We experimentally and theoretically analyze the manifestations of quantum interference and multiple ionization channels (multiple orbitals) in high-harmonic spectra of aligned N${}_2$O molecules. Increasing the probe wavelength from 1.17 to 1.46 $\mu$m, we demonstrate the gradual disappearance of multielectron effects and quantitatively explain the observation through calculations. We thus identify a minimum in the high-harmonic spectrum of N${}_2$O caused only by its structure. By comparing its position with that measured in the isoelectronic CO${}_2$ molecule for similar axis distributions, we find a difference of 10 eV, confirmed by ab initio quantum scattering calculations. Quantum interference in photorecombination is thus shown to be sensitive to subtle differences in the valence orbital structure of molecules with nearly identical lengths. This property may find applications in time-resolved studies.

The possibility to coherently control a quantum rotor is investigated theoretically. The rotor is realized by an antiferromagnetic spin-1 Bose-Einstein condensate, trapped in the optical field of a Fabry-Pérot resonator. By tuning the pumping field of the resonator, coherent control over the rotor is achieved. The technique is illustrated by the numerical simulation of a protocol that transforms the rotor's ground state into a squeezed state. The detection of the squeezed state via measurement of intensity correlations of the cavity field is proposed.

Bell's inequalities are important to our understanding of quantum foundations and critical to several quantum technologies. A recent work [E. Wolfe and S. F. Yelin, Phys. Rev. A 86, 012123 (2012)] derived three parametrized families of two-particle, two-setting Bell inequalities. These inequalities are important as they theoretically explore a larger volume of allowed quantum correlations over local hidden-variable models than previous results [A. Cabello, Phys. Rev. A 72, 012113 (2005)] by exploiting marginal, or single particle measurements. In this work we subject those predictions to experimental test using nonmaximally entangled photon pairs to optimize the expected violation. We find excellent agreement with the upper bounds predicted by quantum mechanics with violations of the limits imposed by local hidden-variable models as large as almost 30$\sigma$ for the optimal parameters and a significant violation over a wide range of parameters.

Coupled-state cross sections have been determined for electron transfer and excitation to individual states up to $3d$, as well as ionization, in collisions between protons and ground-state hydrogenic ions He${}^{+}$, Li${}^{2+}$, Be${}^{3+}$, B${}^{4+}$, and C${}^{5+}$ at intermediate and higher proton energies using two-center 120- and 281-Sturmian bases, extending previous smaller basis results by the author [Phys. Rev. A 35, 3799 (1987)] at intermediate energies, and reexamining scaling rules with target nuclear charge $Z$.

We have evaluated the dominant contribution of the electron correlations to the single $K$-shell ionization cross section of heliumlike atomic systems by impact of high-energy electrons. This study is performed consistently within the framework of nonrelativistic perturbation theory. The results obtained are also applicable for the case of high-energy collisions with arbitrary charged particles whose mass is much less than that of the target. The formula for cross section is represented in the form of a universal scaling. A comparison of our numerical calculations with available experimental data is made.

We calculate the transient absorption of an isolated attosecond pulse (IAP) by helium atoms subject to a delayed infrared (IR) laser pulse. With the central frequency of the broad attosecond spectrum near the ionization threshold, the absorption spectrum is strongly modulated at the sub-IR-cycle level. Given that the absorption spectrum results from a time-integrated measurement, we investigate the extent to which the delay dependence of the absorption yields information about the attosecond dynamics of the atom-field energy exchange. We find two configurations in which this is possible. The first involves multiphoton transitions between bound states that result in interference between different excitation pathways. The second involves the modification of the bound state absorption lines by the IR field, which we find can result in a subcycle time dependence only when ionization limits the duration of the strong-field interaction.

We study the interplay of electronic-structure and multielectron effects in high-harmonic spectra of the two isoelectronic molecules CO${}_2$ and N${}_2$O as a function of the near-infrared driving wavelength and intensity. We show that the minima observed in the spectra of aligned molecules ($\langle {\cos }^2\theta \rangle =0.54–0.64$) are intensity dependent at the shortest investigated wavelength (1.17 $\mu$m) but become gradually intensity independent at longer wavelengths (1.36 and 1.46 $\mu$m). These results combined with detailed theoretical modeling show that the contributions of several ionization channels are important in the spectra recorded with short-wavelength driving pulses because the minima are located in the cutoff region where the contributions from different channels are comparable. In spectra recorded with longer driving wavelengths, the minima are located in the plateau region of the spectrum and are caused by the electronic structure of the molecules. Our results also rationalize and reconcile previous results and provide general guidelines for future studies.

