Vibronic coupling plays a pivotal role in molecular spectroscopy. We present a theoretical study on vibrationally resolved x-ray photoelectron spectra (XPS) of seven azines (${\mathrm{C}}_x{\mathrm{H}}_y{\mathrm{N}}_z$; pyridine, three diazines, two triazines, and one tetrazine) at the nitrogen $1s$ edge, to explore the vibronic coupling effects as influenced by consecutive replacement of the CH group with a N atom. Franck-Condon simulations were performed with the Duschinsky rotation effect included, where the electronic structure was calculated by the density functional theory. Validations on pyrimidine show good agreement with the experiment, weak functional dependence, and weak mode mixing effect. We observed an evident blue shift in binding energies with the increasing number of N atoms in this series, together with molecule-dependent vibronic fine structures. These molecules have either ${\mathrm{C}}_{\text{2v}}$ or ${\mathrm{C}}_{\text{s}}$ molecular symmetry at the optimized core-ionized geometries. Franck-Condon-active vibrational modes were identified to be low frequency (500–1650 ${\mathrm{cm}}^{-1}$), totally symmetric ($A_1$ or $A^{\prime }$), in-plane ring deformation modes. Core ionization on ${\mathrm{N}}^{*}$ always leads to elongation of the ${\mathrm{N}}^{*}-\mathrm{N}$ bond length, accompanied by an increase of the $\angle \mathrm{C}-{\mathrm{N}}^{*}-X$ bond angle ($X=\mathrm{C}$, N). Our study predicts accurate theoretical reference spectra for the azine family and provides useful information on the properties of the core-ionized states as influenced by the structural change of $\mathrm{CH}\leftrightarrow \mathrm{N}$ replacement.

We study bosonic systems in the regime in which the two-body system has a shallow bound state or, equivalently, a large value of the two-body scattering length. Using the effective field theory framework as a guidance, we construct a series of potential terms which have a decreasing importance in the description of the binding energy of the systems. The leading-order potential terms consist in a two-body term, usually attractive, plus a three-body term, usually repulsive; this last term is required to prevent the collapse of systems with more than two particles. At this order, the parametrization of the two-body potential is done to obtain a correct description of the scattering length, which governs the dynamics in this regime, whereas the three-body term fixes a three-body datum. We investigate the role of the cutoff in the leading-order description, and we extend the exploration beyond the leading order by including the next-to-leading-order terms in both the two- and three-body potentials. We use the requirement of the stability of the $N$-body system, whose energy is variationally estimated, to introduce the three-body forces. The potential parametrization, as a function of the cutoff, is fixed to describe the energy of ${}^4\mathrm{He}$ clusters up to seven particles within the expected accuracy. Finally, we also explore the possibility to describe at the same time the atom-dimer scattering length.

The range of entanglement in quantum dots, under the effect of the Coulomb interaction and the system's size, is investigated. As ququart systems, naturally described by the Fermi-Hubbard model, we show that quantum dots supply long-range entanglement up to the third neighbor. In conjunction with that and using the lower bound of concurrence, we show that the Coulomb interaction can be adjusted to create and increase entanglement between distant parties as well. A rigorous description of the pairs is given in terms of a local half-filled state associated with each pair with an electron number $N=2$ and a spin $S=0$. A thorough study of this state provides a proper explanation related to the pairwise entanglement, namely, its amount and its behavior under the effect of the Coulomb interaction together with the system's size. In addition, we show that the confinement state of quantum dots is genuinely four-partite entangled with a maximum amount for the smallest size $L=4$.

We propose and investigate a method to optimize recurrence entanglement purification protocols. The approach is based on a numerical search in the whole set of $\text{SU}\left(4\right)$ matrices with the aid of a quasi-Newton algorithm. Our method evaluates average concurrences where the probabilistic occurrence of mixed entangled states is also taken into account. We show for certain families of states that optimal protocols are not necessarily achieved by bilaterally applied controlled-not gates. As we discover several optimal solutions, the proposed method offers some flexibility in experimental implementations of entanglement purification protocols and interesting perspectives in quantum information processing.

