We present a combined experimental and theoretical study of the fragmentation of ${\mathrm{HDO}}^{2+}$ molecular ions produced by electron-impact ionization of ${\mathrm{HDO}}^{+}$ in the collision energy range 20–2500 eV. Experimental absolute partial inclusive cross sections for the production of ${\mathrm{OD}}^{+},{\mathrm{OH}}^{+}$, and ${\mathrm{O}}^{+}$ are reported and compared successfully to theoretical predictions. Ab initio methods are used to calculate the electron-impact ionization cross sections of the cationic ground state and first excited state leading to the first seven dicationic states. Dissociation probabilities of each channel are obtained by performing classical molecular dynamics on fitted dicationic potential energy surfaces. The predictive character of the theoretical modeling allows us to estimate that the nonmeasured dissociation channel giving a neutral oxygen atom contributes to 30% of the total ionization cross section. The isotopic ratio ${\mathrm{OD}}^{+}/{\mathrm{OH}}^{+}$ deduced from the experiment is $\left(3.1\pm 0.2\right)$ on average, constant in the 30–2500 eV energy range. The calculated isotopic ratio is found to be strongly dependent on the vibrational excitation of the target. Good agreement with the experimental value is obtained for a vibrational excitation corresponding to a temperature of about 2500 K, which is compatible with typical characteristics of electron cyclotron resonance (ECR) ion sources.

We consider a general problem of inelastic collision of particles interacting with power-law potentials. Using quantum-defect theory we derive an analytical formula for the energy-dependent complex scattering length, valid for arbitrary collision energy, and use it to analyze the elastic and reactive collision rates. Our theory is applicable for both universal and nonuniversal collisions. The former corresponds to the unit reaction probability at short range, while in the latter case the reaction probability is smaller than one. In the high-energy limit we present a method that allows us to incorporate quantum corrections to the classical reaction rate due to the shape resonances and the quantum tunneling.

The dynamics of Bose-Einstein condensates in asymmetric double wells is studied. We construct a two-mode model and analyze the properties of the corresponding phase-space diagram, showing in particular that the minimum of the phase-space portrait becomes shifted from the origin as a consequence of the nonvanishing overlap between the ground and excited states from which the localized states are derived. We further incorporate effective interaction corrections in the set of two-mode model parameters. Such a formalism is applied to a recent experimentally explored system, which is confined by a toroidal trap with radial barriers forming an arbitrary angle between them. We confront the model results with Gross-Pitaevskii simulations for various angle values finding a very good agreement. We also analyze the accuracy of a previously employed simple model for moving barriers, exploring a possible improvement that could cover a wider range of trap asymmetries.

We demonstrate and characterize an experimental technique to directly image the momentum distribution of a strongly interacting two-dimensional quantum gas with high momentum resolution. We apply the principles of Fourier optics to investigate three main operations on the expanding gas: focusing, collimation, and magnification. We focus the gas in the radial plane using a harmonic confining potential and thus gain access to the momentum distribution. We pulse a different harmonic potential to stop the rapid axial expansion, which allows us to image the momentum distribution with high resolution. Additionally, we propose a method to magnify the mapped momentum distribution to access interesting momentum scales. All these techniques can be applied to a wide range of experiments, and in particular to study many-body phases of quantum gases.

We introduce two types of toroidal metamaterials which are invisible to surface plasmon polaritons (SPPs) propagating on a metal surface. The former is a toroidal handlebody bridging remote holes on the metal surface: It works as a kind of plasmonic counterpart of electromagnetic wormholes. The latter is a toroidal ring lying on the metal surface: This bridges two disconnected metal surfaces, i.e., it connects a thin metal cylinder to a flat metal surface with a hole. Full-wave numerical simulations demonstrate that an electromagnetic field propagating inside these metamaterials does not disturb the propagation of SPPs at the metal surface. A multilayered design of these devices is proposed, based on effective medium theory for a set of reduced parameters: The former plasmonic analog of the electromagnetic wormhole requires homogeneous isotropic magnetic layers, while the latter merely requires dielectric layers.

Tunable optical solitons due to intrapulse stimulated Raman scattering are well known for their ultrashort pulse width, superb pulse quality, and broadband tunability. Consequently they are suitable for a variety of applications, especially multiphoton microscopy (MPM). Recent progress in MPM demands two-color or multicolor excitation to match the absorption peaks of multiple fluorophores. Here we propose a scheme to generate two-color solitons with tunable wavelength separation, based on the composition of a higher-order soliton. The wavelength of the most energetic soliton can be easily tuned through energy tuning, while qualitative analysis indicates that stretching the input pulse width raises soliton order and narrows the wavelength separation between the two solitons. Numerical simulations show that under realistic experimental conditions, wavelength-separation tuning of hundreds of nanometers can be achieved through prechirping the input pulse. This wavelength-separation-tunable two-color source may find application in various modalities of MPM.

