We take a closer look at the fundamental Casimir-Polder (CP) interaction between quantum particles and dispersive dielectric surfaces with surface polariton or plasmon resonances. Linear response theory shows that in the near-field, van der Waals regime the free-energy shift of a particle contains a thermal component that depends exclusively on the excitation of the evanescent surface polariton (plasmon or phonon) modes. Our work makes evident the link between particle surface interaction and near-field thermal emission and demonstrates how this can be used to engineer Casimir-Polder forces. We also examine how the exotic effects of surface waves are washed out as the distance from the surface increases. In the case of molecules or excited-state atoms, far-field approximations result in a classical dipole-dipole interaction which depends on the surface reflectivity and the mean number of photons at the frequency of the atomic or molecular transition. Finally we present numerical results for the CP interaction between Cs atoms and various dielectric surfaces with a single polariton resonance and discuss the implications of temperature and retardation effects for specific spectroscopic experiments.

Dispersion interactions such as the van der Waals interaction between atoms or molecules derive from quantum fluctuations of the electromagnetic field and can be understood as the exchange of virtual photons between the interacting partners. Any modification of the environment in which those photons propagate will thus invariably lead to an alteration of the van der Waals interaction. Here we show how the two-body dispersion interaction inside a cylindrical waveguide can be made to decay asymptotically exponentially and how this effect sensitively depends on the material properties and the length scales of the problem, eventually leading to the possibility of controllable interactions. Further, we discuss the possibility to detect the retarded van der Waals interaction by resonant enhancement of the interaction between Rydberg atoms in the light of long-range potentials due to guided modes.

The effects induced during the process of inner-shell photoionization followed by Auger decay are investigated for the photoionization of Kr $3d_{5/2}$ in an extremely low photoelectron energy regime. Both the photoelectron energy and relative angular distributions are calculated with a recently developed quantum-mechanical method where all the Coulomb interactions between each pair of particles are fully considered. The photoelectron energy distribution is asymmetric with respect to the initial photoelectron energy, and its peak shifts to a lower energy. As the absorbed photon energy is reduced, the peak of the photoelectron energy distribution moves to the negative energy range. As a manifestation of these dynamics, peak structures are observed in the relative angular distribution. We also simulate the process with a classical-trajectory Monte Carlo method, obtaining similar results to the quantum-mechanical ones.

We develop the eikonal perturbation theory to describe the strong-field ionization by finite laser pulses. This approach in the first order with respect to the binding potential (the so-called generalized eikonal approximation) avoids a singularity at the potential center. Thus, in contrast to the ordinary eikonal approximation, it allows one to treat rescattering phenomena in terms of quantum trajectories. We demonstrate how the first Born approximation and its domain of validity follow from eikonal perturbation theory. Using this approach, we study the coherent interference patterns in photoelectron energy spectra and their modifications induced by the interaction of photoelectrons with the atomic potential. Along with these first results, we discuss the prospects of using the generalized eikonal approximation to study strong-field ionization from multicentered atomic systems and to study other strong-field phenomena.

We investigate the nonlinear optical features of a graphene sheet embedded in an active cavity and we show that, when tuned near its lasing threshold, the cavity is able to isolate the spatially localized graphene nonlinearity thus producing a very strong nonlinear device response with multivalued features. As opposed to standard situations where the small thickness of the graphene sheet hampers its remarkable nonlinear optical properties to be exploited, in our scheme the strong nonlinear optical regime is mainly triggered by the very intrinsic planar localization of graphene nonlinearity. The proposed strategy for exploiting graphene nonlinearity through its unleashing could open novel routes for conceiving ultraefficient nonlinear photonic devices.

We apply a simple analytical method based on a unitary transformation to calculate the Bloch-Siegert (BS) shift over the entire driving-strength range. In quantitative comparison with the numerically exact BS shift obtained by Floquet formalism as well as the previous BS results, we confirm that our calculated results are not only accurate in the weak-driving regime but also correct in the strong-driving limit. In the intermediate strong-driving regime, the calculated values of the BS shift are nearly the same as the exact ones. It turns out that our calculation for the BS shift is beyond perturbation. Meanwhile, we demonstrate the signatures caused by the BS shift by monitoring the excited-state population and the probe-pump spectrum under the experiment accessible conditions. In particular, we find that when the driving frequency is fixed at the transition frequency of the system, the line shape of the probe-pump spectrum becomes asymmetric with the increase of the driving strength, which may be verified experimentally.