Many quantum algorithms seek to output a specific bitstring solving the problem of interest—or a few if the solution is degenerate. It is the case for the quantum approximate optimization algorithm (QAOA) in the limit of large circuit depth, which aims to solve quadratic unconstrained binary optimization problems. Hence, the expected final state for these algorithms is either a product state or a low-entangled superposition involving a few bitstrings. What happens in between the initial $N$-qubit product state ${|0\rangle }^{\otimes N}$ and the final one regarding entanglement? Here, we consider the QAOA algorithm for solving the paradigmatic MaxCut problem on different types of graphs. We study the entanglement growth and spread resulting from randomized and optimized QAOA circuits and find that there is a volume-law entanglement barrier between the initial and final states. We also investigate the entanglement spectrum in connection with random matrix theory. In addition, we compare the entanglement production with a quantum annealing protocol aiming to solve the same MaxCut problems. Finally, we discuss the implications of our results for the simulation of QAOA circuits with tensor network-based methods relying on low-entanglement for efficiency, such as matrix product states.

We experimentally implement the Sz.-Nagy dilation algorithm to simulate open quantum dynamics on a nuclear magnetic resonance quantum processor. The Sz.-Nagy algorithm enables the simulation of the dynamics of an $n$-qubit system using $n+1$ qubits. We experimentally simulate the action of three nonunitary processes, namely, a phase damping channel acting independently on two qubits, a two-qubit correlated amplitude damping channel, and a magnetic-field-gradient pulse acting on an ensemble of two coupled nuclear spin-$\frac{1}{2}$ particles. To evaluate the quality of the experimentally simulated quantum process, we perform convex-optimization-based full quantum process tomography to reconstruct the quantum process from the experimental data and compare it with the target quantum process to be simulated.

Generative modeling is a promising task for near-term quantum devices, which can use the stochastic nature of quantum measurements as a random source. So-called Born machines are purely quantum models and promise to generate probability distributions in a quantum way, inaccessible to classical computers. This paper presents an application of Born machines to Monte Carlo simulations and extends their reach to multivariate and conditional distributions. Models are run on (noisy) simulators and IBM Quantum superconducting quantum hardware. More specifically, Born machines are used to generate muonic force carrier (MFC) events resulting from scattering processes between muons and the detector material in high-energy physics collider experiments. MFCs are bosons appearing in beyond-the-standard-model theoretical frameworks, which are candidates for dark matter. Empirical evidence suggests that Born machines can reproduce the marginal distributions and correlations of data sets from Monte Carlo simulations.

Resonance-enhanced multiphoton ionization process of the argon atom by an 800-nm 30-fs linearly polarized laser field is investigated at intensities range from 1.1 to $4.55\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. At $4.55\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ intensity the experimental photoelectron energy spectrum is in a good agreement with the time-dependent Schrödinger equation (TDSE) calculation where the double structure originating from dressed $4p-4d$ coupled transition is clearly identified. At lower intensity of $1.1\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, the resonant ionization process via the $4f$ state is observed, however, the expected peak (jet) at ${90}^{\circ }$ in the photoelectron angular distribution from the zeroth order above threshold ionization, has vanished completely. Such behavior is attributed to the destructive interference phenomenon in the coherent contributions of different partial waves of the photoelectron, namely, $\epsilon d$ and $\epsilon g$ states, and has been confirmed in our TDSE calculations.

Extracting single photoionization time delays associated with atomic (or molecular) species from attosecond timescale two-photon experiments usually relies on the theoretical description of continuum-continuum transitions. The available models for those processes predict a universal phase contribution, independent of the angular quantum numbers of final states. However, a recent experimental-theoretical study [Fuchs et al., Optica 7, 154 (2020)] determined a sizable time-delay dependence on the angular momentum of near-threshold photoelectrons. In this paper, we present an analytical model for the two-photon two-color transition matrix amplitudes that reproduces the phase dependence on the angular quantum number of final states. Finally, we show that our analytical model can also describe the generalized Fano's propensity rules [Busto et al., Phys. Rev. Lett. 123, 133201 (2019)] for laser-assisted photoionization.

