Enantioselective orientation of chiral molecules excited by a shaped picosecond laser pulse and a delayed femtosecond pulse is considered. Using quantum-mechanical simulations, we demonstrate a strong field-free enantioselective orientation along the laser propagation direction. In addition, we use a classical model to reproduce the enantioselective orientation. Moreover, the analysis of the corresponding classical system allows understanding the qualitative features of the induced enantioselective orientation. The strong enantioselective orientation may be used for the separation of chiral enantiomers using inhomogeneous electrostatic fields.

We study the Lambertian illumination of dielectric media with monochromatic light by numerically solving Maxwell's equations in three dimensions. We compute the average energy density inside spherical and cubic objects with a uniform refractive index. The Lambertian illumination of objects with uniform refractive index reproduces the known enhancement of the average energy density equal to the third power of the refractive index if the surface is sufficiently textured. Furthermore, inhomogeneous refractive index distributions with scattering areas much smaller than the wavelength, of the order of the wavelength, and larger than the wavelength are studied. The average energy density inside these media is compared to effective medium theories. Our investigation shows that the average energy density enhancement inside scattering media with scatterers much smaller than the wavelength can be modeled accurately with effective medium approximations.

The Dicke model exhibits a variety of phase transitions. The quantum phase transition from the normal phase to the super-radiant phase is marked by a dramatic change in the scaling of the participation ratio. We find that the ground state in the super-radiant phase exhibits multifractality manifest in the participation ratio scaling as the square root of the full Hilbert-space dimension. The thermal phase transition temperature, for which an exact analytical expression may be obtained using the partition function, is strikingly captured by the mutual information between two spins. In the excited-state quantum phase transition within the super-radiant phase, there are two cutoff energies; the central energy bands between the lower and upper cutoff energies show distinctly different behaviors. While the level statistics of the central band is Wigner-Dyson, the lower and upper bands show mixed behavior that is closer to Poisson than Wigner-Dyson. This finding is corroborated with the aid of several eigenvector properties: von Neumann entanglement entropy between spins and bosons, the mean photon number, concurrence between two spins, and participation ratio.

We demonstrate that a circuit comprising two unstable LC resonators coupled via a gyrator supports an exceptional point of degeneracy (EPD) with purely real eigenfrequency. Each of the two resonators includes either a capacitor or an inductor with a negative value, showing a purely imaginary resonance frequency when not coupled to the other via the gyrator. With external perturbation imposed on the system, we show analytically that the resonance frequency response of the circuit follows the square-root dependence on perturbation, leading to possible sensor applications. Furthermore, the effect of small losses in the resonators has been investigated, and we show that losses lead to instability. In addition, the EPD occurrence and sensitivity are demonstrated by showing that the relevant Puiseux fractional power series expansion describes the eigenfrequency bifurcation near the EPD. The EPD has the great potential to enhance the sensitivity of a sensing system by orders of magnitude.

We present a classically tractable model that leads to optimized low-depth quantum circuits leveraging separable-pair approximations. The obtained circuits are well suited as a baseline circuit for emerging quantum hardware and can, in the long term, provide significantly improved initial states for quantum algorithms. The associated wave functions can be represented with linear memory requirement, which allows classical optimization of the circuits and naturally defines a minimum benchmark for quantum algorithms. In this work we employ directly determined pair-natural orbitals within a basis-set-free approach. This leads to accurate representation of the one- and many-body parts for weakly correlated systems and we explicitly illustrate how the model can be integrated into other quantum algorithms for stronger correlated systems.

We investigate a class of quantum error-correcting codes to correct errors on both qubits and higher-state quantum systems represented as qudits. These codes arise from an original graph-theoretic representation of sets of quantum errors. In this framework, we represent the algebraic conditions for error correction in terms of edge avoidance between graphs providing a visual representation of the interplay between errors and error-correcting codes. Most importantly, this framework supports the development of quantum codes that correct against a predetermined set of errors, in contrast to current methods. A heuristic algorithm is presented, providing steps to develop codes that correct against an arbitrary noisy channel. We benchmark the correction capability of reflexive stabilizer codes for the case of single-qubit errors by comparison to existing stabilizer codes that are widely used. In addition, we present two instances of optimal encodings: an optimal encoding for fully correlated noise, which achieves a higher encoding rate than previously known, and a minimal encoding for single-qudit errors on a four-state system..