We propose a method to generate a source of spin-polarized cold atoms which are continuously extracted and guided from a magneto-optical trap using an atom-diode effect. We show that it is possible to create a pipe-like potential by overlapping two optical beams coupled with the two transitions of a three-level system in a ladder configuration. With alkali-metal atoms, and in particular with ${}^{87}$Rb, a proper choice of transitions enables both the potential generation and optical pumping, thus polarizing the sample in a given Zeeman state. We extend the Dalibard and Cohen-Tannoudji dressed-atom model of radiative forces to the case of a three-level system. We derive expressions for the average force and the different sources of momentum diffusion in the resonant, nonperturbative regime. We show using numerical simulations that a significant fraction of the atoms initially loaded can be guided over several centimeters with output velocities of a few meters per second. This would produce a collimated continuous source of slow spin-polarized atoms suitable for atom interferometry.

We examine processes of polarization-modulational instability (PMI) in two categories of weakly birefringent optical fibers, namely, fibers whose nonlinearity is comparable to that of a standard telecom fiber, and high-index glass fibers whose nonlinearity is enhanced by several orders of magnitude as compared to that of a standard fiber. We show that the fourth-order dispersion (FOD) has a strong impact on PMI processes in both types of fibers, both at the qualitative and quantitative levels. At the qualitative level, the FOD enriches the phase diagram with nonconventional processes that generate two pairs of sidebands in certain parameter regions, while in other regions we obtain a single pair of sidebands whose frequency is independent of the pump power. The highly nonlinear birefringent fibers cause a pump depletion of a magnitude such that the frequency of the PMI sidebands becomes unstable and undergoes a continual drift. We demonstrate the existence of conditions in which the PMI process takes place in a manner similar to that of a process coupled with a photon reservoir, which feeds in situ the PMI process by continuously providing photons in compensation for those absorbed by the fiber.

We study the dynamics of single photonic and atomic excitations in the Jaynes-Cummings-Hubbard (JCH) model where the cavities are arranged in an Apollonian network (AN). The existence of a gapped field normal frequency spectrum along with strongly localized eigenstates on the AN highlights many of the features provided by the model. By numerically diagonalizing the JCH Hamiltonian in the single excitation subspace, we evaluate the time evolution of fully localized initial states, for many energy regimes. We provide a detailed description of the photonic quantum walk on the AN and also address how an effective Jaynes-Cummings interaction can be achieved at the strong hopping regime. When the hopping rate and the atom-field coupling strength is of the same order, the excitation is relatively allowed to roam between atomic and photonic degrees of freedom as it propagates. However, different cavities will contribute mostly to one of these components, depending on the detuning and initial conditions, in contrast to the strong atom-field coupling regime, where atomic and photonic modes propagate identically.

We propose a simple realization of a quantum simulator of the Riemann-Hurwitz (RH) $\zeta$ function based on a truncation of its Dirichlet representation. We synthesize a nearest-neighbor-interaction Hamiltonian, satisfying the property that the temporal evolution of the autocorrelation function of an initial bare state of the Hamiltonian reproduces the RH function along the line $\sigma +i\omega t$ of the complex plane, with $\sigma >1$. The tight-binding Hamiltonian with engineered hopping rates and site energies can be implemented in a variety of physical systems, including trapped-ion systems and optical waveguide arrays. The proposed method is scalable, which means that the simulation can be, in principle, arbitrarily accurate. Practical limitations of the suggested scheme, arising from a finite number of lattice sites $N$ and from decoherence, are briefly discussed.

In polyatomic molecules with a $\Pi$ electronic ground state, the rovibrational spectrum can be strongly modified by the Renner-Teller effect (the coupling between bending vibrational mode and electrons in the degenerate $\Pi$ state). The linear form of the C${}_3$H molecule has particularly strong Renner-Teller interactions and a very-low-lying vibronic ${\Sigma }_{1/2}^{+}$ level, which corresponds to the excited bending vibrational mode. This leads to the increased sensitivities of the microwave and submillimeter transition frequencies to the possible variation of the fine structure constant $\alpha$ and electron-to-proton mass ratio $\mu$.

We study the task of distilling entanglement by a coherent superposition operation $t\widehat{a}+r{\widehat{a}}^{†}$ applied to a continuous-variable state under a thermal noise. In particular, we compare the performances of two different strategies; i.e., the non-Gaussian operation $t\widehat{a}+r{\widehat{a}}^{†}$ is applied before or after the noisy Gaussian channel. This is closely related to a fundamental problem of whether Gaussian or non-Gaussian entanglement can be more robust under a noisy channel and also provides a useful insight into the practical implementation of entanglement distribution for a long-distance quantum communication. We specifically look into two entanglement characteristics, the logarithmic negativity as a measure of entanglement and the teleportation fidelity as a usefulness of entanglement, for each distilled state. We find that the non-Gaussian operation after (before) the thermal noise becomes more effective in the low- (high-) temperature regime.

Spectral properties of an arbitrary matrix can be characterized by the entropy of its rescaled singular values. Any quantum operation can be described by the associated dynamical matrix or by the corresponding superoperator. The entropy of the dynamical matrix describes the degree of decoherence introduced by the map, while the entropy of the superoperator characterizes the a priori knowledge of the receiver of the outcome of a quantum channel $\Phi$. We prove that for any map acting on an $N$-dimensional quantum system the sum of both entropies is not smaller than $\ln N$. For any bistochastic map this lower bound reads $2\ln N$. We investigate also the corresponding Rényi entropies, providing an upper bound for their sum, and analyze the entanglement of the bi-partite quantum state associated with the channel.

