We report experiments in which positronium (Ps) atoms, optically excited to Rydberg states with principal quantum numbers $n$ in the range 18–25, were selectively ionized by both static and pulsed electric fields. The experiments were modeled using Monte Carlo simulations that include tunnel ionization rates calculated for hydrogen and scaled by the Ps reduced mass. Our measurements exhibit a small disagreement with the calculated tunnel ionization rates. Despite this we show that the electric fields in which different Ps states are ionized are sufficiently separated to allow selective field-ionization methods to be used in typical experimental conditions.

We experimentally and theoretically investigate strong-field ionization of noble-gas atoms in elliptically polarized laser fields at 2000 nm. The ellipticity dependence of the ion yields has been measured at a series of intensities and compared to various theoretical model predictions [i.e., the Ammosov-Delone-Krainov (ADK), strong-field approximation (SFA), and Coulomb-Volkov distorted-wave approximation (CVA) models]. The SFA and ADK calculations match each other, which alludes to the negligible contribution of nonadiabatic effect. In the meantime, they both deviate significantly from the measurements. On the other hand, the good agreement between CVA calculations and measurements reveals the salient role played by the Coulomb potential in atomic ionization. The Coulomb potential effect is further confirmed by comparison of CVA calculations and measurements at 800 nm. Our work demonstrates the significant Coulomb potential effect on the ionization yields, the experimental evidence of which has rarely been documented.

The standard closed orbit theory is extended to investigate the photodetachment of a negative ion near a moving wall. The photodetachment cross section of the ${\mathrm{H}}^{-}$ ion in a quantum well with one expanding wall is specifically put forward. In contrast to the photodetachment of the negative ion in a static quantum well, the returning kinetic energy of the detached electron is different from its initial value after collision with the moving wall, therefore an additional modulation factor appears in the oscillating cross section, which depends on the electron's initial outgoing and returning momentum. Four different types of closed orbits are found for the detached electron in a quantum well with one expanding wall, and their connections with the oscillating cross section are analyzed quantitatively. The calculation results suggest that the photodetachment cross section of this system depends on the speed of the moving wall and the initial distances from the ion to the walls sensitively. The method used in this paper is universal and can be extended to study the photodetachment of the nonhydrogenic negative ion near a moving wall, whether the wall is moving with constant velocity or oscillatory. The results provide insight into the behavior of photodetachment dynamics of the negative ion in the presence of a moving boundary and may have potential interest for future experimental researches.

In Phys. Rev. A 93, 051402(R) (2016), we proposed a method to introduce spin into attoscience with spin-polarized electrons produced by a bicircular laser field. In the present paper we further develop this method. We calculate focal-averaged photoelectron spectra and the corresponding spin asymmetry parameter by using the improved strong-field approximation. We show that the spin asymmetry of the rescattered electrons, which is time dependent on the attosecond timescale, can become especially large for particular peak laser intensities. The pertinent jumps in the spin asymmetry occur when the peak intensity changes so that certain above-threshold ionization channel-closing intensities corresponding to the different ionization continua of Xe become covered by the intensity distribution.

Strong-field ionization of benzene, fluorobenzene, benzonitrile, and 1-chloro-2-fluorobenzene is studied within the framework of real-space and real-time time-dependent density functional theory. Analysis of the ionization rates as a function of the molecule orientation reveals a significant contribution from multiple inner Kohn-Sham orbitals that depends on the electronic structure and on the orbital symmetries of the molecule, as well as on the polarization and intensity of the external laser field. Calculated photoelectron angular distributions at different molecular orientations and in response to laser fields with different degrees of ellipticity further demonstrate the spatial dependency of the orbital ionization rates.

We investigate computationally a method for ultrafast preparation of alkali-metal atoms in their Rydberg states using a three-dimensional model potential in the single active electron approximation. By optimizing laser pulse shapes that can be generated with modern waveform synthesizers, we propose pulses for controlling the population transfer from the ground state to a preselected set of Rydberg states. Dynamical processes under the optimized pulses are shown to be much more complicated than in the traditional optical two-photon preparation of Rydberg states.

