We introduce a class of metamaterials with uniformly balanced gain and loss associated with complex permittivity and permeability constants. The refractive index of such a balanced pseudopassive metamaterial is real. An unbounded uniform pseudopassive medium has transport characteristics similar to those of its truly passive and lossless counterpart with the same real refractive index. However, bounded pseudopassive samples show some unexpected scattering features which can be further emphasized by including such elements in a photonic structure.

We theoretically investigate the nonadiabatic spectral redshift of high-order harmonics with the help of a VUV pulse. It is found that the nonadiabatic spectral redshift of high-order harmonics can be observed when a weak VUV pulse is properly added in the falling part of the fundamental laser due to the nonadiabatic response of the dipole to rapid change of laser intensity. Further time-frequency analysis shows that the high-order harmonics are mainly generated in the falling part of the fundamental pulse. This is because the VUV pulse enhances the ionization in the falling part of the fundamental pulse by the 1s-2p transition of $\text{H}{e}^{+}$. In addition, this scheme is also used to observe the nonadiabatic spectral blueshift of high-order harmonics by changing the time delay between the fundamental laser and the VUV pulse.

We show that the resonances of the Stark effect in hydrogen reported by Fernández-Menchero and Summers [Phys. Rev. A 88, 022509 (2013)] are considerably less accurate than the number of digits appear to suggest. In particular, the imaginary part of the lowest resonance is several orders of magnitude greater than it should be. We compare the results of that paper with those provided by the Riccati-Padé method, perturbation theory, and an asymptotic expansion for the resonance width. The inaccuracy of those results can be traced back to the lack of precision in the calculation of the matrix elements of the secular equation. We carry out a more accurate calculation with the same method and show that the agreement with earlier results of other authors is greatly improved.

The numerical values of the resonances shown in L. Fernández-Menchero and H. P. Summers, L. Fernández-Menchero and H. P. Summers, Phys. Rev. A 88, 022509 (2013) are less accurate than all the decimal ciphers that appear. The preceding Comment repeated the calculations and proved a global accuracy of the method in the order of ${10}^{-7}\mathrm{a}.\mathrm{u}.$ for the used basis set. The table of numerical results has been rewritten, respecting the accuracy of the method.

It is shown that negative ions are ejected from gas-phase water molecules when bombarded with positive ions at keV energies typical of solar-wind velocities. This finding is relevant for studies of planetary and cometary atmospheres, as well as for radiolysis and radiobiology. Emission of both ${\mathrm{H}}^{-}$ and heavier (${\mathrm{O}}^{-}$ and $\mathrm{O}{\mathrm{H}}^{-}$) anions, with a larger yield for ${\mathrm{H}}^{-}$, was observed in $6.6\text{−}\mathrm{keV}{}^{16}{\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}$ collisions. The experimental setup allowed separate identification of anions formed in collisions with many-body dynamics from those created in hard, binary collisions. Most of the anions are emitted with low kinetic energy due to many-body processes. Model calculations show that both nucleus-nucleus interactions and electronic excitations contribute to the observed large anion emission yield.

Parametric down-conversion in a nonlinear crystal-generated photon pair in which photons have orthogonal polarization. The pair was emitted in a cone shape for each polarization and collimated by a lens to be doughnut shaped. The doughnutlike photon pair runs collinear to the pump laser. The group delay and walk-off between the orthogonal polarization photons in the pair were compensated passing through a half-wave plate and a nonlinear crystal. Then, the photon doughnut pair was coupled into a $1\times 2$ fiber together with a pump laser, i.e., the intense pump laser was used as a guide for alignment. After the alignment was finished, a cut filter was inserted in the optical path to cut the pump laser transmitting the photon doughnut pair. The coincidence measurement of the output of the $1\times 2$ fiber showed that the photon pairs are entangled in polarization.

We propose a definitive test of whether plates involved in Casimir experiments should be modeled with ballistic or diffusive electrons—a prominent controversy highlighted by a number of conflicting experiments. The unambiguous test we propose is a measurement of the Casimir force between a disordered quasi-two-dimensional metallic plate and a three-dimensional metallic system at low temperatures, in which disorder-induced weak-localization effects modify the well-known Drude result in an experimentally tunable way. We calculate the weak-localization correction to the Casimir force as a function of magnetic field and temperature and demonstrate that the quantum interference suppression of the Casimir force is a strong, observable effect. The coexistence of weak-localization suppression in electronic transport and Casimir pressure would lend credence to the Drude theory of the Casimir effect, while the lack of such correlation would indicate a fundamental problem with the existing theory. We also study mesoscopic disorder fluctuations in the Casimir effect and estimate the width of the distribution of Casmir energies due to disorder fluctuations.

