In this study, angle-resolved energetic-ion yield spectra were measured in the N $1s$ excitation region of ${\mathrm{N}}_2$O. A Franck-Condon analysis based on ab initio two-dimensional potential energy surfaces of the core-excited Rydberg states, which were calculated by the symmetry-adapted cluster-configuration interaction method, reproduced observed vibrational excitations specific to the individual Rydberg states well and enabled quantitative assignments. Geometric changes in the terminal nitrogen ${\mathrm{N}}_{\mathrm{t}}$ $1s$ and the central nitrogen ${\mathrm{N}}_{\mathrm{c}}$ $1s$ excited states with respect to the $3p$π, $3p$σ, and $4s$σ transitions were analyzed. The coupling of these valence and Rydbergs states was examined based on the second moment analysis. Irregular Rydberg-state behavior in the ${\mathrm{N}}_{\mathrm{c}}$ $1s^{-1}$ $4s$σ state was observed.

We report two-step photoionization studies from the 5$p$ ${}^2$$P_{3/2}$ excited state of rubidium using two dye lasers simultaneously pumped by a common Nd:YAG laser in conjunction with a thermionic diode ion detector. The photoionization cross section at the first ionization threshold is measured as 18.8 ± 3 Mb and at excess energies of 0.013, 0.106, 0.229, and 0.329 eV is measured as 15, 13.6, 12.6, and 12.5 Mb, respectively. The measured value of the photoionization cross section at the threshold is used to calibrate the oscillator strengths of the 5$p$ ${}^2$$P_{3/2}$ → $n$d ${}^2$$D_{5/2}$ (22 ⩽ $n$ ⩽ 52) Rydberg transitions.

We present an alternative explanation of the high-order-harmonic-generation experimental results published recently by Seres et al. [Nature Phys. 6, 455 (2010)]. We show that the physical interpretation can be comprehensively done in the frame of classical theory of high-order harmonic generation without referring to a parametric effect in the XUV domain. The experimental conditions explored by Seres et al. indeed correspond to the case of long-pulse, low-infrared-energy laser beams for which tight focusing is necessary to reach the minimum intensity required for high atomic response. The positive atomic dispersion can compensate for the Gouy phase and explains the behavior of the experimental variation of the harmonic signal presented. We will show that our interpretation explains not only the global behavior of the curves but also the second-order variation of the signal as a function of experimental parameters.

An assembly of noninteracting atoms may become correlated upon interaction with entangled photons, and certain elements of their joint density matrix can then show collective resonances. We explore experimental signatures of these resonances in the nonlinear response of a pair of two-level atoms. We find that these resonances are canceled out in stimulated signals such as pump-probe and two-photon absorption due to the destructive interference of two-photon-absorption and emission pathways in the joint two-particle space. However, they may be observed in photon statistics (Hanbury-Brown–Twiss) measurements through the attenuation of two-time intensity correlations.

The dynamics of a vibronic system in a lossy two-mode cavity is studied, with the first mode being resonant to the electronic transition and the second one being nearly resonant due to Raman transitions. We derive analytical solutions for the dynamics of this system. For a properly chosen detuning of the second mode from the exact Raman resonance, we obtain conditions that are closely related to the phenomenon of Rabi resonance as it is well known in laser physics. Such resonances can be observed in the spontaneous emission spectra, where the spectrum of the second mode in the case of weak Raman coupling is enhanced substantially.

We present a complete and exhaustive theory of signal-to-noiseratio in bipartite ghost imaging with classical (thermal) and quantum (twin beams) light. The theory is compared with experiment for both twin beams and thermal light in a certain regime of interest.

The stochastic variational method is used to compute the binding energy for positrons attached to the repulsive ${\mathrm{H}}_2$($A$ ${}^3{\Sigma }_u$) state. Attachment occurs for internuclear separations between 1.616 $a_0$ and $1.818$ $a_0$. At these distances the vertical ionization potential for the ${\mathrm{H}}_2$($A$ ${}^3{\Sigma }_u$) state is close to the positronium binding energy of 0.250 a.u. The maximum attachment energy occurs at 1.67 $a_0$ and is 0.003532 a.u.

