We present a theoretical study of the delay-dependent Autler-Townes (AT) splitting in transient absorption spectroscopy of an isolated attosecond pulse in helium atoms subject to a delayed infrared (IR) pulse. We concentrate on cases in which the IR pulse is resonant with the helium $1s2p\text{−}1s2s$ transition and provide a time-domain perspective of the dynamics in the delay-dependent pump-probe system. We identify several interesting delay-dependent features in the transient absorption spectrum such as AT splitting, oscillation between absorption and emission at the resonant absorption frequency, and sub-IR-cycles oscillations. We then explain the origins of these features in the time domain in terms of a strongly driven two-level system, in the language of population transfer and coherent control.

Because the commonly adopted viewpoint that the Keldysh parameter $\gamma$ determines the dynamical regime in strong field physics has long been demonstrated to be misleading, one can ask what happens as relevant physical parameters, such as laser intensity and frequency, are varied while $\gamma$ is kept fixed. We present results from our one- and fully three-dimensional quantum simulations of high-order-harmonic generation (HHG) from various bound states of hydrogen with $n$ up to 40, where the laser intensities and the frequencies are scaled from those for $n=1$ in order to maintain a fixed Keldysh parameter $\gamma <1$ for all $n$. We find that as we increase $n$ while keeping $\gamma$ fixed, the position of the cutoff scales in a well-defined manner. Moreover, a secondary plateau forms with a new cutoff, splitting the HHG plateau into two regions. The first of these subplateaus is composed of lower harmonics, and has a higher yield than the second one. The latter extends up to the semiclassical $I_p+3.17U_p$ cutoff. We find that this structure is universal, and the HHG spectra look the same for all $n\gtrsim 10$ when plotted as a function of the scaled harmonic order. We investigate the $n$, $l$, and momentum distributions to elucidate the physical mechanism leading to this universal structure.

We present an ab-initio derivation of a hierarchy of efficient nonclassicality criteria for sources of photons. The operational criteria are explicitly proposed for the linear optical multiport and feasible multiplexed single-photon detectors. The lowest criterion is equivalent to the frequently used $g^{\left(2\right)}\left(0\right)$ autocorrelation measurement testing the anticorrelation effect of photons. We also derive a hierarchy of criteria for the detectors capable to at least partially estimate photon-number distribution. We prove the usefulness of both hierarchies to detect the nonclassical states from the noisy multimode single-photon sources and the nonclassical states from the multiphoton sources.

A promising platform for the emerging field of x-ray quantum optics consists of Mössbauer nuclei embedded in thin-film cavities probed by near-resonant x-ray light, as used in a number of recent experiments. Here we develop a quantum optical framework for the description of experimentally relevant settings involving nuclei embedded in x-ray waveguides. We apply our formalism to two settings of current experimental interest based on the archetype Mössbauer isotope ${}^{57}$Fe. For the present experimental conditions, we derive compact analytical expressions and show that the alignment of medium magnetization, as well as incident and detection polarization, enable the engineering of advanced quantum optical level schemes. The model encompasses nonlinear and quantum effects which could become accessible in future experiments.

In a recent paper [Phys. Rev. A 85, 052508 (2012)], Gidopoulos and Lathiotakis discuss difficulties in solving the optimized effective potential (OEP) equation in a finite basis. We show that these difficulties are resolved by choosing basis sets that fulfill a balance condition. In particular, the nonanalyticity of the finite-basis OEP equation disappears in basis-set convergence. In connection with the basis-set balance, we also discuss the role of small eigenvalues. The approximate treatment of them with a common energy denominator suggested by the authors requires care in order to avoid spurious oscillations in the potential. We briefly discuss an alternative scheme that overcomes the aforementioned difficulties and enables OEP calculations with a minimal orbital basis.