We study the behavior of an unusual doubly quantized vortex in a harmonically trapped Bose gas at nonzero temperatures by using the time-dependent Hartree-Fock-Bogoliubov equations. This structure, which exhibits nontrivial features, generates spontaneously in the anomalous fraction when phases corresponding to the singly charged vortex are imposed in the condensed and the anomalous components of the gas. Our numerical calculations show that at low temperature, condensed atoms tend to fill the core of the anomalous vortex. We demonstrate that the decay of this vortex is attributed to dissipation induced by the anomalous fluctuations. Excitation frequency and the radius core of the anomalous vortex are also investigated.

Motivated by the fundamental question of the fate of interacting bosons in flat bands, we consider a two-dimensional Bose gas at zero temperature with an underlying quartic single-particle dispersion in one spatial direction. This type of band structure can be realized using the NIST scheme of spin-orbit coupling [Y.-J. Lin, K. Jiménez-Garcia, and I. B. Spielman, Nature (London) 471, 83 (2011)], in the regime where the lower-band dispersion has the form ${\varepsilon }_{\mathit{k}}\sim k_x^4/4+k_y^2+...$, or using the shaken lattice scheme of Parker et al. [C. V. Parker, L.-C. Ha, and C. Chin, Nat. Phys. 9, 769 (2013)]. We numerically compare the ground-state energies of the mean-field Bose-Einstein condensate (BEC) and various trial wave functions, where bosons avoid each other at short distances. We discover that, at low densities, several types of strongly correlated states have an energy per particle ($\epsilon$), which scales with density ($n$) as $\epsilon \sim n^{4/3}$, in contrast to $\epsilon \sim n$ for the weakly interacting Bose gas. These competing states include a Wigner crystal, quasicondensates described in terms of properly symmetrized fermionic states, and variational wave functions of Jastrow type. We find that one of the latter has the lowest energy among the states we consider. This Jastrow-type state has a strongly reduced, but finite, condensate fraction, and true off-diagonal long-range order, which suggests that the ground state of interacting bosons with quartic dispersion is a strongly correlated condensate reminiscent of superfluid helium-4. Our results show that even for weakly interacting bosons in higher dimensions, one can explore the crossover from a weakly coupled BEC to a strongly correlated condensate by simply tuning the single-particle dispersion or density.

We study the photon statistics properties of few-photon transport in an optomechanical system where an optomechanical cavity couples to two empty cavities. By analytically deriving the one- and two-photon currents in terms of a zero-time-delayed two-order correlation function, we show that a photon blockade can be achieved in both the single-photon strong-coupling regime and the single-photon weak-coupling regime due to the nonlinear interacting and multipath interference. Furthermore, our systems can be applied as a quantum optical diode, a single-photon source, and a quantum optical capacitor. It is shown that this the photon transport controlling devices based on photon antibunching does not require the stringent single-photon strong-coupling condition. Our results provide a promising platform for the coherent manipulation of optomechanics, which has potential applications for quantum information processing and quantum circuit realization.

We experimentally investigate cold thermal atoms in a single sidedly pumped optical ring resonator for temperatures between 0.4 and $9\mu \mathrm{K}$. The threshold for collective atomic recoil lasing (CARL) is recorded for various pump-cavity detunings. The resulting stability diagram is interpreted by simulating the classical CARL equations. We find that the stability diagram for thermal atoms shows the same asymmetry as observed for Bose-Einstein condensates in previous experiments. Whereas for condensates the asymmetry is well explained by a Dicke-type quantum model we here discuss a simplified classical model. It complements the quantum model and provides an intuitive explanation based on the change in the long-range atomic interaction with pump-cavity detuning.

We examine a lossless Kerr-phase gate in a semiconductor quantum-well system via the tunneling interference effect for weak fields. We show that there exists a magic detuning for the signal field, at which the absorption or amplification for the probe field can be eliminated by increasing the tunneling interference effect. Simultaneously, the probe field will acquire a $-\pi$ phase shift at the exit of the medium. We demonstrate with numerical simulations that a lossless Kerr-phase gate is achieved, which may result in many applications in information science and telecommunication.

