It is shown that the electron-impact direct double ionization (DDI) process can be presented as a sequence of two- and three-step processes. Contribution from ionization-ionization, ionization-excitation-ionization, and excitation-ionization-ionization processes is taken into account. A good agreement with experimental measurements is obtained for double ionization cross sections of ${\text{O}}^{1+}$, ${\text{O}}^{2+}$, ${\text{O}}^{3+}$, ${\text{C}}^{1+}$, and ${\text{Ar}}^{2+}$ ions. We show that distribution of the energy of scattered and ejected electrons, which participate in the next step of ionization, strongly affects DDI cross sections.

Three-component fermionic optical lattice systems are investigated in dynamical mean-field theory for the Hubbard model. Solving the effective impurity model by means of continuous-time quantum Monte Carlo simulations in the Nambu formalism, we find that the $s$-wave superfluid state proposed recently is indeed stabilized in the repulsively interacting case and appears along a first-order phase boundary between metallic and paired Mott states in the paramagnetic system. The BCS-BEC (Bose-Einstein condensation) crossover in the three-component fermionic system is also addressed.

In this Comment, we argue that the theoretical grounds used by Mansuripur and Zakharian in their paper [Phys. Rev. A 80, 023823 (2009)] about the analysis of the force on the end face of a nanofilament exerted by an outgoing light pulse are erroneous. Alternative grounds for the analysis of the force are presented.

We respond to a Comment on our paper [Phys. Rev. A 80, 023823 (2009)], which appears to have stemmed from a misunderstanding of the various energy-momentum tensors of classical electrodynamics. It is shown that each stress tensor, when used in conjunction with the corresponding force-density and momentum-density expressions, yields results that are consistent with Maxwell's equations and with the conservation laws.

In previous years several theoretical papers discussed if time can be an emergent property deriving from quantum correlations. Here, to provide insight into how this phenomenon can occur, we present an experiment that illustrates Page and Wootters' mechanism of “static” time [D. N. Page and W. K. Wootters, Phys. Rev. D 27, 2885 (1983)], and Gambini et al. for subsequent refinements [R. Gambini et al., Phys. Rev. D 79, 041501(R) (2009)]. A static, entangled state between a clock system and the rest of the universe is perceived as evolving by internal observers that test the correlations between the two subsystems. We implement this mechanism using an entangled state of the polarization of two photons, one of which is used as a clock to gauge the evolution of the second: An “internal” observer that becomes correlated with the clock photon sees the other system evolve, while an “external” observer that only observes global properties of the two photons can prove it is static.

The Compton and Thomson radiation spectra generated in collisions of an electron beam with a powerful laser beam are studied in the framework of quantum and classical electrodynamics, respectively. We show that there are frequency regimes where both radiation spectra are nearly identical, which for Compton scattering relates to the process which preserves the electron spin. Although the radiation spectra are nearly identical, the corresponding probability amplitudes exhibit different global phases. This has pronounced consequences, which we demonstrate by investigating temporal power distributions in both cases. We show that, contrary to Thomson scattering, it is not always possible to synthesize short laser pulses from Compton radiation. This happens when the global phase of the Compton amplitude varies in a nonlinear way with the frequency of emitted photons. We also demonstrate that, while the Compton process driven by a nonchirped laser pulse can generate chirped bursts of radiation, this is not the case for the Thomson process. In principle, both processes can lead to a generation of coherent frequency combs when single or multiple driving laser pulse collide with electrons. Once we synthesize these combs into short bursts of radiation, we can control them, for instance, by changing the time delay between the driving pulses.

Generalized probabilistic theories (GPTs) provide a framework in which one can formulate physical theories that includes classical and quantum theories, but also many other alternative theories. In order to compare different GPTs, we advocate an approach in which one views a state in a GPT as a resource and quantifies the cost of interconverting between different such resources. We illustrate this approach on polygon theories [New J. Phys. 13, 063024 (2011).] that interpolate (as the number $n$ of edges of the polygon increases) between a classical trit (when $n=3$) and a real quantum bit (when $n=\infty$). Our main results are that simulating the transmission of a single $n$-gon state requires more than one qubit, or more than $log(log(n\left)\right)$ bits, and that $n$-gon states with $n$ odd cannot be simulated by $n^{\prime }$-gon states with $n^{\prime }$ even (for all $n,n^{\prime }$). These results are obtained by showing that the classical capacity of a single $n$-gon state with $n$ even is 1 bit, whereas it is larger than 1 bit when $n$ is odd; by showing that transmitting a single $n$-gon state with $n$ even violates information causality; and by studying the communication complexity cost of the nondeterministic not-equal function using $n$-gon states.