The Comment by Friedrich et al. does not dispute the central result of our paper [Phys. Rev. A 85, 052508 (2012)] that nonanalytic behavior is present in long-established mathematical pathologies arising in the solution of finite basis optimized effective potential (OEP) equations. In the Comment, the terms “balancing of basis sets” and “basis-set convergence” imply a particular order towards the limit of a large orbital basis sets where the large-orbital-base limit is always taken first, before the large-auxiliary-base limit, until overall convergence is achieved, at a high computational cost. The authors claim that, on physical grounds, this order of limits is not only sufficient, but also necessary in order to avoid the mathematical pathologies. In response to the Comment, we remark that it is already written in our paper that the nonanalyticity trivially disappears with large orbital basis sets. We point out that the authors of the Comment give an incorrect proof of this statement. We also show that the order of limits towards convergence of the potential is immaterial. A recent paper by the authors of the Comment proposes a partial correction for the incomplete orbital basis error in the full-potential linearized augmented-plane-wave method. Similar to the correction developed in our paper, this correction also benefits from an effectively complete orbital basis, even though only a finite orbital basis is employed in the calculation. This shows that it is unnecessary to take, in practice, the limit of an infinite orbital basis in order to avoid mathematical pathologies in the OEP. Our paper is a significant contribution in that direction with general applicability to any choice of basis sets. Finally, contrary to an allusion in the abstract and assertions in the main text of the Comment that unphysical oscillations of the OEP are supposedly attributed to the common energy denominator approximation, in fact, such anomalies arise with the full treatment of the small eigenvalues of the density response function. This characteristic of the finite basis OEP is well known in the literature but also is clearly demonstrated in our paper.

We propose that a coherently prepared four-level atomic medium can provide a versatile platform for realizing parity-time ($\mathcal{PT}$) symmetric optical potentials. Different types of $\mathcal{PT}$-symmetric potentials are proposed by appropriately tuning the exciting optical fields and the pertinent atomic parameters. Such reconfigurable and controllable systems may open up new avenues in observing $\mathcal{PT}$-related phenomena with appreciable gain-loss contrast in coherent atomic media.

We give a simple intuition for why and when entanglement is needed for quantum-enhanced precision measurements, and which type of entanglement is useful. We give a simple construction that shows how entanglement transforms parallel estimation strategies into sequential ones of the same precision. We employ this argument to provide some useful results and to rederive in a simpler form some known results.

We present a stochastic path integral formalism for continuous quantum measurement that enables the analysis of rare events using action methods. By doubling the quantum state space to a canonical phase space, we can write the joint probability density function of measurement outcomes and quantum state trajectories as a phase space path integral. Extremizing this action produces the most likely paths with boundary conditions defined by preselected and postselected states as solutions to a set of ordinary differential equations. As an application, we analyze continuous qubit measurement in detail and examine the structure of a quantum jump in the Zeno measurement regime.

Employing the stochastic wave function method, we study quantum features of stochastic entropy production in nonequilibrium processes of open systems. It is demonstrated that continuous measurements on the environment introduce an additional, nonthermal contribution to the entropy flux, which is shown to be a direct consequence of quantum fluctuations. These features lead to a quantum definition of single trajectory entropy contributions, which accounts for the difference between classical and quantum trajectories and results in a quantum correction to the standard form of the integral fluctuation theorem.

We extend the formalism of cluster-state quantum secret sharing, as presented by Markham and Sanders [Phys. Rev. A 78, 042309 (2008)] and Keet et al. [Phys. Rev. A 82, 062315 (2010)], to the continuous-variable regime. We show that both classical and quantum information can be shared by distributing continuous-variable cluster states through either public or private channels. We find that the adversary structure is completely denied from the secret if the cluster state is infinitely squeezed, but some secret information would be leaked if a realistic finitely squeezed state is employed. We suggest benchmarks to evaluate the security in the finitely squeezed cases. For the sharing of classical secrets, we borrow techniques from the continuous-variable quantum key distribution to compute the secret-sharing rate. For the sharing of quantum states, we estimate the amount of entanglement distilled for teleportation from each cluster state.