We experimentally demonstrate and theoretically explain the onset of optical frequency combs in a simple cavity-enhanced second-harmonic-generation system, exploiting second-order nonlinear interactions. Two combs are simultaneously generated around the fundamental pump frequency, with a spectral bandwidth up to about 10 nm, and its second harmonic. We observe different regimes of generation, depending on the phase-matching condition for second-harmonic generation. Moreover, we develop an elemental model which provides a deep physical insight into the observed dynamics. Despite the different underlying physical mechanism, the proposed model is remarkably similar to the description of third-order effects in microresonators, revealing a potential variety of new effects to be explored and laying the groundwork for a novel class of highly efficient and versatile frequency comb synthesizers based on second-order nonlinear materials.

We study a bosonic matter excitation coupled to a single-mode cavity field via electric dipole. Counter-rotating and $A^2$ terms are included in the interaction model, $\mathbf{A}$ being the vector potential of the cavity field. In the ultrastrong coupling regime the vacuum of the bare modes is no longer the ground state of the Hamiltonian and contains a nonzero population of polaritons, the true normal modes of the system. If the parameters of the model satisfy the Thomas-Reiche-Kuhn sum rule, we find that the two polaritons are always equally populated. We show how this prediction could be tested in a quenching experiment, by rapidly switching on the coupling and analyzing the radiation emitted by the cavity. A refinement of the model based on a microscopic minimal coupling Hamiltonian is also provided, and its consequences on our results are characterized analytically.

We provide a complete and exact quantum description of coherent light scattering in a one-dimensional multimode transmission line coupled to a two-level emitter. Using a recently developed scattering approach, we discuss transmission properties, the power spectrum, the full counting statistics, and the entanglement entropy of transmitted and reflected states of light. Our approach takes into account spatial parameters of an incident coherent pulse as well as waiting and counting times of a detector. We describe the time evolution of the power spectrum and observe deviations from the Poissonian statistics for reflected and transmitted fields. In particular, the statistics of reflected photons can change from sub-Poissonian to super-Poissonian for increasing values of the detuning, while the statistics of transmitted photons is strictly super-Poissonian in all parametric regimes. We study the entanglement entropy of some spatial part of the scattered pulse and observe that it obeys the area laws and that it is bounded by the maximal entropy of the effective four-level system.

We study the closeness of an experimental unitary bosonic network with only partially indistinguishable bosons in an arbitrary mixed input state, in particular an experimental realization of the BosonSampling, to the ideal bosonic network, where the measure of closeness of two networks is the trace distance between the output probability distributions. An upper bound on the trace distance to the ideal bosonic network is proven and also a bound on the difference between probabilities of an output configuration. Moreover, the upper bound on the trace distance is tight, provided that a physically transparent distinguishability conjecture is true. For a small distinguishability error, it is shown that a realistic device with $N$ bosons is at a constant trace distance to ideal BosonSampling under the $O\left(N^{-1}\right)$ scaling of the mismatch of internal states of bosons.

Using a relation between a biorthogonal set of equiseparable bases and the weak values of the density matrix we derive an explicit formula for its tomographic reconstruction completely analogous to the standard mutually unbiased bases expansion.

Intense terahertz-frequency pulses induce coherent rotational dynamics and orientation of polar molecular ensembles. Exact numerical methods for rotational dynamics at room temperature are computationally not feasible for the vast majority of molecular rotors: the asymmetric top molecules at ambient temperatures. We exemplify the use of random phase wave functions (RPWFs) by calculating the terahertz-induced rotational dynamics of sulfur dioxide at ambient temperatures and high-field strengths and show that the RPWF method gains efficiency with the increase in temperature and in the terahertz-field strengths. The present method provides wide-ranging computational access to rotational dynamical responses of molecules at experimental conditions that are far beyond the reach of exact numerical methods.

We use a few-cycle, carrier-envelope-phase (CEP) stabilized laser system to generate high-harmonic emission in argon, neon, and carbon dioxide. The high-harmonic spectra consist of discrete harmonic orders whose positions shift as a function of the CEP. Near the cutoff harmonic, the peaks are separated by two photon orders, and can correspond to either even or odd harmonics of the driving laser frequency, depending on the value of the CEP. In the plateau region, harmonic orders are separated by only one photon order. We develop a simple model which predicts the observed behavior. We use the observed dependence of the harmonic peaks as a function of CEP as a method to measure the statistical CEP fluctuations of the laser system. The measured rms fluctuation of 0.17 radians agrees with optical measurements. The high-harmonic approach to measuring CEP stability has the advantage that it is less sensitive to laser intensity fluctuations than are optical methods.