In this paper we present an accurate analysis of the shape of an isolated rovibrational ammonia line from the strong ${\nu }_2$ band around 10 μm, recorded by laser absorption spectroscopy. Experimental spectra obtained under controlled temperature and pressure are confronted to various models that take into account Dicke narrowing or speed-dependent effects. Our results show a deviation from the Voigt profile probably dominated by speed-dependent broadening and shifting. Accurate line-shape parameters of the ${\nu }_2$ saQ(6,3) line are obtained. Our current project aiming at measuring the Boltzmann constant, $k_B$, by laser spectroscopy will immediately benefit from such knowledge. We anticipate that an optical determination of $k_B$ with a competitive uncertainty of a few parts per million is now reachable.

The recent adiabatic saddle-point method of Shearer et al. [Phys. Rev. A 84, 033409 (2011)] is applied to study strong-field photodetachment of H${}^{-}$ by few-cycle linearly polarized laser pulses of frequencies near the two-photon detachment threshold. The behavior of the saddle points in the complex-time plane for a range of laser parameters is explored. A detailed analysis of the influence of laser intensities [($2\times {10}^{11}$)–(6.5 $\times$ 10${}^{11}$) W/cm${}^2$], midinfrared laser wavelengths (1800–2700 nm), and various values of the carrier envelope phase (CEP) on (i) three-dimensional probability detachment distributions, (ii) photoangular distributions (PADs), (iii) energy spectra, and (iv) momentum distributions are presented. Examination of the probability distributions and PADs reveal main lobes and jetlike structures. Bifurcation phenomena in the probability distributions and PADs are also observed as the wavelength and intensity increase. Our simulations show that the (i) probability distributions, (ii) PADs, and (iii) energy spectra are extremely sensitive to the CEP and thus measuring such distributions provides a useful tool for determining this phase. The symmetrical properties of the electron momentum distributions are also found to be strongly correlated with the CEP and this provides an additional robust method for measuring the CEP of a laser pulse. Our calculations further show that for a three-cycle pulse inclusion of all eight saddle points is required in the evaluation of the transition amplitude to yield an accurate description of the photodetachment process. This is in contrast to recent results for a five-cycle pulse.

The feasibility of generating correlated photon pairs at variable frequencies is investigated. For this purpose we consider the interaction of an off-resonant laser field with a two-level system possessing broken inversion symmetry. We show that the system generates nonclassical photon pairs exhibiting strong intensity-intensity correlations. The intensity of the applied laser tunes the degree of correlation while the detuning controls the frequency of one of the photons, which can be in the terahertz domain. Furthermore, we observe the violation of a Cauchy-Schwarz inequality characterizing these photons.

We estimate optimal thresholds for surface codes in the presence of loss via an analytical method developed in statistical physics. The optimal threshold for the surface code is closely related to a special critical point in a finite-dimensional spin glass, which is disordered magnetic material. We compare our estimations to the heuristic numerical results reported in earlier studies. Further application of our method to the depolarizing channel, a natural generalization of the noise model, unveils its wider robustness even with loss of qubits.

We study the weak-value amplification (WVA) in a phase measurement with an optical interferometer in which shot noise limits the sensitivity. We compute the signal and the shot noise including the full-order interaction terms of the WVA, and show that the shot-noise contribution to a phase shift in a pointer variable is always larger than the final variance of the pointer variable. This yields the difference in estimating the noise level up to a factor of 1.5. To clarify an advantage for practical uses of the WVA, we discuss the signal-to-noise ratio and its optimization in the presence of the shot noise.

We consider two classes of non-Gaussian entangled states generated from the product of number states by the action of the beam splitter or the two-mode squeezer. It is shown that, for many of these states, the covariance matrix is compatible with the covariance matrix of separable Gaussian states and their separability cannot be verified by the measurements of the first and second moments of canonical variables. We identify a couple of continuous-variable entanglement criteria with higher-order moments to verify these non-Gaussian entanglements.

We present the design and implementation of a trapped ion-cavity QED system. A single ytterbium ion is confined by a micron-scale ion trap inside a 2-mm optical cavity. The ion is coherently pumped by near-resonant laser light while the cavity output is monitored as a function of pump intensity and cavity detuning. We observe a Purcell enhancement of scattered light into the solid angle subtended by the optical cavity, as well as a three-peak structure arising from strongly driving the atom. This system can be integrated into existing atom-photon quantum network protocols and is a pathway toward an efficient atom-photon quantum interface.