We present a theoretical scheme for the generation of four quantized modes in a three-level double-$\Lambda$ atomic system driven by two counterpropagating far-detuned pump beams and find the entanglement between the four modes. By using the second-order perturbation method and the phase-matching condition for the four-wave mixing processes, the effective Hamiltonian is derived, which clearly illustrates the generation of four light beams and their entanglement. The dependence of the four-mode entanglement on interaction time, pump detuning, strength of interaction force, and the ratio of Rabi frequency of two pump beams is analyzed with inseparability. The result presented here provides a method for the experimental generation of multipartite entanglement in an atomic system.

We find a phase-matching condition for enhancement of sensitivity in a Mach-Zehnder interferometer illuminated by an arbitrary state in one input port and an odd (even) state in the other port. Under this condition, the Fisher information becomes maximal with respect to the relative phase of two modes, and the phase sensitivity is enhanced. For the case with photon losses, we further find that the phase-matching condition remains unchanged with a coherent state and a coherent superposition state as the input states.

We experimentally demonstrated the Hong-Ou-Mandel (HOM) interference between two photons after visible-to-telecommunication wavelength conversion. In the experiment, we prepared a heralded single photon by using spontaneous parametric down conversion (SPDC) and the other photon from a weak laser source at 780 nm. We converted the wavelength of both photons to the telecommunication wavelength of 1522 nm by using difference-frequency generation in a periodically poled lithium niobate, and then observed the HOM interference between the photons. The observed visibility is $0.76\pm 0.12$ which clearly shows the nonclassical interference of the two photons. The high-visibility interference is an important step for fiber-based quantum communications of photons generated from visible-photon emitters.

The shift and broadening of the resonance lines in the spectrum of antiprotonic helium atoms $\overline{p}{\mathrm{He}}^{+}$ located in fluid and superfluid ${}^4$He have been estimated. The contributions to the shift and broadening from collective degrees of freedom in liquid ${}^4$He have been evaluated using the phenomenological response function. The shift due to collisions of $\overline{p}{\mathrm{He}}^{+}$ with ${}^4$He atoms has been calculated in the quasistatic limit using the experimental pair-correlation function. It has been shown that an implanted $\overline{p}{\mathrm{He}}^{+}$ atom establishes a good probe of liquid-helium properties, since this atom practically does not change the target structure.

Transmission spectra of gases confined (but not adsorbed) within the pores of a 1.4-cm-thick silica xerogel sample have been recorded between 2.5 and 5 μm using a high-resolution Fourier transform spectrometer. This was done for pure CO, CO${}_2$, N${}_2$O, H${}_2$O, and CH${}_4$ at room temperature and pressures of a few hectopascals. Least-squares fits of measured absorption lines provide the optical-path lengths within the confined ($L^C$) and free ($L^F$) gas inside the absorption cell and the half width at half maximum ${\Gamma }^C$ of the lines of the confined gases. The values of $L^C$ and $L^F$ retrieved using numerous transitions of all studied species are very consistent. Furthermore, $L^C$ is in satisfactory agreement with values obtained from independent measurements, thus showing that reliable information on the open porosity volume can be retrieved from an optical experiment. The values of ${\Gamma }^C$, here resulting from collisions of the molecules with the inner surfaces of the xerogel pores, are practically independent of the line for each gas and inversely proportional to the square root of the probed-molecule molar mass. This is a strong indication that, for the studied transitions, a single collision of a molecule with a pore surface is sufficient to change its rotational state. A previously proposed simple model, used for the prediction of the line shape, leads to satisfactory agreement with the observations. It also enables a determination of the average pore size, bringing information complementary to that obtained from nitrogen adsorption porosimetry.

