A numerically cheap way to obtain structural information about clusters of rare gas atoms at low temperatures is developed. The semiclassical frozen-Gaussian imaginary-time propagator is extended such that it can account for the mean values of all interatomic distances in the cluster and their variances. To reduce the required numerical effort an approximation for the mean values is developed which preserves the quality of the results offered by the semiclassical ansatz. The method is applied to the ${\text{Ar}}_6$ cluster. It is found that the cluster dissociates almost in one step to six free atoms when the temperature is increased. Precursors of the dissociation are only observable in the distances of the atoms via the appearance of a second isomer. The process is almost classical. However, the method is able to resolve small differences in the temperatures at which the dissociation takes place and in the mean distances of the bound configuration.

In the free three-dimensional space, we consider a pair of identical $\uparrow$ fermions of some species or in some internal state and a pair of identical $\downarrow$ fermions of another species or in another state. There is a resonant $s$-wave interaction (that is, of zero range and infinite scattering length) between fermions in different pairs and no interaction within the same pair. We study whether this $2+2$ fermionic system can exhibit (as the $3+1$ fermionic system) a four-body Efimov effect in the absence of three-body Efimov effect, that is, the mass ratio $\alpha$ between $\uparrow$ and $\downarrow$ fermions and its inverse are both smaller than 13.6069…. For this purpose, we investigate scale invariant zero-energy solutions of the four-body Schrödinger equation, that is, positively homogeneous functions of the coordinates of degree $s-7/2$, where $s$ is a generalized Efimov exponent that becomes purely imaginary in the presence of a four-body Efimov effect. Using rotational invariance in momentum space, it is found that the allowed values of $s$ are such that $M\left(s\right)$ has a zero eigenvalue; here the operator $M\left(s\right)$, that depends on the total angular momentum $\ell$, acts on functions of two real variables (the cosine of the angle between two wave vectors and the logarithm of the ratio of their moduli), and we write it explicitly in terms of an integral matrix kernel. We have performed a spectral analysis of $M\left(s\right)$, analytical and for an arbitrary imaginary $s$ for the continuous spectrum and numerical and limited to $s=0$ and $\ell \le 12$ for the discrete spectrum. We conclude that no eigenvalue of $M\left(0\right)$ crosses zero over the mass ratio interval $\alpha \in [1;13.6069\cdots ]$, even if, in the parity sector ${(-1)}^{\ell }$, the continuous spectrum of $M\left(s\right)$ has everywhere a zero lower border. As a consequence, there is no possibility of a four-body Efimov effect for the $2+2$ fermions. We also enunciated a conjecture for the fourth virial coefficient of the unitary spin-$1/2$ Fermi gas, inspired from the known analytical form of the third cluster coefficient and involving the integral over the imaginary $s$ axis of $s$ times the logarithmic derivative of the determinant of $M\left(s\right)$ summed over all angular momenta. The conjectured value is in contradiction with the experimental results.

Point defects in self-assembled crystals, such as vacancies and interstitials, attract each other and form stable clusters. This leads to a phase separation between perfect crystalline structures and defect conglomerates at low temperatures. We propose a method that allows one to tune the effective interactions between point defects from attractive to repulsive by means of external periodic fields. In the quantum regime, this allows one to engineer strongly correlated many-body phases. We exemplify the microscopic mechanism by considering dipolar quantum gases of ground-state polar molecules and weakly bound molecules of strongly magnetic atoms trapped in a weak optical lattice in a two-dimensional configuration. By tuning the lattice depth, defect interactions turn repulsive, which allows us to deterministically design a novel supersolid phase in the continuum limit.

A thin conducting sheet—graphene, for example—transmits, absorbs, and reflects radiation. A sheet that is very thin, even vanishingly so, can still produce 50% absorption at normal incidence if it has conductivity corresponding to half the impedance of free space. We find that, regardless of the sheet conductivity, there exists a combination of polarization and angle of incidence that achieves this impedance-half-matching condition. If the conducting medium can be inverted, the conductivity is formally negative and the sheet amplifies the incident radiation. To the extent that a negative half-match in a thin sheet can be maintained, enormous single-pass gain in both transmission and reflection is possible. Known semiconductors (e.g., gallium nitride) have the optical properties necessary to give large amplification in a structure that is, remarkably, both thin and nonresonant.

