Experimental cross sections for $m$-fold photodetachment ($m=2–5$) of oxygen anions via $K$-shell excitation and ionization were measured in the photon-energy range of 525–1500 eV using the photon-ion merged-beams technique at a synchrotron light source. The measured cross sections exhibit clear signatures of direct double detachment, including double $K$-hole creation. The shapes of the double-detachment cross sections as a function of photon energy are in accord with Pattard's [J. Phys. B 35, L207 (2002)] empirical scaling law. We have also followed the complex de-excitation cascades that evolve subsequently to the initial double-detachment events by systematic large-scale cascade calculations. The resulting theoretical product charge-state distributions are in good agreement with the experimental findings.

We show that in the presence of an extreme terahertz (THz) field an additional weak field in the optical frequency range can modulate ionization probability and thereby generate high-order harmonics. The extreme THz pump suppresses recollisions, thereby also fully suppressing the recollision-based harmonics. Nevertheless, high-order harmonics are effectively generated via an alternative mechanism, the so-called Brunel mechanism, based on ionization and subsequent acceleration in the driving field (but not on reabsorption). Using ab initio simulations for the hydrogen atom and short few-cycle optical driver, we show the appearance of a coherent carrier-envelope-phase-insensitive supercontinuum formed by such harmonics, compressible into an isolated pulse with 100-attosecond-scale duration.

A photodissociation of interstitial ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ in single-crystalline Si by a laser field at 532 nm is reported. It is shown that in the course of Raman scattering measurements with a power density of around $2\times {10}^6$ W/${\mathrm{cm}}^2$ the ${\mathrm{H}}_2({\mathrm{D}}_2$) signal decreases with a characteristic time of a few minutes. At temperatures below 100 K the molecule transforms into a metastable state known as ${\mathrm{H}}_2^{*}$, which includes one hydrogen atom located at the bond-centered site and the second one at the antibonding position along the same $\langle 111\rangle$ direction. At temperatures above 100 K both dimers dissociate, whereby the products of fragmentation, inaccessible for the Raman scattering, are proposed to be atomic interstitials.

We reveal the existence of polarizing quantum phases for the enantiomers of cold, interacting chiral molecules in an optical helicity lattice by means of an extended Bose-Hubbard model. These recently proposed lattices have sites with alternating helicity which exert a discriminatory force on chiral molecules with different handedness. In our study of the phase diagram we find that a strong dipolar repulsion between molecules results in the separation of left and right enantiomers.

In this article, we study the formation of the bound states in the continuum (BICs) in a two-channel Fano-Anderson model. We employ the Green's function formalism, together with the equation of motion method, to analyze the relevant observables, such as the transmission coefficient and the density of states. Most importantly, our results show that the system hosts true BICs for the case of a symmetric configuration with degenerate impurity levels, and a complete transmission channel is then suppressed. Finally, we argue that the proposed mechanism could be relevant for the realization of BICs in electronic and photonic systems.

We revisit the idea of quantum nondemolition measurement (QND) of optical quanta via a resonantly enhanced Kerr nonlinearity taking into account quantum backaction. We show that the monolithic microcavities enable QND measurement of the number of quanta in a weak signal field using a classical probe field spatially overlapping with the signal. The phase of the probe field acquires information about the signal number of quanta without altering it due to the cross-phase modulation effect. We find the exact solution to the Heisenberg equations of motion of this system and calculate the measurement error, accounting for the optical losses in the measurement path. We identify a realistic approximation to obtain the explicit form of the final conditional quantum state of the signal field, accounting for the undesirable self-phase modulation effect and designing the optimal homodyne measurement of the probe beam to evade this effect. We show that the best modern monolithic microcavities allow achieving the measurement imprecision several times better than the standard quantum limit.

Tripartite mutual information (TMI) is an efficient observable to quantify the ability of a scrambler for the unitary time-evolution operator with a quenched many-body Hamiltonian. In this paper we give numerical demonstrations of the TMI in disorder-free (translationally invariant) spin models with three-body and four-body multiple-spin interactions. The dynamical behavior of the TMI of these models does not exhibit a linear light cone for sufficiently strong interactions. In early-time evolution, the TMI displays a distinct negative increase fitted by a logarithmiclike function. This is in contrast to the conventional linear light-cone behavior present in the $XXZ$ model and its near-integrable vicinity. The late-time evolution of the TMI in finite-size systems is also numerically investigated. The multiple-spin interactions make the system nearly integrable and weakly suppress the spread of information and scrambling. The observation of the late-time value of the TMI indicates that the scrambling nature of the system changes by interactions and this change can be characterized by a phase-transition-like behavior of the TMI, reflecting the system's integrability and violating the eigenstate thermalization hypothesis.