In device-independent quantum key distribution (DIQKD) the security is not based on any assumptions about the intrinsic properties of the devices and the quantum signals but on the violation of a Bell inequality. We introduce a DIQKD scenario in which an optimal Bell inequality is constructed from the performed measurement data rather than fixing beforehand a specific Bell inequality. Our method can be employed in a general way, for any number of measurement settings and any number of outcomes. We provide an implementable DIQKD protocol and perform finite-size security key analysis for collective attacks. We compare our approach with related procedures in the literature and analyze the robustness of our protocol. We also study the performance of our method in several Bell scenarios as well as for random measurement settings.

Experiments on an electron-beam ion trap (EBIT) and calculations using flexible atomic code (FAC) are carried out to study the charge-state evolution from ${\mathrm{W}}^{5+}$ to ${\mathrm{W}}^{7+}$. The ${\mathrm{W}}^{7+}$ line at 574.47 nm is observed with an electron-beam energy of about 48 eV, which is far below the ionization potentials of ${\mathrm{W}}^{5+}$ (65 eV) and ${\mathrm{W}}^{6+}$ (122 eV). Multicharge-state collisional-radiative (CR) calculations for ${\mathrm{W}}^{5+}, {\mathrm{W}}^{6+}$, and ${\mathrm{W}}^{7+}$ are performed with level-to-level processes with configuration interaction (CI), including direct ionization, collision excitation, radiative recombination, charge exchange, radiative transition, and autoionization. The CI strongly influences the calculated ionization cross sections for metastable levels. The CR-simulated spectra agree well with the experiments, and the calculated effective ionization cross section for ${\mathrm{W}}^{6+}$ has the same trend as the available experimental data [M. Stenke et al., J. Phys. B: At. Mol. Opt. Phys. 28, 2711 (1995)]. The metastable levels (∼40 eV for ${\mathrm{W}}^{5+}$; ∼40 and ∼85 eV for ${\mathrm{W}}^{6+}$) significantly contribute to the ionization through excitation-autoionization at rather low energies (<50 eV) in the EBIT plasma. These metastable levels could have a considerable influence on the charge-state evolution of tungsten ions in edge fusion plasma.

Energy eigenvalues of ground and singly excited $1sns$ (${}^{1,3}S$) ($n=2-5$, being the principal quantum number) states of a He atom in a quantum dot have been investigated in detail by incorporating explicitly correlated Hylleraas-type wave functions in the framework of the Ritz variational method. The quantum dot environment is simulated by considering the influence of the finite oscillator (FO) potential. We have examined the behavior of different energy components contributing to the total energy. In this regard, the Hund's spin multiplicity rule for $1sns$ (${}^{1,3}S$) ($n=2-5$) states of a He atom has been examined in depth in terms of observed unusual ordering of the electron repulsion and total energy. The energy contribution due to the total correlation (in the presence of both radial and angular correlation) effect, the radial correlation limit and angular correlation limit of a He atom under different strengths of the FO potential have been studied. As a quantitative replication of our results, we have introduced and verified the Hellmann-Feynman theorem and virial theorem for both a He atom and its first ionization threshold, i.e., ${\mathrm{He}}^{+}$ ($1s,{}^{2}S$) ion under the influence of the FO potential. The expectation value of different radial quantities $r_1, r_1^2, r_{12},r_{12}^2, r_{<}=min(r_1,r_2),r_{>}=max(r_1,r_2)$, angular quantities, e.g., interparticle angles ${\theta }_1, {\theta }_{12}$, one-electron delta function $\delta \left({\vec{r}}_1\right)$, and two-electron delta function $\delta \left({\vec{r}}_{12}\right)$ have been determined for different strengths of the FO potential. The effect of the FO potential on the quantum similarity index and dissimilarity between the ${\mathrm{He}}^{+}$ ($1s,{}^{2}S$) and He ($1s^2,{}^{1}S$ and $1s2s,{}^{1,3}S$) as well as between He ($1s2s,{}^{1}S$) and He ($1s2s,{}^{3}S$) has been examined. The von Neumann, linear, and Shannon information entropy for the ground state He atom have been computed to ascertain the characteristic features of the electronic entanglement and the charge distribution under the FO potential.