We derive the coupling-constant expression for the kinetic energy (KE) functional of the pair-density-functional theory. Such the expression can be effectively used in developing the approximate KE functional. Indeed, on the basis of this expression, we present an approximate form of the KE functional by means of the perturbation theory. Furthermore, as the first step of an alternative way of obtaining the approximate KE functional, we derive several relations and bounds for the KE functional by investigating how the KE functional changes with the nonuniform scaling, rotation, and translation of electron coordinates. Such relations and bounds can be utilized as restrictive conditions on the approximate form. Thus, the coupling-constant expression, relations and bounds obtained here provide useful tools for devising various types of the approximate KE functional that is indispensable for advancing the pair-density-functional theory.

Each Kohn-Sham eigenenergy can be mapped onto a difference between the interacting ground-state energies of successive integer-electron systems via a coupling-constant integration. Occupied Kohn-Sham energies can be mapped onto vertical ionization energies and virtual Kohn-Sham energies can be mapped onto vertical electron affinities. This mapping is unique for nondegenerate Kohn-Sham energies, but degenerate Kohn-Sham levels can be mapped onto multiple ionization energies or electron affinities. Exact expressions for the first ionization and electron affinity energies lead to exact formal expressions for the difference between the Kohn-Sham gap and the fundamental gap.

The effects of ultrahigh-intensity laser radiation on dynamical processes such as electron scattering, bremsstrahlung emission, and pair production, have received growing theoretical interest as laser intensities in the laboratory continue to increase. Recently, for example, a calculation was published that predicted resonant increases of more than four orders of magnitude in bremsstrahlung emission in the presence of intense optical laser radiation [A. A. Lebed and S. P. Roshchupkin, Phys. Rev. A 81, 033413 (2010)]. The analysis in that paper was limited to laser intensities of $\le$${10}^{17}$ W/cm${}^2$, and it was applied only to bremsstrahlung emissions at the laser frequency. In the present paper, we extend this Lebed and Roshchupkin analysis in order to assess the possibility of achieving some enhancement in bremsstrahlung emissions at significantly higher harmonics of the optical laser photon energies ($\sim$6 keV) and thereby to appraise whether or not enhanced bremsstrahlung emissions may have played a hidden role in producing the population inversions and kilovolt x-ray amplifications that have been seen experimentally [A. B. Borisov et al., J. Phys. B 40, F307 (2007)]. In those experiments, light from a KrF laser was focused onto a gas of xenon clusters to intensities $\gtrsim$${10}^{19}$ W/cm${}^2$. A model of the expansion and ionization dynamics of a xenon cluster when heated by such laser intensities has been constructed [Tz. B. Petrova et al., High Energy Density Phys. 8, 209 (2012)]. It is capable of replicating the x-ray gains seen experimentally, but only under the assumption that sufficiently high inner-shell photoionization rates are generated in the experiments. We apply this model to show that such photoionization rates are achievable, but only if there are enhancements of the Bethe-Heitler bremsstrahlung emission rate of three to four orders of magnitude. Our extended analysis of the Lebed and Roshchupkin work shows that, for there to be emissions (whether or not enhanced by four orders of magnitude) at high-order KrF-laser harmonic energies, laser intensities $\gtrsim$${10}^{19}$ W/cm${}^2$ must be reached. Thus, further extensions of these calculations (or experimental measusurements) are needed to determine whether the enhancement factors that are predicted for small laser harmonics at laser intensities $\lesssim$${10}^{17}$ W/cm${}^2$ can be extrapolated to large laser harmonics at laser intensities $\gtrsim$${10}^{19}$, which are shown in our work to be needed in order to produce high laser harmonic kilovolt emissions.

We study properties of isolated integrable quantum systems after a sudden quench starting from thermal states. We show that, even if the system is initially in thermal equilibrium at finite temperature, the diagonal entropy after a quench remains a fraction of the entropy in the generalized ensembles introduced to describe integrable systems after relaxation. The latter is also, in general, different from the entropy in thermal equilibrium. Furthermore, we examine the difference in the distributions of conserved quantities in the thermal versus the generalized ensembles after a quench and show that they are also, in general, different from each other. This explains why these systems fail to thermalize. A finite-size scaling analysis is presented for each quantity, which allows us to make predictions for thermodynamically large lattice sizes.

This paper develops, in precise quantum electrodynamic terms, photonic attributes of the “optical chirality density,” one of several measures long known to be conserved quantities for a vacuum electromagnetic field. The analysis lends insights into some recent interpretations of chiroptical experiments, in which this measure, and an associated chirality flux, have been treated as representing physically distinctive “superchiral” phenomena. In the fully quantized formalism the chirality density is promoted to operator status, whose exploration with reference to an arbitrary polarization basis reveals relationships to optical angular momentum and helicity operators. Analyzing multimode beams with complex wave-front structures, notably Laguerre-Gaussian modes, affords a deeper understanding of the interplay between optical chirality and optical angular momentum. By developing theory with due cognizance of the photonic character of light, it emerges that only the spin-angular momentum of light is engaged in such observations. Furthermore, it is shown that these prominent measures of the helicity of chiral electromagnetic radiation have a common basis in differences between the populations of optical modes associated with angular momenta of opposite sign. Using a calculation of the rate of circular dichroism as an example, with coherent states to model the electromagnetic field, it is discovered that two terms contribute to the differential effect. The primary contribution relates to the difference in left- and right-handed photon populations; the only other contribution, which displays a sinusoidal distance dependence corresponding to the claim of nodal enhancements, is connected with the quantum photon number-phase uncertainty relation. From the full analysis, it appears that the term “superchiral” can be considered redundant.