Rydberg blockade at $n\sim 300$, is examined using strontium $n{}^{1}F_3$ Rydberg atoms excited in an atomic beam in a small volume defined by two tightly focused crossed laser beams. The observation of blockade for such states is challenging due to their extreme sensitivity to stray fields and the many magnetic sublevels associated with $F$ states which results in a high local density of states. Nonetheless, with a careful choice of laser polarization to selectively excite only a limited number of these sublevels, sizable blockade effects are observed on an $\sim 0.1$ mm length scale extending blockade measurements into the near-macroscopic regime and enabling study of the dynamics of strongly coupled many-body high-$n$ Rydberg systems under carefully controlled conditions.

Surprisingly often decoherence is due to classical fluctuations of ambient fields and may thus be described in terms of random unitary (RU) dynamics. However, there are decoherence channels where such a representation cannot exist. Based on a simple and intuitive geometric measure for the distance of an extremal channel to the convex set of RU channels we are able to characterize the set of true quantum phase-damping channels. Remarkably, using the Caley-Menger determinant, our measure may be assessed directly from the matrix representation of the channel. We find that the channel of maximum quantumness is closely related to a symmetric, informationally complete positive operator-valued measure on the environment. Our findings are in line with numerical results based on the entanglement of assistance.

We present methods for the direct characterization of quantum dynamics in which both the principal and ancilla systems undergo noisy processes. Using a concatenated error detection code, we discriminate between located and unlocated errors on the principal system in what amounts to filtering of ancilla noise. The example of composite noise involving amplitude damping and depolarizing channels is used to demonstrate the method, while we find the rate of noise filtering is more generally dependent on code distance. Our results indicate the accuracy of quantum process characterization can be greatly improved while remaining within reach of current experimental capabilities.

The dipole polarizabilities, dipole hyperpolarizabilities, quadrupole moments, and quadrupole polarizabilities of the $5s^2{}^{1}S$ and $5s5p{}^3{P}^o$ states of ${\mathrm{In}}^{+}$ and Sr are calculated by using the finite-field method. The electron correlation effect and the basis set convergence are investigated in the relativistic coupled-cluster and configuration interaction calculations in order to obtain polarizabilities of high accuracy. Comparative study of the fully and scalar relativistic calculations reveals the effect of the spin-orbit coupling on the dipole polarizabilities of ${\mathrm{In}}^{+}$ and Sr. The blackbody-radiation shifts of the clock transition $5s^2{}^1{S}_0-5s5fp{}^3{P}_0^o$ due to the dipole polarizabilities, quadrupole polarizabilities, and dipole hyperpolarizabilities are evaluated to be 0.017, $8.33\times {10}^{-10}$, and $1.93\times {10}^{-17}$ Hz for ${\mathrm{In}}^{+}$ and 2.09 and $5.82\times {10}^{-8}$, and $1.69\times {10}^{-15}$ Hz for Sr. The blackbody-radiation shifts from the quadrupole polarizabilities and dipole hyperpolarizabilities are significantly less than that from the dipole polarizabilities and therefore can be safely omitted for the quoted ${10}^{-18}$ uncertainty of the optical frequency standard of ${\mathrm{In}}^{+}$ and Sr.

Excitation energies, term designations, $g$ factors, transition rates, and lifetimes of ${\mathrm{U}}^{2+}$ are determined using a relativistic configuration interaction (CI) + linearized-coupled-cluster (LCC) approach. The CI-LCC energies are compared with CI + many-body-perturbation-theory (MBPT) and available experimental energies. Close agreement has been found with experiment, within hundreds of ${\mathrm{cm}}^{-1}$. In addition, lifetimes of higher levels have been calculated for comparison with three experimentally measured lifetimes, and close agreement has been found within the experimental error. CI-LCC calculations constitute a benchmark test of the CI + all-order method in complex relativistic systems such as actinides and their ions with many valence electrons. The theory yields many energy levels, $g$ factors, transition rates, and lifetimes of ${\mathrm{U}}^{2+}$ that are not available from experiment. The theory can be applied to other multivalence atoms and ions, which would be of interest to many applications.