Using the ab initio computed potential energy surface for the electronic interaction of the ${\mathrm{MgH}}^{+}({}^1\mathrm{\Sigma }$) ion with the He(${}^{1}S$) atom, we calculate the relevant state-changing rotationally inelastic collision cross sections from a quantum treatment of the multichannel scattering problem. We focus on the quantum dynamics at the translationally low energies for the present partners discussed in earlier, cold ion trap experiments (see below), which we wish to model in detail. The corresponding state-changing rates computed between the lower rotational states of the molecular ion are employed to describe the time-evolution kinetics followed by recent experiments on Coulomb-crystallized ${\mathrm{MgH}}^{+}({}^1\mathrm{\Sigma }$), where the ions are rotationally cooled by micromotion tuning after being uploaded into the trap of He as a buffer gas. The present computational modeling of the final ion rotational temperatures in the experiments turns out to agree very well with the observations and points at a fast equilibration between rotational and thermal temperatures of the ions.

We investigate the interaction-driven instability of a quadratic band crossing arising for ultracold bosonic atoms loaded into a two-dimensional optical lattice. We consider the case when the degenerate point becomes a local minimum of both crossing energy bands such that it can support a stable Bose–Einstein condensate. A repulsive contact interaction among the condensed bosons induces a spontaneously time-reversal-symmetry broken superfluid phase and a topological gap is opened in the excitation spectrum. We propose two concrete realizations of the desired quadratic band crossing in lattices with either fourfold or sixfold rotational symmetries via suitable tuning of the unit cell leading to reduced Brillouin zones and correspondingly folded bands. In either case, topologically protected edge excitations are found for a finite system.

Quantum-mechanical linear-response theory is used to calculate the diffraction of light from a small hole in a thin flat screen. The field-induced dynamics of the charged particles (electrons) is obtained by modeling a screen without a hole as a two-level quantum well, with jellium behavior for the in-plane electron motion. Local-field corrections are calculated in a self-field approximation to a coupled-antenna theory. Particular attention is devoted to frequency resonance effects in the local field. A generalization to a screen with a hole is suggested, replicating the homogeneous jellium surface electron density by a space varying density in the vicinity of the hole. Quantum-mechanical expressions for the electric dipole moment $\mathbf{p}\left(\omega \right)$, the magnetic dipole moment $\mathbf{m}\left(\omega \right)$, and the electric quadrupole moment $\mathbf{Q}\left(\omega \right)$ of the so-called aperture current density are derived and the light scattering from these moments is studied. From the general theory results for $\mathbf{p}\left(\omega \right)$, $\mathbf{m}\left(\omega \right)$, and $\mathbf{Q}\left(\omega \right)$ in three cases are given: (i) no induced electron motion perpendicular to the plane of the screen [leading to $\mathbf{p}\left(\omega \right)=\mathit{0}]$, (ii) resonance excitation of the electron system, and (iii) a circular hole. This paper presents an extension of the quantum-mechanical diffraction theory developed in two recent papers of ours [J. Jung and O. Keller, Phys. Rev. A 90, 043830 (2014); Phys. Rev. A 92, 012122 (2015)].

The upside-down $-x^4$, $-x^6$, and $-x^8$ potentials with appropriate $\mathcal{PT}$-symmetric boundary conditions have real, positive, and discrete quantum-mechanical spectra. This paper proposes a macroscopic quantum-mechanical scattering experiment in which one can observe and measure these bound-state energies directly. An important subtlety of the scattering experiment is that the incoming (and outgoing) waves are not plane waves but rather must be carefully engineered so as to enforce the $\mathcal{PT}$-symmetry of the potential.

