Using time-dependent density functional theory (TDDFT) calculations, we compare tunneling ionization of the $a{}^{4}F$ ground state and the $a{}^{6}D$ first excited state of vanadium in laser fields of intensities between 1.4 and $4.0\times {10}^{13}\mathrm{W}{\mathrm{cm}}^{-2}$. The calculated ionization yields of the ground state of vanadium were already shown to agree well with experimental results [Chu and Groenenboom, Phys. Rev. A 94, 053417 (2016)]. We find that the tunneling ionization rate of the sextet state is lower than that of the quartet state. This is surprising, since the ionization potential of the sextet is lower than that of the quartet state. This finding, however, is consistent with the experimental observation that niobium, whose ground state is $a{}^6{D}_{1/2}$, has a much smaller ionization yield than vanadium ($a{}^4{F}_{3/2}$), even though their ionization potentials are extremely close [Smits et al., Phys. Rev. Lett. 93, 213003 (2004)]. Our calculations demonstrate the existence of exchange blockade for the higher spin state. It arises from a strong field dynamic effect that mixes the highest and second highest electrons in the same set of unoccupied spin orbitals, which causes an isotropic attractive potential that confines the electrons close to the core.

Several recent experiments have reported a variety of new collective behaviors of two atoms in a cavity. An important experimental result was that the back-reaction in a high-quality cavity leads to subradiance rather than superradiance. Further investigation by changing the parameter domain has shown the possibility of hyperradiance. In this paper we present a model of hyperradiance which is based on the idea of vacuum Rabi splittings when the driving field is detuned from the atom or cavity resonance so that it hits one of the dressed states. We show that this hyperradiance is accompanied by significant nonclassicality of the output. Some aspects, but not nonclassicality, of the hyperradiance when the driving field is very weak can be understood semiclassically. Our mechanism of hyperradiance applies to atoms coupled symmetrically to the cavity mode, whereas in the work of M. Pleinert et al. [Optica 4, 779 (2017)] atoms have to be coupled almost asymmetrically to the cavity.

A detailed stability and bifurcation analysis of spin-polarized vertical-cavity surface-emitting lasers (VCSELs) is presented. We consider both steady-state and dynamical regimes. In the case of steady-state operation, we carry out a small-signal (asymptotic) stability analysis of the steady-state solutions for a representative set of spin-VCSEL parameters. Compared with full numerical simulation, we show this produces surprisingly accurate results over the whole range of pump ellipticity, and spin-VCSEL bias up to 1.5 times the threshold. We then combine direct numerical integration of the extended spin-flip model and standard continuation technique to examine the underlying dynamics. We find that the spin VCSEL undergoes a period-doubling or quasiperiodic route to chaos as either the pump magnitude or polarization ellipticity is varied. Moreover, we find that different dynamical states can coexist in a finite interval of pump intensity, and observe a hysteresis loop whose width is tunable via the pump polarization. Finally we report a comparison of stability maps in the plane of the pump polarization against pump magnitude produced by categorizing the dynamic output of a spin VCSEL from time-domain simulations, against supercritical bifurcation curves obtained by the standard continuation package auto. This helps us better understand the underlying dynamics of the spin VCSELs.

We report bound states in the radiation continuum (BICs) in a single infinitely long dielectric rod with periodically stepwise modulated permittivity alternating from ${\epsilon }_1$ to ${\epsilon }_2$. For ${\epsilon }_2=1$ in air the rod is equivalent to a stack of dielectric disks with permittivity ${\epsilon }_1$. Because of rotational and translational symmetries the BICs are classified by orbital angular momentum $m$ and the Bloch wave vector $\beta$ directed along the rod. For $m=0$ and $\beta =0$ the symmetry protected BICs have definite polarization and occur in a wide range of the radius of the rod and the dielectric permittivities. More involved BICs with $m\ne 0,\beta =0$ exist only for a selected radius of the rod at a fixed dielectric constant. The existence of robust Bloch BICs with $\beta \ne 0,m=0$ is demonstrated. Asymptotic limits to a homogeneous rod and to very thin disks are also considered.

We report the experimental observation of high-order sideband transitions at the single-photon level in a quantum circuit system of a flux qubit ultrastrongly coupled to a coplanar waveguide resonator. With the coupling strength reaching 10% of the resonator's fundamental frequency, we obtain clear signatures of higher order red-sideband and first-order blue-sideband transitions, which are mainly due to the ultrastrong Rabi coupling. Our observation advances the understanding of ultrastrongly coupled systems and paves the way to study high-order processes in the quantum Rabi model at the single-photon level.