We theoretically study orbital alignment in x-ray-ionized atoms and ions, based on improved electronic-structure calculations starting from the Hartree-Fock-Slater model. We employ first-order many-body perturbation theory to improve the Hartree-Fock-Slater calculations and show that the use of first-order-corrected energies yields significantly better transition energies than originally obtained. The improved electronic-structure calculations enable us also to compute individual state-to-state cross sections and transition rates and, thus, to investigate orbital alignment induced by linearly polarized x rays. To explore the orbital alignment of transiently formed ions after photoionization, we discuss alignment parameters and ratios of individual state-resolved photoionization cross sections for initially neutral argon and two exotic electronic configurations that may be formed during x-ray multiphoton ionization dynamics induced by x-ray free-electron lasers. We also present how the orbital alignment is affected by Auger-Meitner decay and demonstrate how it evolves during a sequence of one photoionization and one Auger-Meitner decay. Our present work establishes a step toward investigation of orbital alignment in atomic ionization driven by high-intensity x rays.

We theoretically investigate the magneto-optical rotation (MOR) effect in cesium atoms in the Voigt geometry, in which an off-resonance linearly polarized laser beam serves as both pump and probe. By calculating the detailed evolution of atomic multipole moments truncated to second-rank, alignment-to-orientation conversion (AOC) effects are observed in two hyperfine ground states. The mechanisms responsible for this effect are demonstrated. The tensor AC-Stark shift produced by the optical pumping generates a nonlinear effect, resulting in atomic alignment directly coupled to orientation, which enables spin orientation to be obtained. Simultaneously, spin-exchange collisions lead to atomic alignment and orientation transfer between two ground-state manifolds. Additionally, we present the analytical expression of atomic spin polarization described by atomic multipole moments, and the contributions of the AOC effect to the optical-rotation signals are discussed in different light power regimes. Our results can be helpful for guiding MOR experiments by refining and optimizing the parameters.

Out-of-time-order correlators (OTOCs) can be used to probe how quickly a quantum system scrambles information when the initial conditions of the dynamics are changed. In sufficiently large quantum systems, one can extract from the OTOC the quantum analog of the Lyapunov coefficient that describes the timescale on which a classical chaotic system becomes scrambled. OTOCs have been applied only to a very limited number of toy models, such as the Sachdev-Ye-Kitaev model connected with black hole information scrambling, but they could find much wider applicability for information scrambling in quantum systems that allow comparison with experiments. The vibrations of polyatomic molecules are known to undergo a transition from regular dynamics at low energy to facile energy flow at sufficiently high energy. Molecules therefore represent ideal quantum systems to study scrambling in many-body systems of moderate size (here 6 to 36 degrees of freedoms). By computing quantum OTOCs and their classical counterparts we quantify how information becomes “scrambled” quantum mechanically in molecular systems. Between early “ballistic” dynamics, and late “saturation” of the OTOC when the full density of states is explored, there is indeed a regime where a quantum Lyapunov coefficient can be defined for all molecules in this study. Comparison with experimental rate data shows that slow scrambling as measured by the OTOC can reach the timescale of molecular reaction dynamics. Even for the smallest molecules we discuss, the Maldacena bound remains satisfied by regularized OTOCs, but not by unregularized OTOCs, highlighting that the former are more useful for discussing information scrambling in this type of moderate-size quantum system.

The polarization beam splitter (PBS) is a crucial photonic element to separately extract transverse-electric and transverse-magnetic polarizations from propagating light fields. Here, we propose a concise, continuously tunable, and all-optical partial PBS in a vector optomechanical system which contains two orthogonal polarized cavity modes with degenerate frequency. The results show that one can manipulate the polarization states of different output fields by tuning the polarization angle of the pumping field and the system functions as a partial PBS when the pump laser polarizes vertically or horizontally. As a significant application of the tunable PBS, we propose a scheme of implementing quantum walks in resonator arrays without the aid of other auxiliary systems. Furthermore, we investigate the optomechanically induced Faraday effect in a vector optomechanical system which enables arbitrary tailoring of the input lights and the behaviors of polarization angles of the output fields in the undercoupling, critical coupling, and overcoupling regimes. Our findings prove the optomechanical system is a potential platform to manipulate the polarization states in multimode resonators and boost the process of applications related to polarization modulation.