The relative dissociation fractions for the production of fragment ions and ion pairs of SF${}_6$ are studied for H${}^{-}$ and C${}^{-}$ impact in the energy range from 15 to 30 keV. Recoil ions (${\mathrm{SF}}_4^{+}$, SF${}_3^{+}$, SF${}_2^{+}$, SF${}^{+}$, S${}^{+}$, F${}^{+}$, SF${}_4^{2+}$, SF${}_2^{2+}$) and ion pairs $({\mathrm{SF}}_3^{+}+{\mathrm{F}}^{+},{\mathrm{SF}}_2^{+}+{\mathrm{F}}^{+},{\mathrm{SF}}^{+}+{\mathrm{F}}^{+},{\mathrm{S}}^{+}+{\mathrm{F}}^{+}$, F${}^{+}+{\mathrm{F}}^{+})$ are detected and identified in coincidence with scattered projectiles in two charge states ($q=0$ and $q=+1$) by using a time-of-flight spectrometer. The relative dissociation fractions are energy dependent for both single-electron-loss (SL) channel and double-electron-loss (DL) channel processes for certain negative ions. It is also found that the relative dissociation fractions for DL are larger than those for SL. In addition, the degree of fragmentation will become greater with a larger mass number of the projectiles at the same impact energy for the same electron-loss channel. A comparison of the time-of-flight spectra is made between that under negative-ion impact and that under electron impact, and it is found that the probability of production of SF${}_n^{+}$ ions with $n$ odd is higher than that of similar ions with $n$ even, and the probability of production of SF${}_n^{2+}$ ions with $n$ even is higher than that of similar ions with$n$ odd under H${}^{-}$, C${}^{-}$, positive-ion, and electron impact. We analyze this interesting phenomenon from the bond-dissociation energies of SF${}_n^{+}$ and SF${}_n^{2+}$. We also analyze the coincident time-of-flight spectra of two fragment ions resulting from double ionization of SF${}_6$ by H${}^{-}$ and C${}^{-}$ impact and describe the major dissociation pathways of SF${}_6^{2+}$ for H${}^{-}$ and C${}^{-}$ impact in the energy range from 15 to 30 keV.

We investigate the photoionization of an atom by a high-frequency strong coherent laser pulse, where the energy of a single photon is sufficient to ionize the system. Ab initio theory exemplified by numerical calculations of the photoionization of the hydrogen atom illustrates that the laser pulse induces a time-dependent ac (or dynamic) Stark shift of the binding energy of the system due to interaction with the electronic continuum. The energy shift follows the pulse envelope. Because of this shift, the energy of the emitted photoelectron also follows the intensity envelope of a short laser pulse. This results in a strong dynamic interference of the photoelectrons of the same kinetic energy emitted at different times, when the pulse arrives and expires, respectively. The dynamic interference leads to interesting modifications of the photoionization process and results in a multiple peak structure in the photoelectron spectrum. The present work provides a detailed theoretical and numerical study of the results reported in Demekhin and Cederbaum [Phys. Rev. Lett. 108, 253001 (2012)]. The working equations are derived explicitly and an extended analysis of the theory is provided. Additional results are presented. In particular, we demonstrate that the dynamic interference can be controlled by the intensity, duration, and shape of the pulse envelope.

We introduce a method using the concept of instantaneous frequency, as determined with the help of a Hilbert transform, to achieve an accurate time-frequency analysis of the dipole issued from the numerical resolution of the time-dependent Schrödinger equation for any model system driven by an intense laser field. It permits the quantitative extraction of the emission times of the harmonics that coincide with the recollision times of the laser-driven electronic wave packet with the ionic core, for the first and the second return.

In the present article we substantially expand on our recent study about the fabrication of mesoscale polymeric templates of disordered photonic network materials [Haberko and Scheffold, Opt. Expr. 21, 1057 (2013)]. We present a detailed analysis and discussion of important technical aspects related to the fabrication and characterization of these fascinating materials. Compared to our initial report we were able to reduce the typical structural length scale of the seed pattern from $a=3.3\mu$m to $a=2\mu$m, bringing it closer to the technologically relevant fiber-optic communications wavelength range around $\lambda \sim 1.5\mu$m. We have employed scanning electron microscopy coupled with focused ion beam cutting to look inside the bulk of the samples of different heights. Moreover, we demonstrate the use of laser scanning confocal microscopy to assess the real space structure of the samples fabricated by direct laser writing. We address in detail questions about scalability, finite size effects, and geometrical distortions. We also study the effect of the lithographic voxel shape, that is, the ellipsoidal shape of the laser pen used in the fabrication process. To this end we employ detailed numerical modeling of the scattering function using a discrete dipole approximation scheme.