The direct state measurement (DSM) based on the weak measurement has the advantage of simplicity, versatility, and directness. However, the weak measurement will introduce an unavoidable error in the reconstructed quantum state. We modify the DSM by replacing the weak coupling between the system and the pointer by a strong one, and present two procedures for measuring quantum states, one of which can give the wave function or the density matrix directly. We can also measure the Dirac distribution of a discrete system directly. Furthermore, we propose quantum circuits for realizing these procedures, and the main body of the circuits consists of Toffoli gates. By numerical simulation, we find that our scheme can eliminate the biased error effectively.

Repeat-until-success (RUS) circuits can approximate a given single-qubit unitary with an expected number of $T$ gates of about $\frac{1}{3}$ of what is required by optimal, deterministic, ancilla-free decompositions over the Clifford + $T$ gate set. In this work, we introduce a more general and conceptually simpler circuit decomposition method that allows for synthesis into protocols that probabilistically implement quantum circuits over several universal gate sets including, but not restricted to, the Clifford + $T$ gate set. The protocol, which we call probabilistic quantum circuits with fallback (PQF), implements a walk on a discrete Markov chain in which the target unitary is an absorbing state and in which transitions are induced by multiqubit unitaries followed by measurements. In contrast to RUS protocols, the presented PQF protocols are guaranteed to terminate after a finite number of steps. Specifically, we apply our method to the Clifford + $T$, Clifford + $V$, and Clifford + $\pi /12$ gate sets to achieve decompositions with expected gate counts of ${log}_b(1/\varepsilon )+O\{ln[ln(1/\varepsilon )]\}$, where $b$ is a quantity related to the expansion property of the underlying universal gate set.

Time-reversal symmetry breaking can enhance or suppress the probability of success for quantum state transfer (QST), and remarkably it can be used to implement the directional QST. In this paper we study the QST on a ring with time-reversal asymmetry. We show that the system will behave as a quantum state turnplate under some proper parameters, which may serve as time controlled quantum routers in complex quantum networks. We propose realizing the quantum state turnplate in the coupled resonator optical waveguide by controlling the coupling strength and the phase.

A coinless, discrete-time quantum walk possesses a Hilbert space whose dimension is smaller compared to the widely studied coined walk. Coined walks require the direct product of the site basis with the coin space; coinless walks operate purely in the site basis, which is clearly minimal. These coinless quantum walks have received considerable attention recently because they have evolution operators that can be obtained by a graphical method based on lattice tessellations and they have been shown to be as efficient as the best known coined walks when used as a quantum search algorithm. We argue that both formulations in their most general form are equivalent. In particular, we demonstrate how to transform the one-dimensional version of the coinless quantum walk into an equivalent extended coined version for a specific family of evolution operators. We present some of its basic, asymptotic features for the one-dimensional lattice with some examples of tessellations, and analyze the mixing time and limiting probability distributions on cycles.

We use the perturbed relativistic coupled-cluster (PRCC) theory to compute the electric dipole polarizabilities $\alpha$ of Zn, Cd, and Hg. The computations are done using the Dirac-Coulomb-Breit Hamiltonian with the Uehling potential to incorporate vacuum polarization corrections. To assimilate the self-energy corrections we use the model self-energy operator of Shabaev et al. [Phys. Rev. A 88, 012513 (2013)]. The triple excitations are included perturbatively in the PRCC theory and nonperturbatively in the unperturbed sector. Our results for $\alpha$ for all three elements are in excellent agreement with the experimental data. The other highlight of the results is the orbital energy corrections from Breit interactions. In the literature we could only get the data of Hg [E. Lindroth et al., J. Phys. B 22, 2447 (1989)], which are a near perfect match with our results. We also present the linearized equations of the cluster amplitudes, including the triple excitations, with the angular factors.