The long-range interaction between an antiprotonic helium atom $\overline{p}{\mathrm{He}}^{+}$ and helium atom in its ground state is studied. We calculate the dispersion coefficients $C_6$ using the complex coordinate rotation (CCR) formalism in order to comply with the resonant nature of metastable states of the antiprotonic helium. We present as well numerical data on static dipole polarizabilities of antiprotonic helium states. The obtained coefficients $C_6$ may be used to estimate the collisional shift and broadening of transition lines in a low-density precision spectroscopy of the antiprotonic helium.

We compare the physical information contained in the Temkin-Poet (TP) scattering wave function representing electron-impact ionization of hydrogen, calculated by the convergent close-coupling (CCC) and generalized Sturmian function (GSF) methodologies. The idea is to show that the ionization cross section can be extracted from the wave functions themselves. Using two different procedures based on hyperspherical Sturmian functions we show that the transition amplitudes contained in both GSF and CCC scattering functions lead to similar single-differential cross sections. The single-continuum channels were also a subject of the present studies, and we show that the elastic and excitation amplitudes are essentially the same as well.

We present an accurate theoretical model for the charge dependence of kinetic energy transferred in collisions between slow highly charged ions (HCIs) and the atoms in a carbon solid. The model is in excellent agreement with experimental kinetic-energy-loss data for carbon nanomembrane and thin carbon foil targets. This study fills a notable gap in the literature of charged-particle energy loss in the regime of low incident velocity ($v_p\lesssim 2.188\times {10}^6$ m/s) where charge states greatly exceed the equilibrium values.

We theoretically study the control of photoelectron interference of atoms ionized by a two-color laser field with parallel polarizations. Based on both the quantum-trajectory Monte Carlo model and an ab initio simulation with numerically solving the time-dependent Schrödinger equation, we show that the photoelectron angular distributions can be controlled by changing the relative phase of the two frequency components. The high-energy and low-energy photoelectrons show different modulations with respect to the relative phase. Tracing back to the initial coordinates (the ionization time with respect to the laser phase and the initial momentum) of photoelectrons, we have studied the physical origin of the asymmetric structure in photoelectron angular distributions and the dynamics of controlling photoelectron emission using the two-color laser fields. We show that the long tail of the Coulomb potential plays a minor role in the position of the high-energy cutoff in the backward scattering, whereas it is of great importance in the formation of the asymmetric structures of the low-energy photoelectrons.

We propose a protocol for measurement of the phonon number distribution of a harmonic oscillator based on selective mapping to a discrete spin-1/2 degree of freedom. We consider a system of a harmonically trapped ion, where a transition between two long-lived states can be driven with resolved motional sidebands. The required unitary transforms are generated by amplitude-modulated polychromatic radiation fields, where the time-domain ramps are obtained from numerical optimization by application of the chopped random basis algorithm (CRAB). We provide a detailed analysis of the scaling behavior of the attainable fidelities and required times for the mapping transform with respect to the size of the Hilbert space. As one application we show how the mapping can be employed as a building block for experiments which require measurement of the work distribution of a quantum process.

Free-electron lasers (FELs) produce short and very intense light pulses in the XUV and x-ray regimes. We investigate the possibility to drive Rabi oscillations in xenon with an intense FEL pulse by using the unusually large dipole strength of the giant dipole resonance (GDR). The GDR decays within less than 30 as due to its position, which is above the $4d$ ionization threshold. We find that intensities around ${10}^{18}\mathrm{W}/{\mathrm{cm}}^2$ are required to induce Rabi oscillations with a period comparable to the lifetime. The pulse duration should not exceed 100 as because xenon will be fully ionized within a few lifetimes. Rabi oscillations reveal themselves also in the photoelectron spectrum in the form of Autler-Townes splittings extending over several tens of electronvolts.