Full-field imaging through scattering media is fraught with many challenges. Despite many achievements, current imaging methods are too slow to deal with fast dynamics, e.g., in biomedical imaging. We present an ultrafast all-optical method where a highly multimode self-imaging laser cavity is built around the reflective object to be imaged and the scattering medium. We show that the intracavity laser light from the object is mainly focused onto specific regions of the scattering medium where the phase variations are low. Thus, round-trip loss within the laser cavity is minimized, thereby overcoming most of the scattering effects. Our method can deal with temporal variations that occur on timescales as short as several cavity round trips, typically 100 ns in our laser cavity.

In this paper we consider a scheme for achieving long-time sustainable Bell states of two distant qubits mediated by a one-dimensional waveguide whose outputs are subjected to time-continuous photon counting or homodyne detection. In both of the detection cases, it is shown that different Bell states can be obtained, for different initial states, in the long-time regime. In particular, we find that, in the case of photon counting, a cyclic jump among Bell states can be formed once the first photon is registered. While in the homodyne-detection case we further reveal that any steady Bell state can be achieved independent of detection efficiency, with a probability of 50% . The underlying physical reason for this is also analyzed. Our scheme is advantageous over previous studies in which only transient or intermittent Bell states can be generated. The long-time Bell states of distant qubits can be used for constructing quantum networks.

Realizing universal topological quantum computers requires the manipulation of non-Abelian topological orders in a physical system, which presents great challenges. Conversely, the rapid development in circuit-based quantum computing offers a reliable quantum simulation approach to study these topological orders. The preliminary problem is how to identify distinct topological orders. Here, we develop a framework based on the quantum scattering circuit to directly and efficiently measure the modular transformation matrix, which is widely deemed as the fingerprint of a given topological order. The information of the modular transformation matrix is encoded in the probe qubit, and the readout merely requires single-qubit Pauli measurements. We further implement the scheme in a nuclear magnetic resonance quantum simulator to emulate the string-net model, where an Abelian ${\mathbb{Z}}_2$ toric code and a non-Abelian Fibonacci order emerge. In particular, the latter is predicted to be the simplest candidate for universal topological quantum computers. The two topological orders are unambiguously distinguished by the experimentally measured modular transformation matrices. As an experimental demonstration of a non-Abelian topological order with efficient readout, our work may open avenues toward investigating topological orders in circuit-based quantum simulators.

In electron-electron interactions in electromagnetic systems, retardation in the exchange of a virtual photon is essentially important as the first-order quantum electrodynamics correction. However, the retardation effect is generally so small that it is buried in unretarded electric and magnetic interactions and thus has yet to be directly probed. Here, we present a giant contribution of the retardation effect in an electron-electron interaction via observing strong electric-dipole-allowed radiative transition rates. The relative transition rates are obtained for two dominant radiative transitions from the $1s2s^{2}2p_{1/2}2p_{3/2}$ inner-shell excited state of boronlike tungsten and bismuth ions to $1s^{2}2s^{2}2p_{1/2}$ and $1s^{2}2s^{2}2p_{3/2}$, and it was found that the transition rate ratio between the two transitions is affected by the retardation effect up to more than 100%.

We report a precision measurement of a tune-out wavelength for ${}^{87}\mathrm{Rb}$ using circularly polarized light. A tune-out wavelength characterizes a zero in the electric polarizability of the atom. For circularly polarized light, the total polarizability depends on both the scalar and vector polarizability components. This shifts the location of the tune-out wavelength and makes it sensitive to different combinations of atomic dipole matrix elements than the scalar polarizability alone. Using ${\sigma }_{-}$ polarized light with a purity of 0.9931(1), we observe a tune-out wavelength of 785.1522(3) nm, which agrees with theoretical expectations when small contributions from the core electrons and off-resonant valence states are taken into account.

We analyze exact ground state (GS) separability in general $N$-particle systems with two-site couplings. General necessary and sufficient conditions for full separability in the form of one- and two-site eigenvalue equations are first derived. The formalism is then applied to a class of $\mathrm{SU}\left(n\right)$-type interacting systems where each constituent has access to $n$-local levels, and where the total number parity of each level is preserved. Explicit factorization conditions for parity-breaking GSs are obtained, which generalize those for $XYZ$ spin systems and correspond to a fundamental GS multilevel parity transition where the lowest $2^{n-1}$ energy levels cross. We also identify a multicritical factorization point with exceptional high degeneracy proportional to $N^{n-1}$, arising when the total occupation number of each level is preserved, in which any uniform product state is an exact GS. Critical entanglement properties (such as full range pairwise entanglement) are shown to emerge in the immediate vicinity of factorization. Illustrative examples are provided.