In terms of the metric-adjusted skew information (an important and versatile class of quantum Fisher information), which generalizes the seminal Wigner-Yanase skew information arising naturally from the study of quantum measurement, we propose a family of coherence measures of states relative to quantum channels, and reveal their basic properties such as unitary covariance, convexity, and monotonicity. Furthermore, we evaluate these coherence measures of states relative to several prototypical quantum channels, and make a comparative study for this family of coherence measures with relative entropy of coherence. This provides a general approach to coherence of states relative to quantum channels, which also captures decoherence on the states caused by quantum channels and asymmetry of states relative to quantum channels.

Regarding microwave frequency standards based on trapped ions, we report on the determination of the hyperfine splittings and the Landé $g_J$ factors of ${}^{111,113}\mathrm{Cd}^{+}$. The hyperfine splittings and Landé $g_J$ factors of the $5s{}^2{S}_{1/2}$ and $5p{}^2{P}_{1/2,3/2}$ states of ${}^{111,113}\mathrm{Cd}^{+}$ are calculated using the multiconfiguration Dirac-Hartree-Fock scheme. Furthermore, the hyperfine splittings of the $5p{}^2{P}_{3/2}$ state of ${}^{111,113}\mathrm{Cd}^{+}$ are derived based on the laser-induced fluorescence spectroscopy. The ${\mathrm{Cd}}^{+}$ ions are confined in a linear Paul trap and sympathetically cooled by laser-cooled ${\mathrm{Ca}}^{+}$ ions. The calculated values and the derived values are cross-checked, thereby guaranteeing the reliability of our results. The results provided in this paper can improve the signal-to-noise ratio of the clock transition and the accuracy of the second-order Zeeman shift correction, thus further improving the stability and accuracy of the microwave frequency standards based on trapped ${}^{111,113}\mathrm{Cd}^{+}$ ions.

We describe a quantum droplet of a Bose-Bose mixture squeezed by an external harmonic force in one spatial direction. Our approach is based on the self-consistent method formulated in Zin et al. (arXiv:2202.10109) The true spatial droplet profile in the direction of confinement is accounted for; however, the local density approximation is assumed in the free directions. We define a numerical approach to find the beyond-mean-field contribution to the chemical potential (Lee-Huang-Yang chemical potential)—the quantity that determines the droplet's profile. In addition to the numerical approach, we find the Lee-Huang-Yang potential in the analytic form in two limiting cases: a perturbative result for a strong confinement and a semiclassical expression when confinement is very weak.

In a lossless periodic structure, a bound state in the continuum (BIC) is characterized by a real frequency and a real Bloch wave vector for which there exist waves propagating to or from infinity in the surrounding media. For applications, it is important to analyze the high-$Q$ resonances that either exist naturally for wave vectors near that of the BIC or appear when the structure is perturbed. Existing theories provide quantitative results for the complex frequency (and the $Q$ factor) of resonant modes that appear or exist due to structural perturbations or wave vector variations. When a periodic structure is regarded as a periodic waveguide, eigenmodes are often analyzed for a given real frequency. In this paper, we consider periodic waveguides with a BIC and study the eigenmodes for a given real frequency near the frequency of the BIC. It turns out that such eigenmodes near the BIC always have a complex Bloch wave number, but they may or may not be leaky modes that radiate out power laterally to infinity. These eigenmodes can also be the so-called complex modes that decay exponentially in the lateral direction. Our study is relevant for applications of BICs in periodic optical waveguides, and it is also helpful for analyzing photonic devices operating near the frequency of a BIC.

This work reports on light scattering by a homogeneous dielectric sphere with a periodically time-varying radius. The off-shell inelastic scattering $T$ matrix, which describes the dynamically changing particle, is evaluated using the Floquet method, and some remarkable phenomena, emerging in the strong- and weak-coupling regimes, are discussed. In particular, the limits of validity of the approximate quasistatic solution are established through comparison with the results of fully dynamic calculations, and the scattering in the strong-coupling regime is analyzed in terms of the general behavior of parametrically driven oscillators. Additionally, the influence of damping of the sphere vibrations on the optical spectra is also investigated.

We investigate the dynamical evolution of the spontaneous emission of a two-level quantum emitter near a graphene nanodisk. We employ the macroscopic quantum electrodynamics methodology to study the reversible population dynamics of the excited state of the quantum emitter in the non-Markovian limit. Our results indicate that the quantum-emitter–nanodisk interaction enters the strong light-matter coupling regime, as manifested in the excited-state probability where Rabi oscillations and population trapping effects are observed. The level of non-Markovianity is estimated by computing three different well-established measures and relate them to the quantum speedup due to the strong-coupling interaction of the emitter with the graphene nanodisk. Importantly, high values of the non-Markovianity and quantum speedup measures are achieved when the emitter-nanodisk interaction is strong, whereas decreasing coupling strength leads to smaller values.