Nonadiabatic holonomic quantum computation uses non-Abelian geometric phases to implement a universal set of quantum gates that are robust against control imperfections and decoherence. Until now, a number of three-level-based schemes of nonadiabatic holonomic computation have been put forward, and several of them have been experimentally realized. However, all these works are based on the same class of nonadiabatic paths, which originates from the first nonadiabatic holonomic proposal. Here, we propose a universal set of nonadiabatic holonomic gates based on an extended class of nonadiabatic paths. We find that nonadiabatic holonomic gates can be realized with paths shorter than the known ones, which provides the possibility of realizing nonadiabatic holonomic gates with less exposure to decoherence. Furthermore, inspired by the form of this type of paths, we find a way to eliminate decoherence from nonadiabatic holonomic gates without resorting to redundancies.

It is important for performance studies in quantum technologies to analyze quantum circuits in the presence of noise. We introduce an error probability tensor, a tool to track generalized Pauli error statistics of qudits within quantum circuits composed of qudit Clifford gates. Our framework is compatible with qudit stabilizer quantum error-correcting codes. We show how the error probability tensor can be applied in the most general case, and we demonstrate an error analysis of bipartite qudit repeaters with quantum error correction. We provide an exact analytical solution of the error statistics of the state distributed by such a repeater. For a fixed number of degrees of freedom, we observe that higher dimensional qudits can outperform qubits in terms of distributed entanglement.

We present a family of correlation constraints that apply to all multipartite quantum systems of finite dimension. The size of this family is exponential in the number of subsystems. We obtain these relations by defining and investigating the generalized state inversion map. This map provides a systematic way to obtain local unitary invariants of degree two in the state and is directly linked to the shadow inequalities proved by Rains [IEEE Trans. Inf. Theory 46, 54 (2000)]. The constraints are stated in terms of linear inequalities for the linear entropies of the subsystems. For pure quantum states they turn into monogamy relations that constrain the distribution of bipartite entanglement among the subsystems of the global state.

Interqubit coupling and qubit connectivity in a processor are crucial for achieving high-fidelity multiqubit gates and efficient implementation of quantum algorithms. Typical superconducting processors employ relatively weak transverse interqubit coupling which is activated via frequency tuning or microwave drives. Here, we propose a class of multimode superconducting circuits which realize multiple transmon qubits with all-to-all longitudinal coupling. These “artificial molecules” directly implement a multidimensional Hilbert space that can be easily manipulated due to the always-on longitudinal coupling. We describe the basic technique to analyze such circuits, compute the relevant properties, and discuss how to optimize them to create efficient small-scale quantum processors with universal programmability.

In a combined theoretical and experimental study, we give evidence that the self-organized electric potential in tapered glass capillaries has the strength to focus a low-energy ion beam. Similar to Einzel lenses, the on-axis injected beam is focused by an axis-symmetric potential, generated by the charge accumulated in the insulating capillary. We argue that for capillaries with large aspect ratio, the mechanism responsible for the focusing in our experiment is different from the one shown in earlier experiments. We found that the potential inside the capillary had to reach about 70% of the extraction potential of the ion source in order to be strong enough to focus the beam through the capillary. With increasing injected current intensities, the transmitted current density is shown to increase up to a factor 10 with respect to the injected one. An original experimental setup is used to monitor the accumulated total charge in the capillary linking the latter to the transmitted fraction of the beam. This way, we can clearly identify the different stages of the transmission in real time, and in particular the Coulomb blocking, and explain why it occurred inevitable in this setup. The experimental data are corroborated by our simulations, which allow a valuable and comprehensive insight into the dynamics of the self-organized Coulomb potential. The latter controls the focusing effect and explains many features such as why the transmitted fraction increases with the injected intensity.