We present the hypersatellite Auger spectrum of neon which contains the decay transitions of the $K^{-2}, K^{-2}V, K^{-2}{L}^{-1}, K^{-2}{L}^{-1}V$, and $K^{-1}{L}^{-1}V$ states. The Auger decays of these types of core-hole states show different line shapes and linewidths which can be distinguished due to the high experimental resolution available for the present experiments. The individual Auger transitions are assigned based on various approaches. Complementary calculations are performed to assign the $K^{-2}{L}^{-1}\to K^{-1}{L}^{-3}$ and the $K^{-2}{L}^{-1}V\to K^{-1}{L}^{-3}V$ Auger transitions. Based on the obtained results we were able to derive the ratio for the double to single ionization yield.

We study the generation of nonlocal correlations by atomic interactions in a pair of bimodal Bose-Einstein condensates in state-dependent potentials including spatial dynamics. The wave functions of the four components are described by combining a Fock state expansion with a time-dependent Hartree-Fock ansatz so that both the spatial dynamics and the local and nonlocal quantum correlations are accounted for. We find that despite the spatial dynamics, our protocol generates enough nonlocal entanglement to perform an Einstein-Podolsky-Rosen steering experiment with two spatially separated condensates of a few thousand atoms.

We study a squeezed vacuum field generated in hot Rb vapor via the polarization self-rotation effect. Our previous experiments showed that the amount of observed squeezing may be limited by the contamination of the squeezed vacuum output with higher-order spatial modes, also generated inside the cell. Here, we demonstrate that the squeezing can be improved by making the light interact several times with a less dense atomic ensemble. With optimization of some parameters we can achieve up to $-2.6$ dB of squeezing in the multipass case, which is a 0.6 dB improvement compared to the single-pass experimental configuration. Our results show that, other than the optical depth of the medium, the spatial mode structure and the cell configuration also affect the squeezing level.

Traditional optical frequency conversion model is well improved in this work. In terms of the dyadic Green's function method, a set of coupled-amplitude equations is reduced under a proposed transition layer assumption, accompanying the simultaneous integral equations. The model, as a generalization of the current frequency conversion theory, is aimed at any one-dimensional thin film or bulk nonlinear structure, allowing for arbitrary optical anisotropy and absorption without pumping and propagating limitations. The assumption reasonably simplifies the strict nonlinear boundary conditions and enables the equations to yield exact radiative field solutions. A field-enhanced phase-matching configuration is designed for second harmonic generation in a lossy $\epsilon$-near-zero material. The high contrast of refractive indices between a substrate (silicon) and the material traps the harmonic wave inside and constructs a natural mirror reflection waveguide. A simulation in the lowest guided mode predicts an efficiency enhancement proportional to the relative wave impedance to the fifth power under a resonant condition.

We systematically investigate the bistable behavior and squeezing property of the librational mode of a levitated nonspherical nanoparticle trapped by laser beams. By expanding the librational potential to the fourth order of the librational angle $\theta$, we find that the nonlinear coefficient of this mode is dependent only on the size and material of nanoparticle, but independent of trapping potential shape. The bistability and hysteresis are displayed when the driving frequency is red detuned to the librational mode. In the blue-detuned region, we have studied squeezing of the variance of librational mode in detail, which has potential application for measurement of angle and angular momentum.

We adopt a variational technique to study the dynamics of perturbed dissipative solitons whose evolution is governed by a Ginzburg-Landau equation (GLE). As a specific example of such solitons, we consider a silicon-based active waveguide in which free carriers are generated through two-photon absorption. In this case, dissipative solitons are perturbed by physical processes such as third-order dispersion, intrapulse Raman scattering, self-steepening, and free-carrier generation. To solve the variational problem, we adopt the Pereira-Stenflo soliton as an ansatz since this soliton is the exact solution of the unperturbed GLE. With this ansatz, we derive a set of six coupled differential equations exhibiting the dynamics of various pulse parameters. This set of equations provides considerable physical insight into the complex behavior of perturbed dissipative solitons. Its predictions are found to be in good agreement with direct numerical simulations of the GLE. More specifically, the spectral and temporal shifts of the chirped soliton induced by free carriers and intrapulse Raman scattering are predicted quite accurately. We also provide simple analytic expressions of these shifts by making suitable approximations. Our semianalytic treatment is useful for gaining physical insight into complex soliton-evolution processes.

We propose a method for protecting fragile quantum superpositions in many-particle systems from dephasing by external classical noise. We call superpositions “fragile” if dephasing occurs particularly fast, because the noise couples very differently to the superposed states. The method consists of letting a quantum superposition evolve under the internal thermalization dynamics of the system, followed by a time-reversal manipulation known as Loschmidt echo. The thermalization dynamics makes the superposed states almost indistinguishable during most of the above procedure. We validate the method by applying it to a cluster of spins ½.

