A recently developed ab initio implementation of the muticonfiguration time-dependent Hartree Fock (MCTDHF) method is used to calculate valence and core photoionization cross sections of the Be atom and HF molecule in the Born-Oppenheimer approximation. The cross sections are extracted from the dynamics following excitation by a single subfemtosecond laser pulse. We compare with previously published results and those from time-independent complex Kohn scattering calculations and find that the MCTDHF method calculates both the magnitude of the cross section and the line shapes of the resonant peaks to good accuracy, particularly for valence photoionization. These calculations demonstrate a methodology immediately applicable to calculating multiphoton and ultrafast dynamics.

We show that photoionization cross sections of atoms can be accurately extracted from the high-order-harmonic spectra generated by intense infrared lasers only if the degree of ionization in the gas medium is small, implying that for this purpose the high-order-harmonic generation spectra should be taken at low gas pressure and at low laser intensity experimentally.

We investigate the freezing phenomenon of quantum discord occurring in phase damping noise processes. By relating the expression of the time variation of the discord to the convex function of relative entropy, we obtain the necessary and sufficient conditions of the phenomenon for standard Bell-diagonal states. These conditions are applicable also to the phenomenon occurring in a non-Markovian dephasing process. Moreover, we show that the same condition and phenomenon coincide in an unusual sort of Bell-diagonal states beyond the standard form.

We propose a potentially practical scheme to simulate artificial Abelian gauge field for polaritons using a hybrid quantum system consisting of nitrogen-vacancy center ensembles (NVEs) and superconducting transmission line resonators (TLR). In our case, the collective excitations of NVEs play the role of bosonic particles, and our multiport device tends to circulate polaritons in a behavior like a charged particle in an external magnetic field. We discuss the possibility of identifying signatures of the Hofstadter butterfly in the optical spectra of the resonators and analyze the ground state crossover for different gauge fields. Our work opens new perspectives in quantum simulation of condensed matter and many-body physics using a hybrid spin-ensemble circuit quantum electrodynamics system. The experimental feasibility and challenge are justified using currently available technology.

We consider a system of three ${}^4$He atoms, which is so far the simplest realistic three-body system exhibiting the Efimov effect, in order to analyze deviations from the universal Efimov three-body spectrum. We first calculate the bound states using a realistic two-body potential, and then analyze how they can be reproduced by simple effective models beyond Efimov's universal theory. We find that the nonuniversal variations of the ground and first excited states can be well reproduced by models parametrized with only three quantities: the scattering length and effective range of the original potential, and a small three-body force. Furthermore, the three-body parameter, which fixes the origin of the infinite set of three-body levels, is found to be consistent with recent experimental observations in other atomic species.

High-order harmonic generation (HHG) by an atom in an elliptically polarized laser field is studied numerically and theoretically. We derive equations describing the effect of the driving ellipticity on the HHG efficiency, harmonic ellipticity, and rotation angle for the cases of the ground state with the orbital angular momentum $l=0$ and $l\ne 0$. These equations provide good agreement with the results of our three-dimensional time-dependent Schrödinger equation calculations, as well as with the published experimental data. We show that HHG sensitivity to the driving laser ellipticity increases with the drive wavelength. This predicts high efficiency of the polarization gating schemes using midinfrared laser fields. Furthermore, our study shows significant enhancement of the harmonic ellipticity near the Cooper minimum.

Quantum control landscape theory was formulated to assess the ease of finding optimal control fields in simulations and in the laboratory. The landscape is the observable as a function of the controls, and a primary goal of the theory is the analysis of landscape features. In what is referred to as the kinematic picture of the landscape, prior work showed that the landscapes are generally free of traps that could halt the search for an optimal control at a suboptimal observable value. The present paper considers the dynamical picture of the landscape, seeking the existence of singular controls, especially of a nonkinematic nature along with an assessment of whether they correspond to traps. We analyze the necessary and sufficient conditions for singular controls to be kinematic or nonkinematic critical solutions and the likelihood of their being encountered while maximizing an observable. An algorithm is introduced to seek singular controls on the landscape in simulations along with an associated Hessian landscape analysis. Simulations are performed for a large number of model finite-level quantum systems, showing that all the numerically identified kinematic and nonkinematic singular critical controls are not traps, in support of the prior empirical observations on the ease of finding high-quality optimal control fields.