A collisional inhomogeneous broadening of the ${\text{H}}_2$ $Q\left(1\right)$ line perturbed by Ar was observed for the first time 25 years ago. Several attempts were made to explain the line broadening from ab initio calculations, which, however, resulted in fundamental discrepancies between the theory and experiment. To resolve this problem we investigate two possible sources of these differences. First, we repeat the ab initio calculations of the broadening and shifting speed dependence using in the scattering calculations a highly accurate ab initio ${\text{H}}_2\text{-Ar}$ interaction potential. Second, we replace the previous phenomenological models of velocity-changing collisions with a more physical one based on the interaction potential. This allows us to properly reproduce the experimental broadening over a wide range of temperatures and pressures. We show that this abnormal broadening is affected by strong competition between the velocity-changing collisions and speed-dependent collisional shifting, especially at high pressures.

The ability to coherently rearrange structures at the atomic scale is among the grand challenges of physical science. Some of the primary obstacles are nonadiabatic increases in energy, such as intramolecular vibrational relaxation (IVR) and electronic excitations. Motivated by recent advances in strong terahertz (THz) pulse generation, we investigate the potential of THz to circumvent these obstacles. Employing TDDFT-Ehrenfest simulations, we discover that strong THz pulses can drive isomerization of the LiNC molecule over barriers greater than 0.2 eV with very low ionization rates and, in the best case, less than 3 meV of residual excess energy. This work points to new possibilities in predictively manipulating chemical bonds in molecules and materials.

We solve the problem of tunneling ionization of a multielectron atom in a static electric field by using the Green's function for the Stark-Coulomb problem. This allows us to incorporate the outgoing-wave boundary conditions at infinity. The interaction of the active electron with the atomic residue is described either by a model potential or by the $l$-dependent pseudopotential which prevents virtual transitions to orbitals occupied by inner electrons. The method works well in the broad range of electric fields including the region above the classical ionization threshold (the barrier-suppression region). Calculations of ionization of Ar demonstrate a noticeable difference between the model potential approach and the pseudopotential approach, but both sets of results agree with experimental data.

Electron correlation can break the symmetry of photoelectron angular distribution upon two-step ionization to a doubly degenerate ionic state via a doubly degenerate intermediate state. The interference between the two components of the intermediate state prepared using linearly and circularly polarized light is discussed for cyclopropane as an example.

Initial insight into time delay in Mn photoionization in the region of the $3p\to ,3d$ giant autoionization resonance is gained in the framework of the “spin-polarized” random-phase approximation with exchange. The dramatic effect of the giant autoionization resonance on the time delay of photoemission from the $3d$ and $4s$ valence subshells of the Mn atom is unraveled. Strong sensitivity of the time delay of the $4s$ photoemission to the final-state term of the ion remainder $[{\mathrm{Mn}}^{+}\left(4s^1{,}^{5}S\right)$ vs ${\mathrm{Mn}}^{+}(4s^1,{}^{7}S)]$ is discovered. It is shown that photoionization time delay in the autoionizing resonance region is explicitly associated with the resonance lifetime, which can thus be directly measured in attosecond time-delay experiments. Similar features are expected to emerge in photoionization time delays of other transition-metal and rare-earth atoms with half-filled subshells that possess giant autoionization resonances as well.

A Ramsey-Raman interferometer is used to realize a compact force sensor using cold ${}^{87}\mathrm{Rb}$ atoms trapped in a vertical optical lattice. The sensitivity and accuracy of the force measurements are discussed and the limits in short-term sensitivity evaluated. We reach a relative sensitivity on the Bloch frequency and thus on the gravity acceleration of $3.9\times {10}^{-6}$ at 1 s. We perform an experimental study of the influence of the transverse confinement onto the decay of the interferometer contrast and compare the measurements with a simple semiclassical model. It is shown that vertical gradients of the trapping potential can contribute significantly to the loss of contrast.

We demonstrate the realization of topological band structures by exploiting the intrinsic spin-orbit coupling of dipolar interactions in combination with broken time-reversal symmetry. The system is based on polar molecules trapped in a deep optical lattice, where the dynamics of rotational excitations follows a hopping Hamiltonian which is determined by the dipolar exchange interactions. We find topological bands with Chern number $C=2$ on the square lattice, while a very rich structure of different topological bands appears on the honeycomb lattice. We show that the system is robust against missing molecules. For certain parameters we obtain flat bands, providing a promising candidate for the realization of hard-core bosonic fractional Chern insulators.