Excitation energies of the [Xe]$nd$ ($n=5-9$), [Xe]$ns$ ($n=6-10$), [Xe]$np$ ($n=6-9$), [Xe]$nf$ ($n=4-8$), and [Xe]$ng$ ($n=5-8$) states in La iii, where [Xe] = $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}4d^{10}5s^{2}5p^6$, are evaluated. Electric dipole matrix elements for the allowed transitions between the low-lying [Xe]$nd$, [Xe]$ns$, [Xe]$np$, [Xe]$nf$, and [Xe]$ng$ states in the La iii ion are calculated using the high-precision relativistic all-order method where all single, double, and partial triple excitations of the Dirac-Fock wave functions are included to all orders of perturbation theory. Recommended values are provided for a large number of electric dipole matrix elements, oscillator strengths, transition rates, and lifetimes. Scalar and tensor polarizabilities of the states listed above are evaluated. The uncertainties of the recommended values are estimated. Electric quadrupole and magnetic dipole matrix elements are calculated to determine lifetimes of the $5d_{5/2}$ and $6s$ metastable levels. The ground-state $E1$, $E2$, and $E3$ static polarizabilities are calculated. This work provides recommended values critically evaluated for their accuracy for a number of La iii atomic properties for use in planning and analysis of various experiments as well as theoretical modeling.

Measuring the energy dependence of optical-transition collision shifts has proven extremely difficult, in part because of Doppler broadening and the manifold of overlapping hyperfine components that must be disentangled in the spectra. Here, we demonstrate an approach to these measurements based on spectroscopic isoclinic points. To illustrate the approach's efficacy, we investigated the Kr collision shift of the Rb $D_1$ transition at 795 nm. For the expected Rb-Kr van der Waals interaction, the collision shift should scale like ${(T/T_o)}^{\kappa }$, where $T_{\mathrm{o}}$ is a reference temperature and κ${}_{\mathrm{theo}}$ = 0.31. Exemplifying the difficulty of κ determinations, previous alkali-metal–noble-gas experimental measurements of κ have varied widely, sometimes in striking disagreement with theory (i.e., factor of 2 larger). In the present work, we not only demonstrate a measurement precision better than 10${}^{-10}$/°C, but with our technique we validate the theoretical scaling constant, finding ${\kappa }_{\mathrm{expt}}$ = 0.36 ± 0.06.

We have performed the quantum trajectory analysis for high-order-harmonic generation (HHG) with different driving laser wavelengths. By defining the ratio of HHG yields of the $N$th and first rescattering events ($Y_N/Y_1$), we quantitatively evaluate the HHG contributions from multiple rescatterings. The results show that the HHG yield ratio increases gradually with the increase of the laser wavelength, which demonstrates that high-order rescatterings provide ascendent contributions to HHG at longer wavelength. By calculating the classical electron trajectories, we find significant differences exist in the electron behaviors between the first and high-order rescatterings. Further investigations have demonstrated that the increasing HHG yield ratio is mainly attributed to the relatively smaller contributions from the short path of the first electron rescattering at longer laser wavelength.

Here the problem considered is that of a pair of oscillators coupled to a common heat bath. Many, if not most, discussions of a single operator coupled to a bath have used the independent oscillator model of the bath. However, that model has no notion of separation, so the question of phenomena when the oscillators are near one another compared with when they are widely separated cannot be addressed. Here the Lamb model of an oscillator attached to a stretched string is generalized to illustrate some of these questions. The coupled Langevin equations for a pair of oscillators attached to the string at different points are derived and their limits for large and small separations obtained. Finally, as an illustration of a different phenomenon, the fluctuation force between a pair of masses attached to the string is calculated, with closed-form expressions for the force at small and large separations.

We consider the unitary time evolution of continuous quantum-mechanical systems confined to a cavity in contact with a finite bath of variable size. We define a measure relating to (non-)Markovianity which parallels the standard one for the case of integrable Lindbladian dynamics but has the advantage of being numerically tractable also for large many-particle systems. The relevant time scales are identified which characterize non-Markovian transient behavior, boundary-scattering-induced non-Markovian oscillations at intermediate times, and non-Markovian rephasing events at long time scales. It is shown how these time scales can be controlled by tunable parameters such as the bath size and the strength of the system-bath coupling.