Objectivity constitutes one of the main features of the macroscopic classical world. An important aspect of the quantum-to-classical transition issue is to explain how such a property arises from the microscopic quantum theory. Recently, within the framework of open quantum systems, there has been proposed such a mechanism in terms of the so-called spectrum broadcast structures. These are multipartite quantum states of the system of interest and a part of its environment, assumed to be under an observation. This approach requires a departure from the standard open quantum systems methods, as the environment cannot be completely neglected. In the present paper we study the emergence of such a state structure in one of the canonical models of the condensed-matter theory: the spin-boson model, describing the dynamics of a two-level system coupled to an environment made up by a large number of harmonic oscillators. We pay much attention to the behavior of the model in the non-Markovian regime, in order to provide a testbed to analyze how the non-Markovian nature of the evolution affects the surfacing of a spectrum broadcast structure.

We consider quantum algorithms for the unique sink orientation problem on cubes. This problem is widely considered to be of intermediate computational complexity. This is because there is no known polynomial algorithm (classical or quantum) for the problem and yet it arises as part of a series of problems for which it being intractable would imply complexity-theoretic collapses. We give a reduction which proves that if one can efficiently evaluate the $k\mathrm{th}$ power of the unique sink orientation outmap, then there exists a polynomial time quantum algorithm for the unique sink orientation problem on cubes.

Bulk diamond phonons have been shown to be a versatile platform for the generation, storage, and manipulation of high-bandwidth quantum states of light. Here we demonstrate a diamond quantum memory that stores, and releases on demand, an arbitrarily polarized $\sim 250$ fs duration photonic qubit. The single-mode nature of the memory is overcome by mapping the two degrees of polarization of the qubit, via Raman transitions, onto two spatially distinct optical phonon modes located in the same diamond crystal. The two modes are coherently recombined upon retrieval and quantum process tomography confirms that the memory faithfully reproduces the input state with average fidelity $0.784\pm 0.004$ with a total memory efficiency of $(0.76\pm 0.03)\%$. In an additional demonstration, one photon of a polarization-entangled pair is stored in the memory. We report that entanglement persists in the retrieved state for up to 1.3 ps of storage time. These results demonstrate that the diamond phonon platform can be used in concert with polarization qubits, a key requirement for polarization-encoded photonic processing.

Two different effects of the blackbody-radiation-induced atomic line broadening are compared. The first one (stimulated Raman scattering) has been discussed by many authors; the second one (quadratic level mixing) was discussed in our previous work. It is shown that the mixing effect gives the most significant contribution to the line broadening and it is indicated how to distinguish these two effects in laboratory experiments. The influence of the level mixing on the recombination history of primordial plasma is also discussed.

The release of the highly reactive radical ${\mathrm{OH}}^{+}$ from the fragmentation of water by electron impact is made mostly through the ${\mathrm{OH}}^{+}+{\mathrm{H}}^0$ channel. This channel ejects suprathermal ${\mathrm{OH}}^{+}$ ions with a kinetic energy distribution whose details are unexplored so far due to the difficulty in experimentally characterizing ions ejected with very low kinetic energy without another charged partner. These ions are studied here using the delayed extraction time-of-flight technique (DETOF). The structures and substructures in the kinetic energy distribution of ${\mathrm{OH}}^{+}$ associated with both single and double ionization are identified qualitatively and quantitatively. A comparison with the kinetic energy distribution of the complementary channel ${\mathrm{OH}}^0+{\mathrm{H}}^{+}$, also originating from vacancies in the $1b_2$ orbital, shows marked differences between the two, mainly regarding the relative role between the fragmentation involving the ${\mathrm{H}}_2{\mathrm{O}}^{+}$ ground state or via transitions to repulsive states.

A simple rate-equation description of the heating and cooling of antiparticle clouds under conditions typical of those found in antihydrogen formation experiments is developed and analyzed. We include single-particle collisional, radiative, and cloud expansion effects and, from the modeling calculations, identify typical cooling phenomena and trends and relate these to the underlying physics. Some general rules of thumb of use to experimenters are derived.

We study the quantum dynamics of the Bose-Hubbard model on a ladder formed by two rings coupled by the tunneling effect. By implementing the Bogoliubov approximation scheme, we prove that, despite the presence of the inter-ring coupling term, the Hamiltonian decouples in many independent sub-Hamiltonians ${\widehat{H}}_k$ associated with momentum-mode pairs $\pm k$. Each sub-Hamiltonian ${\widehat{H}}_k$ is then shown to be part of a specific dynamical algebra. The properties of the latter allow us to perform the diagonalization process, to find the energy spectrum and the conserved quantities of the model, and to derive the time evolution of important physical observables. We then apply this solution scheme to the simplest possible closed ladder, the double trimer. After observing that the excitations of the system are weakly populated vortices, we explore the corresponding dynamics by varying the initial conditions and the model parameters. Finally, we show that the inter-ring tunneling determines a spectral collapse when approaching the border of the dynamical-stability region.