The fact that closed timelike curves (CTCs) are permitted by general relativity raises the question as to how quantum systems behave when time travel to the past occurs. Research into answering this question by utilizing the quantum circuit formalism has given rise to two theories: Deutschian-CTCs (D-CTCs) and “postselected” CTCs (P-CTCs). In this paper the quantum circuit approach is thoroughly reviewed, and the strengths and shortcomings of D-CTCs and P-CTCs are presented in view of their nonlinearity and time-travel paradoxes. In particular, the “equivalent circuit model”—which aims to make equivalent predictions to D-CTCs, while avoiding some of the difficulties of the original theory—is shown to contain errors. The discussion of D-CTCs and P-CTCs is used to motivate an analysis of the features one might require of a theory of quantum time travel, following which two overlapping classes of alternate theories are identified. One such theory, the theory of “transition probability” CTCs (T-CTCs), is fully developed. The theory of T-CTCs is shown not to have certain undesirable features—such as time-travel paradoxes, the ability to distinguish nonorthogonal states with certainty, and the ability to clone or delete arbitrary pure states—that are present with D-CTCs and P-CTCs. The problems with nonlinear extensions to quantum mechanics are discussed in relation to the interpretation of these theories, and the physical motivations of all three theories are discussed and compared.

We present a device-independent quantum cryptography protocol for continuous variables. Our scheme is based on the Gottesman-Kitaev-Preskill encoding scheme whereby a qubit is embedded in the infinite-dimensional space of a quantum harmonic oscillator. The application of discrete-variable device-independent quantum key distribution to this encoding enables a continuous-variable analog. Since the security of this protocol is based on discrete variables we inherit by default security against collective attacks and, under certain memoryless assumptions, coherent attacks. We find that our protocol is valid over the same distances as its discrete-variable counterpart, except that we are able to take advantage of high efficiency commercially available detectors where, for the most part, only homodyne detection is required. This offers the prospect of closing the loopholes associated with Bell inequalities.

We investigate the possibility of enhancement of cooling a trapped ion by combining the electromagnetically induced transparency (EIT) effect with the standing-wave coupling. Our study shows that the quantum destructive interference which is caused by the EIT effect and the standing-wave coupling can cancel all the dominant heating effects if appropriate parameters are chosen. The analytical predictions and numerical simulations show that the final temperature can be much lower than the recoil energy. In addition, this fast-cooling scheme is robust against fluctuations of the strength of the laser beams, which makes it more feasible for experimental realization.

We investigate the thermodynamics of a spin-1 Bose gas with fixed magnetization including the quadratic Zeeman energy shift. Our calculations are based on the grand canonical description for the ideal gas and the classical field approximation for atoms with ferromagnetic and antiferromagnetic interactions. We confirm the occurrence of a double phase transition in the system that takes place due to two global constraints. We show analytically for the ideal gas how critical temperatures and condensed fractions are changed by a nonzero magnetic field. The interaction strongly affects the condensate scenario below the second critical temperature. The effect imposed by interaction energies becomes diminished in high magnetic fields where condensation of both ferromagnetic and antiferromagnetic atoms agrees with the ideal gas results.

Recently [Phys. Rev. Lett. 106, 093902 (2011)] it has been shown that $\mathcal{PT}\text{−}\text{symmetric}$ scattering systems with balanced gain and loss undergo a transition from $\mathcal{PT}\text{−}\text{symmetric}$ scattering eigenstates, which are norm preserving, to symmetry broken pairs of eigenstates exhibiting net amplification and loss. In the present work we derive the existence of an invariant nonlocal current which can be directly associated with the observed transition playing the role of an “order parameter.” The use of this current for the description of the $\mathcal{PT}\text{−}\text{symmetry}$ breaking allows the extension of the known phase diagram to higher dimensions incorporating scattering states which are not eigenstates of the scattering matrix.

A dissipative scheme is proposed to generate single- or two-mode squeezed states in a hybrid quantum system consisting of superconducting resonators and nitrogen-vacancy-center ensembles (NVEs). For the single-mode squeezing generation, we consider an array of linearly coupled superconducting resonators, each of which is magnetically coupled to a NVE. In addition, one of the resonators is coupled to a gap-tunable qubit. It is shown that at steady state all the resonators and NVEs in the array can enter into single-mode squeezed states via dissipation of the qubit. For the two-mode squeezing realization, we consider a pair of superconducting resonator chains, which are coupled to two gap-tunable qubits. We find that at steady state all the resonator pairs in the up and down chains will be steered into two-mode squeezed states via dissipation of the qubits. The present scheme has a remarkable feature: only one or two gap-tunable dissipative qubits are locally involved, and no squeezing resource is required.