The entanglement-assisted (EA) formalism is a generalization of the standard stabilizer formalism, and it can transform arbitrary quaternary classical linear codes into entanglement-assisted quantum error correcting codes (EAQECCs) by using of shared entanglement between the sender and the receiver. Using the special structure of linear EAQECCs, we derive an EA-Plotkin bound for linear EAQECCs, which strengthens the previous known EA-Plotkin bound. This linear EA-Plotkin bound is tighter then the EA-Singleton bound, and matches the EA-Hamming bound and the EA-linear programming bound in some cases. We also construct three families of EAQECCs with good parameters. Some of these EAQECCs saturate this linear EA-Plotkin bound and the others are near optimal according to this bound; almost all of these linear EAQECCs are degenerate codes.

We develop a scheme for fault-tolerant quantum computation based on asymmetric Bacon-Shor codes, which works effectively against highly biased noise dominated by dephasing. We find the optimal Bacon-Shor block size as a function of the noise strength and the noise bias, and estimate the logical error rate and overhead cost achieved by this optimal code. Our fault-tolerant gadgets, based on gate teleportation, are well suited for hardware platforms with geometrically local gates in two dimensions.

Excitation energies of the $ns$, $np$, $nd$, and $nf$ ($n\le$ 9) states in Li-like Be${}^{+}$ are evaluated within the framework of relativistic many-body theory. First-, second-, third-, and all-order Coulomb energies and first- and second-order Breit corrections to the energies are calculated. Two alternative treatments of the Breit interaction are investigated. In the first approach, we omit Breit contributions to the Dirac-Fock potential and evaluate Coulomb and Breit-Coulomb corrections through second order perturbatively. In the second approach, we include both Coulomb and Breit contributions on the same footing via the Breit-Dirac-Fock potential and then treat the residual Breit and Coulomb interactions perturbatively. The results obtained from the two approaches are compared and discussed. All-order calculations of reduced matrix elements, oscillator strengths, transition rates, and lifetimes are given for levels up to $n$ = 9. Electric-dipole ($2s–np$), electric-quadrupole ($2s–nd$), and electric-octupole ($2s–nf$) matrix elements are evaluated in order to obtain the corresponding ground-state multipole polarizabilities using the sum-over-states approach. Recommended values are provided for a large number of electric-dipole matrix elements. Scalar and tensor polarizabilities for the $ns$, $n{p}_{1/2}$, $n{p}_{3/2}$, $n{d}_{3/2}$, and $n{d}_{5/2}$ states with $n\le 9$ are also calculated. The scalar hyperpolarizability for the ground $2s$ state is evaluated and compared with the result of a nonrelativistic calculation.

Accurate theoretical predictions including leading QED corrections for $2{{}^{2}P}_{1/2}\text{−}2{{}^{2}S}_{1/2}$ ($D_1$) and $2{{}^{2}P}_{3/2}\text{−}2{{}^{2}S}_{1/2}$ ($D_2$) transition energies have been obtained for ${}^{6,7}$Li isotopes. Our results for the Bethe logarithms $\ln k_0\left(2{}^{2}S\right)=5.178169\left(4\right)$, and for mass polarization corrections $\Delta \ln k_0\left(2{}^{2}S\right)=0.11381\left(3\right)$ are in disagreement with ones obtained recently in the literature. In contrast, results $\ln k_0\left(2{}^{2}P\right)=5.17981\left(7\right)$ and $\Delta \ln k_0\left(2{}^{2}P\right)=0.1113\left(5\right)$ are in good agreement with them. From our theoretical predictions and recent measurements of ${}^{6,7}$Li $D$ lines at NIST, we determine the mean-square charge radius difference between ${}^7$Li and ${}^6$Li nuclei, in agreement with determinations based on the $3{{}^{2}S}_{1/2}\text{−}2{{}^{2}S}_{1/2}$ transition, which demonstrates consistency of atomic spectroscopy determination of fundamental properties of nuclei.

We characterize the effect of quantum interference on the line shapes and measured line positions in atomic spectra. These effects, which occur when the excited-state splittings are of order of the natural line widths, represent an overlooked but significant systematic effect. We show that excited-state interference gives rise to non-Lorentzian line shapes that depend on excitation polarization, and we present expressions for the corrected line shapes. We present spectra of ${}^{6,7}$Li $D$ lines taken at multiple excitation laser polarizations and show that failure to account for interference changes the inferred line strengths and shifts the line centers by as much as 1 MHz. Using the correct line shape, we determine absolute optical transition frequencies with an uncertainty of $\le$25 kHz and provide an improved determination of the difference in mean-square nuclear charge radii between ${}^6$Li and ${}^7$Li. This analysis should be important for a number of high-resolution spectral measurements that include partially resolvable atomic lines.