Rigorous and universal relationships among radial expectation values of any $D$-dimensional quantum-mechanical system are obtained, using Rényi-like position-momentum inequalities in an information-theoretical framework. Although the results are expressed in terms of four moments (two in position space and two in the momentum one), especially interesting are the cases that provide expressions of uncertainty in terms of products $\langle r^a\rangle {}^{1/a}\langle p^b\rangle {}^{1/b}$, widely considered in the literature, including the famous Heisenberg relationship $\langle r^2\rangle \langle p^2\rangle ⩾D^2/4$. Improved bounds for these products have recently been provided, but are always restricted to positive orders $a,b>0$. The interesting part of this work are the inequalities for negative orders. A study of these relationships is carried out for atomic systems in their ground state. Some results are given in terms of relevant physical quantities, including the kinetic and electron-nucleus attraction energies, the diamagnetic susceptibility, and the height of the peak of the Compton profile, among others.

This paper reveals an important role that controllability plays in the complexity of optimizing quantum control dynamics. We show that the loss of controllability generally leads to multiple locally suboptimal controls when gate fidelity in a quantum control system is maximized, which does not happen if the system is controllable. Such local suboptimal controls may attract an optimization algorithm into a local trap when a global optimal solution is sought, even if the target gate can be perfectly realized. This conclusion results from an analysis of the critical topology of the corresponding quantum control landscape, which refers to the gate fidelity objective as a functional of the control fields. For uncontrollable systems, due to SU(2) and SU(3) dynamical symmetries, the control landscape corresponding to an implementable target gate is proven to possess multiple locally optimal critical points, and its ruggedness can be further increased if the target gate is not realizable. These results imply that the optimization of quantum dynamics can be seriously impeded when operating with local search algorithms under these conditions, and thus full controllability is demanded.

We study quantum walks of many noninteracting particles on a beam splitter array as a paradigmatic testing ground for the competition of single- and many-particle interference in a multimode system. We derive a general expression for multimode particle-number correlation functions, valid for bosons and fermions, and infer pronounced signatures of many-particle interferences in the counting statistics.

We explore a geometric approach to generating local $\text{SU}(2)$ and $\text{SL}(2,\mathbb{C})$ invariants for a collection of qubits inspired by lattice gauge theory. Each local invariant or “gauge” invariant is associated with a distinct closed path (or plaquette) joining some or all of the qubits. In lattice gauge theory, the lattice points are the discrete space-time points, the transformations between the points of the lattice are defined by parallel transporters, and the gauge invariant observable associated with a particular closed path is given by the Wilson loop. In our approach the points of the lattice are qubits, the link transformations between the qubits are defined by the correlations between them, and the gauge invariant observable, the local invariants associated with a particular closed path, are also given by a Wilson looplike construction. The link transformations share many of the properties of parallel transporters, although they are not undone when one retraces one’s steps through the lattice. This feature is used to generate many of the invariants. We consider a pure three-qubit state as a test case and find we can generate a complete set of algebraically independent local invariants in this way; however, the framework given here is applicable to generating local unitary invariants for mixed states composed of any number of $d$-level quantum systems. We give an operational interpretation of these invariants in terms of observables.

In this paper the entanglement and quantum phase transition of the anisotropic spin-$1/2$ $\mathit{XY}$ model are studied by using the quantum renormalization-group method. By solving the renormalization equations, we get the trivial and nontrivial fixed points, which correspond to the phase of the system and the critical point, respectively. The concurrence between two blocks are calculated and it is found that when the number of iterations of the renormalization tends to infinity, the concurrence develops two saturated values that are associated with two different phases, i.e., Ising-like and spin-fluid phases. We also investigate the first derivative of the concurrence and find that there exists nonanalytic behaviors at the quantum critical point, which are directly associated with the divergence of the correlation length. To gain further insight, the scaling behaviors of the system are analyzed and it is shown that the maximum value of the first derivative of the concurrence reaches infinity and the critical point is approached as the size of the system increases.