Quantum tunneling is a fundamental quantum-mechanical effect involved in plenty of physical phenomena. Its control would impact these phenomena and the technologies based on them. We show that the quantum tunneling probability through a potential barrier can be increased to approach unity in a quantum Zeno dynamics undergone by the tunneling particle in which the direction of the momentum is frequently monitored. We first model the measurements of the momentum direction as selective von Neumann projections, and then as nonselective, direction-sensitive interactions of the particle with probe particles. Nonselective measurements are more efficient than selective measurements in enhancing the quantum tunneling probability.

Machine learning techniques are increasingly being used in fundamental research to solve various challenging problems. Here we explore one such technique to address an important problem in the quantum communication scenario. While transferring quantum information through a noisy quantum channel, the utility of the channel is characterized by its quantum capacity. Quantum channels, however, display an intriguing property called superadditivity of coherent information. This makes the calculation of quantum capacity a hard computational problem involving optimization over an exponentially increasing search space. In this work, we first utilize a neural network Ansatz to represent quantum states, and then we apply an evolutionary optimization scheme to address this problem. We find regions in the three-parameter space of qubit Pauli channels where coherent information exhibits this superadditivity feature. We characterized the quantum codes that achieve high coherent information, finding several nontrivial quantum codes that outperform the repetition codes for some Pauli channels. For some Pauli channels, these codes display very high superadditivity of the order of 0.01, much higher than the observed values in other well-studied quantum channels. We further compared the learning performance of the neural network Ansatz with the raw Ansatz to find that in the three-shot case, the neural network Ansatz outperforms the raw representation in finding quantum codes of high coherent information. We also compared the learning performance of the evolutionary algorithm with a simple particle swarm optimization scheme, and we show empirical results indicating comparable performance, suggesting that the neural network Ansatz coupled with the evolutionary scheme is indeed a promising approach to finding nontrivial quantum codes of high coherent information.

Transitions of many-particle quantum systems between distinct phases at absolute-zero temperature, known as quantum phase transitions, require an exacting treatment of particle correlations. In this work, we present a general quantum-computing approach to quantum phase transitions that exploits the geometric structure of reduced density matrices. While typical approaches to quantum phase transitions examine discontinuities in the order parameters, the origin of phase transitions—their order parameters and symmetry breaking—can be understood geometrically in terms of the set of two-particle reduced density matrices (2-RDMs). The convex set of 2-RDMs provides a comprehensive map of the quantum system including its distinct phases as well as the transitions connecting these phases. Because 2-RDMs can potentially be computed on quantum computers at nonexponential cost, even when the quantum system is strongly correlated, they are ideally suited for a quantum-computing approach to quantum phase transitions. We compute the convex set of 2-RDMs for a Lipkin-Meshkov-Glick spin model on IBM superconducting-qubit quantum processors. Even though computations are limited to few-particle models due to device noise, comparisons with a classically solvable 1000-particle model reveal that the finite-particle quantum solutions capture the key features of the phase transitions including the strong correlation and the symmetry breaking.

We introduce a technique to store and manipulate single x-ray photons, which relies on dynamically controlled absorption via nuclear hyperfine magnetic splitting. This scheme is inherently suitable for storage, on-demand generation, and dynamical manipulation of single x-ray photons, for instance, the manipulation of the temporal shape, temporal splitting, the interference between x-ray photons, and the control of the polarization. Our approach provides more opportunities in x-ray quantum information.

Quantum-key-distribution protocols are known to be vulnerable against a side channel attack that exploits the time difference in detectors' responses used to obtain key bits. The recommended solution against this timing side channel attack is to use a large time bin width instead of high-resolution timing information. A common notion is that using a large bin width reduces the resolution of detectors' responses, hence supposedly minimizes the information leakage to an eavesdropper. We challenge this conventional wisdom and demonstrate that increasing the bin width does not monotonically reduce the mutual information between the key bits and the eavesdropper's observation of detectors' responses. Instead of randomly increasing the bin width, it should be carefully chosen because the mutual information fluctuates with respect to the bin width. We also examine the effect of full width half maximums (FWHMs) of the detectors' responses on the mutual information and show that decreasing the FWHM increases the mutual information. Lastly, the start time of binning is also shown to be important in the binning process and the mutual information fluctuates periodically with respect to it.

Strong perturbation caused due to the Coulomb interaction with fast highly charged ions has been used to probe the quadrupole plasmon excitation in ${\mathrm{C}}_{60}$ fullerene. The electron-emission spectrum arising from a free ${\mathrm{C}}_{60}$ molecule reveals a double-peak structure due to the decay of the giant plasmon resonance. Using a state-of-the-art theoretical approach describing the electron dynamics by time-dependent density functional theory, with an explicit ionic background these two peaks are unambiguously identified as arising from the dipole and quadrupole modes of the plasmon. Furthermore, the electron angular distribution clearly shows a double-well-type distribution which is well reproduced by quadrupole and plasmon resonances. The dipole and quadrupole excitations combined with a long-range postcollisional Coulomb interaction shows excellent agreement with the observed anisotropic angular distribution. The present paper serves as an independent confirmation of the presence of a substantial quadrupole plasmon contribution in molecular systems.