Nonrecolliding tunneling free-state electron wave packets driven by low-frequency ultrashort laser pulses can help synthesize, as we show in this work, extremely short field waveforms lasting a tiny fraction of the field cycle with intensities orders of magnitude higher than the intensities of high-order-harmonic pulses.

We study the electromagnetically induced transparency (EIT) effect with two individually addressed four-level Rydberg atoms subjected to the interatomic van der Waals interaction. We derive an effectively atomic Raman transition model where two ladders of the usual Rydberg EIT setting terminating at the same upper Rydberg level of long radiative lifetime are turned into a Rydberg EIT $\lambda$ setup via two-photon transitions, leaving the middle levels of each ladder largely detuned from the coupling and probe laser beams. It can hence overcome the limits of applications for EIT with atoms of the ladder-type level configuration involving a strongly decaying intermediate state by inducing coherence between two ground states. By probing one of the atoms, we observe four doublets of absorption induced by the Autler-Townes splitting and the van der Waals interaction. In particular, we find that the location of the EIT center remains unchanged compared to the interatomic-interaction-free case, which demonstrates that the interference among the multiple transition channels is basically destructive. The EIT with controlled Rydberg-Rydberg interaction among a few atoms provides a versatile tool for engineering the propagation dynamics of light.

Quantum theory in a global spacetime gives rise to nonlocal correlations, which cannot be explained causally in a satisfactory way; this motivates the study of theories with reduced global assumptions. Oreshkov, Costa, and Brukner [Nat. Commun. 3, 1092 (2012)] proposed a framework in which quantum theory is valid locally but where, at the same time, no global spacetime, i.e., predefined causal order, is assumed beyond the absence of logical paradoxes. It was shown for the two-party case, however, that a global causal order always emerges in the classical limit. Quite naturally, it has been conjectured that the same also holds in the multiparty setting. We show that, counter to this belief, classical correlations locally compatible with classical probability theory exist that allow for deterministic signaling between three or more parties incompatible with any predefined causal order.

Quantum Gaussian states can be considered as the majority of the practical quantum states used in quantum communications and more generally in quantum information. Here we consider their properties in relation to the geometrically uniform symmetry, a property of quantum states that greatly simplifies the derivation of the optimal decision by means of the square root measurements. In a general framework of the $N$-mode Gaussian states we show the general properties of this symmetry and the application of the optimal quantum measurements. An application example is presented to quantum communication systems employing pulse position modulation. We prove that the geometrically uniform symmetry can be applied to the general class of multimode Gaussian states.

We describe protocols for passive atomic clocks based on quantum interrogation of the atoms. Unlike previous techniques, our protocols are adaptive and take advantage of prior information about the clock's state. To reduce deviations from an ideal clock, each interrogation is optimized by means of a semidefinite program for atomic state preparation and measurement whose objective function depends on the prior information. Our knowledge of the clock's state is maintained according to a Bayesian model that accounts for noise and measurement results. We implement a full simulation of a running clock with power-law noise models and find significant improvements by applying our techniques.

We have calculated the intrinsic electric dipole moments (EDMs) of ${}^{129}\mathrm{Xe}$, ${}^{171}\mathrm{Yb}$, ${}^{225}\mathrm{Ra}$ atoms induced by the nuclear Schiff moment (NSM). We use the coupled perturbed Hartree-Fock [CPHF] framework to perform this calculation. In this work, the effects of electron correlation on atomic EDMs are studied in CPHF framework by gradual inclusion of virtual orbitals of higher symmetries. Individual orbital contributions to the final magnitude of the EDM are presented. The results for the NSM-induced EDM for these atoms show a trend similar to the electron-nucleon tensor-pseudotensor (T-PT)-induced EDM, the study of which was performed elsewhere. This behavior can be attributed to the fact that the influence of virtual orbitals of higher symmetry on the magnitude of EDM is essentially the same for both T-PT and NSM-induced atomic EDMs. However, we also report in this paper, that the most dominant contribution to the atomic EDM of ${}^{129}\mathrm{Xe}$, ${}^{171}\mathrm{Yb}$, ${}^{225}\mathrm{Ra}$ atoms induced by the NSM arises from the $s_{1/2}\text{−}{p}_{3/2}$ pairings rather than the $s_{1/2}\text{−}{p}_{1/2}$ pairings, which were the dominantly contributing pairings for the electron-nucleon tensor-pseudotensor-induced atomic EDM.