The observation of $\mathcal{PT}$ symmetry in a coupled optical waveguide system that involves a complex refractive index has been demonstrated impressively in the experiment by Rüter et al. [Nat. Phys. 6, 192 (2010)]. This is, however, only an optical analog of a quantum system, and it would be highly desirable to observe the manifestation of $\mathcal{PT}$ symmetry and the resulting properties also in a real, experimentally accessible, quantum system. Following a suggestion by Klaiman et al. [Phys. Rev. Lett. 101, 080402 (2008)], we investigate a $\mathcal{PT}$-symmetric arrangement of a Bose-Einstein condensate in a double-well potential, where in one well cold atoms are injected while in the other particles are extracted from the condensate. We investigate, in particular, the effects of the nonlinearity in the Gross-Pitaevskii equation on the $\mathcal{PT}$ properties of the condensate. To study these effects we analyze a simple one-dimensional model system in which the condensate is placed into two $\mathcal{PT}$-symmetric $\delta$-function traps. The analysis will serve as a useful guide for studies of the behavior of Bose-Einstein condensates in realistic $\mathcal{PT}$-symmetric double wells.

We present a diagrammatic technique for calculating quantum-detected nonlinear optical signals using loop diagrams that act in the joint matter plus field space. This formalism, which is based on time-ordered superoperators keeps track of the entangled matter plus field state, making it most suitable for spectroscopy applications. Photon counting is recast as a time and frequency convolution of a bare signal, which is given by a correlation function of matter, and a gating spectrogram.

We present a theoretical framework that describes a wave packet of light prepared in a state of definite photon number interacting with an arbitrary quantum system (e.g., a quantum harmonic oscillator or a multilevel atom). Within this framework we derive master equations for the system as well as for output field quantities such as quadratures and photon flux. These results are then generalized to wave packets with arbitrary spectral distribution functions. Finally, we obtain master equations and output field quantities for systems interacting with wave packets in multiple spatial and/or polarization modes.

We develop for dipole-forbidden transition a dynamical theory of two-photon paired superradiance (PSR). This is a cooperative process characterized by two photons emitted back to back with equal energies. By irradiating the trigger laser from two target ends, with its frequency tuned at the half energy between two levels, a macroscopically coherent state of medium and fields dynamically emerges as time evolves, and a large signal of amplified output occurs with a time delay. The basic semiclassical equations in $1+1$ space-time dimensions are derived for the field-plus-medium system to describe the space-time evolution of the entire system and are numerically solved to demonstrate the existence of both explosive and weak PSR phenomena in the presence of relaxation terms. The explosive PSR event terminates accompanying a sudden release of most of the energy stored in the target. Our numerical simulations are performed using the vibrational transition $X^1{\Sigma }_g^{+}v=1\to 0$ of a para-H${}_2$ molecule and taking many different excited atom number densities and different initial coherences between the metastable and the ground states. In an example with a number density close to $O({10}^{21}$ cm${}^{-3}$) and a high initial coherence, the explosive event terminates several nanoseconds after the trigger irradiation, when the phase relaxation time larger than $O$(10 ns) is taken. After PSR events the system is expected to follow a steady-state solution which is obtained by analytic means and is made of many objects of field condensates endowed with a topological stability.

We present a quantum memory protocol for photons that is based on the direct control of the transition dipole moment. We focus on the case where the light-matter interaction is enhanced by a cavity. We show that the optimal write process (maximizing the storage efficiency) is related to the optimal read process by a reversal of the effective time $\tau =\int dt{g}^2\left(t\right)/\kappa$, where $g\left(t\right)$ is the time-dependent coupling and $\kappa$ is the cavity decay rate. We discuss the implementation of the protocol in a rare-earth-ion–doped crystal where an optical transition can be turned on and off by switching a magnetic field.

Recently, Jones and Kok [Jones and Kok, Phys. Rev. A 82, 022107 (2010)] presented alternative geometric derivations of the Mandelstam-Tamm [Mandelstam and Tamm, J. Phys. (USSR) 9, 249 (1945)] and Margolus-Levitin [Margolus and Levitin, Phys. D 120, 188 (1998)] inequalities for the quantum speed of dynamical evolution. The Margolus-Levitin inequality followed from an upper bound on the rate of change of the statistical distance between two arbitrary pure quantum states. We show that the derivation of this bound is incorrect. Subsequently, we provide two upper bounds on the rate of change of the statistical distance, expressed in terms of the standard deviation of the generator $K$ and its expectation value above the ground state. The bounds lead to the Mandelstam-Tamm inequality and a quantum speed limit which is only slightly weaker than the Margolus-Levitin inequality.

