominant contributions to the final magnitude of the properties of interest. Our results reflect the interplay between the $CP$-conserving Coulomb interaction and a $CP$-violating EDM interaction, which is a nuclear-density-dependent interaction on one hand and on the other hand, its interplay with the simple induced dipole perturbation that varies proportionally with the distance from the nucleus. We also demonstrate the effect of Coulomb perturbation by systematic inclusion of virtuals from different symmetries. The results for these atoms show a very interesting trend and also highlight the contributions from individual orbitals. We have also studied the hyperpolarizability where the Dirac-Fock and the all-order results for the three atomic systems, for systematic inclusion of virtuals, shows an interesting trend. A very good agreement with earlier calculations is achieved for EDM and polarizability.

A method based on the $B$-spline basis set is developed that significantly simplifies the Bethe-logarithm calculations for the atomic hydrogen. Without any auxiliary optimization and extrapolation, this method not only can calculate the Bethe logarithms of low-lying states to high precision using relatively small basis sets, but can also calculate high-lying Rydberg states efficiently. For the ground state, the Bethe logarithm is calculated to 24 significant figures using 450 $B$-spline functions. For high-lying Rydberg states, we can reproduce all the results of Jentschura and Mohr [Phys. Rev. A 72, 012110 (2005)].

A systematic study of the leading isotropic van der Waals coefficients for the alkali-metal atom+molecule and molecule+molecule systems is presented. Dipole moments and static and dynamic dipole polarizabilities are calculated employing high-level quantum chemistry calculations. The dispersion, induction, and rotational parts of the isotropic van der Waals coefficient are evaluated. The known van der Waals coefficients are then used to derive characteristics essential for simple models of the collisions involving the corresponding ultracold polar molecules.

Linear polarization of hard x rays emitted in the process of atomic-field electron bremsstrahlung has been measured with a polarized electron beam. The correlation between the initial orientation of the electron spin and the angle of photon polarization has been systematically studied by means of Compton and Rayleigh polarimetry techniques applied to a segmented germanium detector. The results are in good agreement with those of fully relativistic calculations. The observed correlations are also explained classically and in a unique way manifest that due to the spin-orbit interaction the electron scattering trajectory is not confined to a single scattering plane. The developed photon polarimetry technique with a passive scatterer is very efficient and accurate and thus allows for additional applications. Bremsstrahlung polarization correlations lead to an alternative method of polarimetry of electron beams. Such a method is sensitive to all three components of the electron spin. It can be applied in a broad range of the electron beam energies from $\approx$100 keV up to a few tens of MeV. The results of a measurement at 100 keV are shown. The optimum scheme for electron polarimetry is analyzed and the relevant theoretical predictions are presented.