Motivated by experiments exploring the physics of neutral atoms in artificial magnetic fields, we study the ground state of bosons interacting with long-range dipolar interactions on a two-leg ladder. We focus on the limit where the number of particles per site is large and the interactions are weak. Using two complementary variational approaches, we find rich physics driven by the long-range forces. Generically, long-range interactions tend to destroy the Meissner phase in favor of modulated density wave phases. For example, nearest-neighbor interactions produce an interleg charge density wave phase, where the total density remains uniform but the density on each leg of the ladder is modulating in space, out of phase with one another. Next-nearest-neighbor interactions lead to a fully modulated biased ladder phase, where all of the particles are on one leg of the ladder. This state simultaneously breaks $Z_2$ reflection symmetry and $\mathrm{U}\left(1\right)$ translation symmetry. For values of the flux near $\varphi =\pi$, we find a switching effect for arbitrarily weak interactions, where the density is modulated but the chiral current changes sign on every plaquette. Arbitrarily weak attractive interactions along the rungs destroy the Meissner phase completely, in favor of a modulated density wave phase. Remarkably, varying the rung to ladder hopping produces a cascade of first-order transitions between modulated density wave states with different wave vectors, which manifests itself as discrete jumps in the chiral current. Polarizing the dipoles along the ladder direction yields a region of phase space where a stable biased ladder phase occurs even at arbitrarily weak rung hopping. We discuss the experimental consequences of our work and draw connections between our work and recent experiments on cold atoms in synthetic dimensions.

This paper demonstrates the application of a polarization current approach to estimate the yield of soft x-ray Cherenkov radiation when a charged particle travels in the vicinity of a finite-size screen. To account for resonant anomalous dispersion of the complex permittivity, we apply the formalism for the atomic scattering factor. We derived the simple analytical expression for the radiation field, which allows us to determine all properties for a wide energy range of emitted radiation of a charged particle. The influence of screen sizes on Cherenkov radiation characteristics is determined by macroscopic and microscopic approaches.

We evaluate a method, based on the near-field properties of surface-plasmon polaritons, to significantly enhance the dissipative optomechanical backaction mechanism. Although the large momentum of the surface-plasmon-polariton modes leads to the enhanced sensitivity of the scattering to the mechanical displacement, the overall efficiency will not improve unless an optical antenna efficiently couples the plasmonic modes to the far field. The predicted improvements in both efficiency and bandwidth make this approach uniquely suitable for many new applications.

We present the total transmission effect of inhomogeneous electromagnetic waves between dissipative media. The total transmission of a $p$-polarized plane wave at the interface between two lossless dielectrics happens at the Brewster angle. However, when a dielectric-conductor interface is considered, the effect cannot be achieved and there is only a minimum, different from zero, of the reflection coefficient. We prove that, by considering an inhomogeneous plane wave, the total transmission can be obtained both at the interface between two dissipative media and at the dielectric-conductor interface.

The Manakov model is the simplest multicomponent model of nonlinear wave theory: It describes elementary stable soliton propagation and multisoliton solutions, and it applies to nonlinear optics, hydrodynamics, and Bose-Einstein condensates. It is also of fundamental interest as an asymptotic model in the context of the widely used wavelength-division-multiplexed optical fiber transmission systems. However, although its physical relevance was confirmed by the experimental observation of Manakov (vector) solitons in a planar waveguide in 1996, there have in fact been no quantitative experiments confirming its validity for nonlinear dynamics other than soliton formation. Here, we report experiments in optical fiber that provide evidence of passband and baseband polarization modulation instabilities in a defocusing Manakov system. In the spontaneous regime, we also reveal a unique saturation effect as the pump power increases. We anticipate that such observations may impact the application of this minimal model to describe and understand more complicated phenomena in nature, such as the formation of extreme waves in multicomponent systems.