We study the emergence of universal scaling in the time-evolving momentum distribution of a harmonically trapped three-dimensional Bose-Einstein condensate, parametrically driven to a turbulent state. We demonstrate that the out-of-equilibrium dynamics post excitation is described by a single function due to nearby nonthermal fixed points. The observed behavior connects the dynamics of a quantum turbulent state to several far-from-equilibrium phenomena. We present a controllable protocol to explore universality in such systems, obtaining the associated scaling exponents. Our experimental results thus offer a promising route to investigate the complex dynamics of the quantum turbulent regime.

Vector Bessel beams may have a spatially varying polarization state combined with a Bessel-type transversal intensity profile. Such beams can carry both spin angular momentum and orbital angular momentum (OAM) and be characterized by points on higher-order Poincaré (HOP) spheres. We report a method to produce vector Bessel beams on any point of a HOP sphere of any topological charge by utilizing only a single all-dielectric planar metasurface. Specifically, by a proper choice of the input polarization and distance from the metasurface, the entire HOP sphere surface can be covered. The proposed method is simple and efficient and applies to any $n\mathrm{th}$-order HOP sphere. Furthermore, it yields an extremely high OAM mode purity. Our paper demonstrates a robust approach for generating arbitrary high-quality vector Bessel beams and will likely inspire future applications ranging from optical manipulation to laser beam engineering.

We analyze the interaction of the magnetic quadrupole moment of a neutral particle with magnetic and electric fields around a cylindrical cavity in a rotating reference frame. In contrast to the energy levels yielded by a Coulomb-type potential that stems from the interaction of the magnetic quadrupole moment with a nonuniform magnetic field, we show that the effects of rotation break the degeneracy of the energy levels. Moreover, we show that the effects of rotation can gives rise to an Aharonov-Bohm-type effect. Further, we discuss the appearance of persistent currents and the revival time.

A local, deterministic toy model for quantum mechanics is introduced and discussed. It is demonstrated that the model makes the same predictions as quantum mechanics when averaged over the hidden variables. In this model the dynamics depends on the settings of the measurement device at the time of detection but not on how those settings were chosen. As a consequence, the model has a future input dependence and violates statistical independence but is local. We further show that the model is neither fine tuned nor allows for superluminal signaling and discuss the causality relations.

The classical Kolmogorov-Arnold-Moser (KAM) theorem provides the underlying mechanism for the stability of the solar system under some small chaotic perturbations. Despite many previous efforts, any quantum version of the KAM theorem remains elusive. In this work, we provide a quantum KAM theorem in the context of the anisotropic Dicke model, which is the most important quantum optics model. It describes a single mode of photons coupled to $N$ qubits with both a rotating wave (RW) term and a counter-RW (CRW) term. As the ratio of the CRW over the RW term increases from zero to one, the systems evolves from quantum integrable to quantum chaotic. We establish a quantum KAM theorem to characterize such an evolution quantitatively by both large-$N$ expansion and random matrix theory and find agreement from the two complementary approaches. Connections and differences between the Dicke models and Sachdev-Ye-Kitaev (SYK) or hybrid SYK models are examined. A possible quantum KAM theorem in terms of other quantum chaos criteria such as the quantum Lyapunov exponent is also discussed.