We show that the use of a recently proposed iterative collision model with interenvironment swaps displays a signature of strongly non-Markovian dynamics that is highly dependent on the establishment of system-environment correlations. Two models are investigated: one in which such correlations are canceled iteratively and one in which they are kept all across the dynamics. The degree of non-Markovianity, quantified using a measure based on the trace distance, is found to be much greater for all coupling strengths, when system-environment correlations are maintained.

We consider the process of changing reference frames in the case where the reference frames are quantum systems. We find that, as part of this process, decoherence is necessarily induced on any quantum system described relative to these frames. We explore this process with examples involving reference frames for phase and orientation. Quantifying the effect of changing quantum reference frames provides a theoretical description for this process in quantum experiments, and serves as a first step in developing a relativity principle for theories in which all objects including reference frames are necessarily quantum.

The distance dependence of the anisotropic atom-wall interaction is studied. The central result is the $1/z^6$ quadrupolar anisotropy decay in the retarded Casimir-Polder regime. Analysis of the transition region between nonretarded van der Waals regime (in $1/z^3$) and Casimir-Polder regime shows that the anisotropy crossover occurs at very short distances from the surface, on the order of $0.03\lambda$, where $\lambda$ is the atom characteristic wavelength. Possible experimental verifications of this distance dependence using surface-induced inelastic transitions are discussed.

We study theoretically the Feshbach-optimized photoassociation of the ${}^7\mathrm{Li}$ and ${}^{133}\mathrm{Cs}$ atoms driven by electric and magnetic fields. The results show that the combination of electric and magnetic fields can significantly enhance the photoassociation rate by regulating scattering resonance and increasing the probability density of atom pairs at sufficiently short internuclear distance. The electric field can induce the coupling between the $s$ wave and $p$ wave and makes it possible to directly produce ultracold molecules in the lowest rotational state via one-photon stimulated emission photoassociation at ultralow temperature. Moreover, in the presence of a magnetic field, only a relatively weak electric field ($E\le 100$ kV/cm) is required to realize the scattering resonance for heteronuclear alkali-metal atom pairs.

The B-spline R-matrix method is used to study electron collisions with neutral fluorine over an energy range from threshold to 100 eV. The multiconfiguration Hartree-Fock method in connection with B-spline expansions is employed for an accurate representation of the target wave functions. The sensitivity of the results is checked by comparing data obtained in different approximations, including a large-scale model with over 600 continuum pseudostates. Both correlation and polarization effects are found to be important for accurate calculations of the cross sections. Coupling to the target continuum strongly affects transitions from the ground state, but to a lesser extent the strong transitions between excited states. Cross sections are presented for selected transitions between the lowest 26 states of fluorine, as well as elastic scattering from and ionization of the ground state. The current predictions represent an extensive set of electron scattering data for neutral fluorine.

A scheme to control the many-boson tunneling process from a trap through a potential barrier to open space is devised and demonstrated. The number of ejected particles and their velocities can be controlled by two parameters, the threshold of the potential and the interparticle interaction. Employing a recently developed efficient many-body method, results for two, three, and one hundred interacting bosons in one spatial dimension can be computed numerically exactly, solving thereby an intricate problem of general interest. It is shown that the control scheme devised for the many-boson tunneling process performs very well for the dynamics of the momentum density, the correlations, and the coherence, as well as for the number of particles remaining in the trap. To interpret the many-body tunneling process, a transparent model assembling the many-body process from single-particle emission processes is derived. Analysis of the energetics of available decay channels together with the numerical results rules out the situation of two (or more) bosons tunneling together to open space. The present investigation generalizes the findings reported by us [Proc. Natl. Acad. Sci. USA 109, 13521 (2012)] for many-boson tunneling to open space in the absence of a threshold.

We study an impurity in a one-dimensional potential consisting of a harmonic part and a periodic part using both the time-evolving block decimation (TEBD) algorithm and a variational ansatz. Attractive and repulsive contact interactions with a sea of fermions are considered. We find excellent agreement between TEBD and variational results and use the variational ansatz to investigate higher lattice bands. We conclude that the lowest band approximation fails at sufficiently strong interactions and develop a method for computing the Tan contact parameter.