In parametric systems, squeezed states of radiation can be generated via extra work done by external sources. This eventually increases the entropy of the system despite the fact that squeezing is reversible. We investigate the entropy increase due to squeezing and show that it is quadratic in the squeezing rate and may become important in the repeated operation of tunable oscillators (quantum buses) used to connect qubits in various proposed schemes for quantum computing.

We demonstrate the existence and stability properties of fundamental and ring-profile vortex solitons in a defocusing Kerr medium with an imprinted radially symmetric lattice with a lower-index core covering several lattice rings. The decrease of energy flow with the growth of topological charge is explained using the law of conservation of energy. In contrast to the vortices in bulk media with competing nonlinearities, vortex solitons in radial lattices with defects are stable at lower- or moderate-energy flow. In particular, we reveal that vortex solitons with different charges share a substantially collective stability area. Higher-charged vortices at higher-energy flow suffer a weak oscillatory instability, which allows them to survive very long propagation distances without visible distortions.

The pushing of the Fabry-Perot cavity modes by an intracavity medium of Rb vapor may be tuned with optical pumping. A second laser, propagating through the Rb vapor orthogonal to the intracavity beam, is used to modify the optical transmission of the Rb vapor. We demonstrate that the cavity scale factor may be switched from $S>1$, on one side of its pole along the gain axis, to the other side of the pole, where mode splitting occurs, simply by changing the pumping transition. Continuous tuning of the cavity scale factor and sensitivity may be realized by varying the intensity and/or frequency of the pump beam.

In this Brief Report, we observed the dynamic guiding of 10-keV O${}^{-}$ ions (50 pA/mm${}^2$) transmitting through Al${}_2$O${}_3$ nanocapillaries. The transmitted O${}^{-}$ ions shift gradually to the direction of the capillary axis, and the angular FWHM also increases significantly. However, the transmission rate of O${}^{-}$ ions decreases. The data imply that the charges deposited in capillaries provide a sufficient guiding force to the incident O${}^{-}$ ions.

A simple relationship between recently proposed measures of non-Markovianity and the Loschmidt echo is established, holding for a two-level system (qubit) undergoing pure dephasing due to a coupling with a many-body environment. We show that the Loschmidt echo is intimately related to the information flowing out from and occasionally back into the system. This, in turn, determines the non-Markovianity of the reduced dynamics. In particular, we consider a central qubit coupled to a quantum Ising ring in the transverse field. In this context, the information flux between system and environment is strongly affected by the environmental criticality; the qubit dynamics is shown to be Markovian exactly and only at the critical point. Therefore, non-Markovianity is an indicator of criticality in the model considered here.

The advantages of weak measurements, and especially measurements of imaginary weak values, for precision enhancement, are discussed. A situation is considered in which the initial state of the measurement device varies randomly on each run, and is shown to be in fact beneficial when imaginary weak values are used. The result is supported by numerical calculation and also provides an explanation for the reduction of technical noise in some recent experimental results. A connection to quantum metrology formalism is made.

The decoherence of a qubit is studied for a general class of spectral densities with an arbitrary band gap. The exact dynamics reveals the existence of critical values of the qubit transition frequency, such that relaxations of coherence show discontinuities, jumping from decays with powers arbitrarily larger than unity to powers arbitrarily larger than 0. The critical frequency is the sum of the band-gap frequency and the first negative moment of the spectral density. In the superohmic case, coherence is not entirely lost and the ratio of the trapped population approaches unity as the second negative moment of the spectral density becomes negligible with respect to the scale frequency. Arbitrarily slow decoherence processes are obtained in the critical configuration by approaching the boundary between subohmic and superohmic regimes. The setup of critical configurations constitutes a different way to control qubit decoherence.

Quantum-state transfer in a triangular domain of a two-dimensional, equally spaced, spin lattice with nonhomogeneous nearest-neighbor couplings is analyzed. An exact solution of the one-excitation dynamics is provided in terms of two-variable Krawtchouk orthogonal polynomials that have been recently defined. The probability amplitude for an excitation to transit from one site to another is given. For some values of the parameters, perfect transfer is shown to take place from the apex of the lattice to the boundary hypotenuse.