We investigate the quantum control of two qubits interacting with a Markovian environment and evolving as an $X$ state. The control is implemented via a simple applied field, whose profile is determined by means of the piecewise time-independent quantum control method. The goal is to make either the population or the system concurrence to follow a specified tracking control trajectory. By considering the system under the influence of three kinds of noise—phase damping, amplitude damping, or a combination of both—we unveil the conditions (like energy balance) under which the quantum control is properly achieved and fidelity (monitored through the trace distance) is kept satisfactorily high, even if the qubits interact differently with the dissipative medium. We further find that the effects of phase damping, a type of noise which notoriously destroys coherence and entanglement, can be mitigated if during the control the system is also exposed to amplitude damping. Of potential interest for the usage of entanglement as resource for quantum information, our results indicate that distinct entanglement measures and coherence can be maintained relatively high (even at long times) as a consequence of the tracking quantum control process.

The defining feature of memoryless quantum walks is that they operate on the vertex space of a graph and therefore can be used to produce search algorithms with minimal memory. We present a memoryless walk that can find a unique marked vertex on a two-dimensional lattice. Our walk is based on the construction proposed by Falk, which tessellates the lattice with squares of size $2\times 2$. Our walk uses minimal memory, $O\left(\sqrt{NlogN}\right)$ applications of the walk operator, and outputs the marked vertex with vanishing error probability. To accomplish this, we apply a self-loop to the marked vertex—a technique we adapt from interpolated walks. We prove that with our explicit choice of self-loop weight, this forces the action of the walk asymptotically into a single rotational space. We characterize this space and as a result show that our memoryless walk produces the marked vertex with a success probability asymptotically approaching one.

We propose a fast, robust, and long-distance quantum state transfer (QST) protocol via a splicing Su-Schrieffer-Heeger (SSH) chain, where the interchain couplings vary with the change in the phase parameter and the single or splicing SSH chain can be designed by adjusting it. It is found that the existence of a zero-energy interface state (IFS) not only can improve the speed of QST but also can realize long-distance QST. Furthermore, we give the phase diagram in the parameter space of the transfer time $T$ and the system's size $N$, where the different regions that can successfully implement QST via a single- or splicing-SSH-chain protocol are given. Therefore, we can choose the optimal QST protocol by adjusting only the phase parameter for different transfer times and system sizes. By numerically investigating the resilience of each protocol to static disorder, we reveal that the splicing-SSH-chain protocol is quite robust to both diagonal and off-diagonal disorders and clearly outperforms the single-SSH-chain protocol. By considering the environmental influence, rendering the Hamiltonian non-Hermitian by allowing energy to radiate away, our work shows that the QST protocol assisted by zero-energy IFS is more robust than previously expected and also outperforms the single-SSH-chain protocol.

Long-range interaction between distant spins is an important building block for the realization of a large quantum-dot network in which couplings between pairs of spins can be selectively addressed. Recent experiments on the coherent oscillation of logical states between remote spins facilitated by intermediate electron states were the first step for large-scale quantum information processing. Reaching this ultimate goal requires extensive studies on the superexchange interaction in different quantum-dot spatial arrangements and electron configurations. Here, we consider a linear triple quantum dot with two antiparallel spins in the outer dots forming the logical states while varying numbers of electrons in the middle dot form a mediator, which facilitates the superexchange interaction. We show that the superexchange is enhanced when the number of mediating electrons increases. In addition, we show that forming a four-electron triplet in the mediator dot further enhances the superexchange strength. Our work can be a guide to scale up the quantum-dot array with controllable and dense connectivity.

We investigate the resonance-enhanced few-photon ionization of atomic lithium by linearly polarized light whose frequency is tuned near the $2s\text{−}2p$ transition. Considering the direction of light polarization orthogonal to the quantization axis, the process can be viewed as an atomic “double-slit experiment” where the $2p$ states with magnetic quantum numbers $m_{\ell }=\pm 1$ act as the slits. In our experiment, we can virtually close one of the two slits by preparing lithium in one of the two circularly polarized $2p$ states before subjecting it to the ionizing radiation. This allows us to extract the interference term between the two pathways and obtain complex phase information on the final state. The experimental results show very good agreement with numerical solutions of the time-dependent Schrödinger equation. The validity of the two-slit model is also analyzed theoretically using a time-dependent perturbative approach.