A scale-invariant two-component Fermi gas in a time-dependent isotropic harmonic potential is investigated. The exact time evolution of the density distribution in position space in any spatial dimension is obtained. Two experimentally relevant examples, an abrupt change and a periodic modulation of the trapping frequency, are solved. Small deviations from scale invariance are addressed within first-order perturbation theory. The consequences for experiments with ultracold quantum gases, such as the excitation of a tower of undamped breathing modes and an alternative for measuring the Tan contact, are discussed.

We consider the behavior of bipartite and tripartite nonlocality between fermionic entangled states shared by observers, one of whom uniformly accelerates. We find that while fermionic entanglement persists for arbitrarily large acceleration, the Bell–Clauser-Horne-Shimony-Holt inequalities cannot be violated for sufficiently large but finite acceleration. However, the Svetlichny inequality, which is a measure of genuine tripartite nonlocality, can be violated for any finite value of the acceleration.

We revisit transit time broadening for one of the typical experiment designs in molecular spectroscopy, that of a collimated supersonic beam of particles crossing a focused Gaussian laser beam. In particular, we consider a Doppler-free arrangement of a collimated supersonic beam of Na${}_2$ molecules crossing two counterpropagating laser beams that excite a two-photon transition in a three-level ladder scheme. We propose an analytical two-level model with a virtual intermediate level to show that the excitation line shape is described by a Voigt profile and provide the validity range of this model with respect to significant experimental parameters. The model also shows that line broadening due to the curvature of laser field wave fronts on the particle beam path is exactly compensated by increased transit time of particles farther away from the beam axis, such that the broadening is determined solely by the size of the laser beam waist. The analytical model is validated by comparing it with numerical simulations of density-matrix equations of motion using a split propagation technique and with experimental results.

We present a simulation study of the properties of the Tonks-Girardeau gas, a system of $N$ hard-core bosons confined in a one-dimensional harmonic trap. For this system the ground-state wave function is exactly known based on a Bose-Fermi mapping theorem. We employ Nelsons' interpretation of quantum mechanics in which this $N$-body wave function gives rise to a set of stochastic differential equations for the positions of the $N$ particles, which can be simulated by standard methods. In particular, real space densities and momentum distributions can simply be determined by time averages along particle trajectories. Employing this approach we are able to significantly extend the range of particle numbers $N$ treated numerically compared to earlier approaches, while reproducing all exactly known results for this model. We also show that for the bosons in a harmonic trap, contrary to what has been assumed so far, the momentum distribution reflects the system size scaling of the occupation numbers of the natural orbitals; that is, it can be used to decide on the presence of a Bose-Einstein condensate.

We study the relation between the eigenfrequencies of the Bogoliubov excitations of Bose-Einstein condensates and the eigenvalues of the Jacobian stability matrix in a variational approach that maps the Gross-Pitaevskii equation to a system of equations of motion for the variational parameters. We do this for Bose-Einstein condensates with attractive contact interaction in an external trap and for a simple model of a self-trapped Bose-Einstein condensate with attractive $1/r$ interaction. The stationary solutions of the Gross-Pitaevskii equation and Bogoliubov excitations are calculated using a finite-difference scheme. The Bogoliubov spectra of the ground and excited state of the self-trapped monopolar condensate exhibit a Rydberg-like structure, which can be explained by means of a quantum-defect theory. On the variational side, we treat the problem using an ansatz of time-dependent coupled Gaussian functions combined with spherical harmonics. We first apply this ansatz to a condensate in an external trap without long-range interaction and calculate the excitation spectrum with the help of the time-dependent variational principle. Comparing with the full-numerical results, we find good agreement for the eigenfrequencies of the lowest excitation modes with arbitrary angular momenta. The variational method is then applied to calculate the excitations of the self-trapped monopolar condensates and the eigenfrequencies of the excitation modes are compared.

Dipolar interactions support the formation of intersite soliton molecules in a stack of quasi-one-dimensional (quasi-1D) traps. We show that the stability and properties of individual solitons and soliton molecules in such a geometry crucially depend on the interplay between contact and dipolar interactions. In particular, two different quasi-1D soliton regimes are possible: a one-dimensional (1D) soliton characterized by purely repulsive dipole-dipole interactions (DDI) and a three-dimensional (3D) soliton for which a sufficiently large dipole moment renders the DDI attractive. Furthermore, we find that in contrast to the dimers of polar molecules the soliton dimers exhibit a nontrivial behavior of the elementary excitations that stems from the competition between onsite and intersite DDI. Finally, we prove the existence of soliton trimers in a regime where molecular trimers do not occur. We demonstrate that the soliton molecules that we report are well feasible under realistic experimental conditions.