The formalism is presented for the linear response of a time-dependent (TD) variational coupled cluster (VCC), truncated according to Møller-Plesset perturbation theory, i.e., a TD-VCC[$n$] linear response, where $n$ denotes the order of the corresponding quasienergy with respect to the fluctuation potential. The resulting eigenvalue problem determining the excitation energies is Hermitian and of the simple Tamm-Dancoff form. The VCC[1] excitation energies are equivalent to those of the configuration-interaction singles (CIS) model, while the Casida equation for the TD-Hartree-Fock approach is an approximation to it. The TD-VCC[2] response, the lowest-order method including electron correlation, is discussed in detail and the relations to other second-order methods, such as the CC2 linear response and the algebraic diagrammatic construction at second order [ADC(2)] are explored.

We consider the two-color photoassociation of a quantum degenerate atomic gas into ground-state diatomic molecules via a molecular dark state. This process can be described in terms of a $\Lambda$ level scheme that is formally analogous to the situation in electromagnetically induced transparency in atomic systems and therefore can result in slow-light propagation. We show that the group velocity of the light field depends explicitly on whether the atoms are bosons or fermions, as well as on the existence or absence of a pairing gap in the case of fermions, so that the measurement of the group velocity realizes a nondestructive diagnosis of the atomic state and the pairing gap.

We calculate the band structure of ultracold atoms located inside a laser-driven optical cavity. For parameters where the atom-cavity system exhibits bistability, the atomic band structure develops loop structures akin to the ones predicted for Bose-Einstein condensates in ordinary (noncavity) optical lattices. However, in our case the nonlinearity derives from the cavity back-action rather than from direct interatomic interactions. We find both bi- and tristable regimes associated with the lowest band, and show that the multistability we observe can be analyzed in terms of swallowtail catastrophes. Dynamic and energetic stability of the mean-field solutions is also discussed, and we show that the bistable solutions have, as expected, one unstable and two stable branches. The presence of loops in the atomic band structure has important implications for proposals concerning Bloch oscillations of atoms inside optical cavities [Peden et al., Phys. Rev. A 80, 043803 (2009); Prasanna Venkatesh et al., Phys. Rev. A 80, 063834 (2009)].

We study theoretically interspecies Cooper pairing in a fermionic system with SU(2) $\times$ SU(6) symmetry. We show that, with suitable unitary transformations, the order parameter for the ground state can be reduced to only two nonvanishing complex components. The ground state has a large degeneracy. We find that while some Goldstone modes have linear dispersion, others are quadratic at low frequencies. We compare our results with the case of SU($N$).

We investigate the effects of phase noise and particle loss on the dynamics of a Bose-Einstein condensate in an optical lattice. Starting from the many-body master equation, we discuss the applicability of generalized mean-field approximations in the presence of dissipation as well as methods to simulate quantum effects beyond mean field by including higher-order correlation functions. It is shown that localized particle dissipation leads to surprising dynamics, as it can suppress decay and restore the coherence of a Bose-Einstein condensate. These effects can be applied to engineer coherent structures such as stable discrete breathers and dark solitons.

We present a detailed analysis of stationary light pulses (SLP’s) for the case of inhomogeneous broadening in both optical and spin transitions, which is normally found in solid materials with long-lived spin coherence. By solving the Langevin equations of motion for the density matrix elements under the integral over the entire range of the inhomogeneous broadenings, the necessary conditions for creating the SLP in a solid are obtained. Then the decay and diffusion processes that the SLP undergoes are analyzed. The characteristics of such processes are studied based on the analytic solution of the SLP with a slowly varying envelope. The dependence of SLP lifetime on inhomogeneous broadenings of spin and optical transitions, which can be regarded as the laser linewidth in the repump scheme, has been discussed.

We address recent spin transport experiments in ultracold unitary Fermi gases. We provide a theoretical understanding for how the measured temperature dependence of the spin diffusivity at low $T$ can disagree with the expected behavior of a Fermi liquid (FL), while the spin susceptiblity, following the experimental protocols, is consistent with a FL picture. We show that the experimental protocols for extracting the spin susceptibility implicitly reflect a FL viewpoint; relaxing this leads to consistency within but not proof of a pseudogap-based theory. Our transport calculations yield insight into the observed suppression of the spin diffusion constant at lower $T$.