Critical dressing, the simultaneous dressing of two spin species to the same effective Larmor frequency, is a technique that can, in principle, improve the sensitivity to small frequency shifts. The benefits of spin dressing and thus critical dressing are achieved at the expense of generating a large (relative to the holding field $B_0$,) homogeneous oscillating field. Due to inevitable imperfections of the fields generated, the benefits of spin dressing may be lost from the additional relaxation and noise generated by the dressing field imperfections. In this analysis, the subject of relaxation and frequency shifts are approached with simulations and theory. Analytical predictions are made from a new quasiquantum model that includes gradients in the holding field $B_0={\omega }_0/\gamma$ and dressing field $B_1={\omega }_1/\gamma$ where $B_1$ is oscillating at frequency $\omega$. It is found that irreversible DC gradient relaxation can be canceled by an AC spin-dressing gradient in the Redfield regime. Furthermore, it is shown that there is no linear in $E$ frequency shift generated by gradients in the dressing field. The results are compared with a Monte Carlo simulation coupled with a fifth-order Runge-Kutta integrator. Comparisons of the two methods are presented as well as a set of optimized parameters that produce stable critical dressing for a range of oscillating frequencies $\omega ,$ as well as pulsed frequency modulation parameters for maximum sensitivity.

A way to considerably enhance terahertz radiation, emitted in the interaction of intense midinfrared laser pulses with atomic gases, in both the total energy and the electric-field amplitude is suggested. The scheme is based on the application of a two-color field consisting of a strong circularly polarized midinfrared pulse with wavelengths of $1.6–4\mu \mathrm{m}$ and its linearly or circularly polarized second harmonic of lower intensity. By combining the strong-field approximation for the ionization of a single atom with particle-in-cell simulations of the collective dynamics of the generated plasma, it is shown that the application of such two-color circularly polarized laser pulses may lead to an order-of-magnitude increase in the energy emitted in the terahertz frequency domain as well as in a considerable enhancement in the maximal electric field of the terahertz pulse. Our results support recently reported experimental and numerical findings.

Ionization can serve as a universal probe of excited-state dynamics in molecules, such as internal conversion, dissociation, and isomerization. These processes are of fundamental importance to a wide array of dynamics in biology, chemistry, and physics. In recent years, there has been significant debate about the relative merits of strong-field ionization (SFI), which involves multiphoton absorption, versus weak-field ionization (WFI), where a single photon is absorbed, as probes of these dynamics. SFI is advantageous because it uses wavelengths that are relatively easy to generate, and one can always ionize the molecule with sufficient intensity. However, for SFI it is difficult to calculate observables, such as the time-dependent ion yield, since the calculation of the ionization dynamics including multiphoton processes is computationally expensive and difficult to carry out for many molecular geometries. WFI has the advantage that calculations of observables are tractable. However, the generation and implementation of the appropriate wavelengths (photon energies) can be challenging, and the fixed energy of the probe can lead to technical complications in following the dynamics from excited states back down to the ground state. Here we present a quantitative comparison of the two approaches for following the excited-state dynamics of two molecules, diiodomethane and uracil. The combination of internal conversion and dissociation in these molecules provides an ideal comparison of WFI and SFI as a probe. We compare the measurements with calculations of the dynamics. Our work indicates that while SFI and WFI provide qualitatively similar information about the excited-state dynamics, only WFI results can be compared quantitatively with present-day calculations.

We propose a method to selectively populate a large-angular-momentum state of ultracold atoms (each with an orbital angular momentum $l\approx 2\hslash$) in the Mott regime of a two-dimensional optical lattice. This is done by periodically modulating the lattice amplitude and implementing an additional rotated rectangular lattice of shorter wavelength. The specific pulse sequences are designed using a four-level model for each well and are implemented sequentially. The results are confirmed with numerical simulations of the full Schrödinger equation. These methods are another step in constructing a modular toolbox of operations for creating higher orbital states in optical lattices.

We demonstrate the occurrence of a narrow transparency resonance, due to the combined action of a coherent mechanical pump and a quantum well, placed in an otherwise nonresonant, macroscopic optomechanical cavity. The presence of a quantum well creates a broad absorption dip at the mechanical resonance frequency in the output probe intensity profile. Subsequent application of mechanical driving gives rise to a very narrow transparency window at the line center. Further, we show that by varying the phase and strength of the external mechanical pump, this macroscopic hybrid optomechanical cavity can be rendered either a perfect absorber or completely transparent for the output fields. These features are illustrated by studying the probe transmission profile and the second-order sideband generation, for specific choice of parameters.