We theoretically investigated the ionization dynamics of a hydrogen atom in circular excited states irradiated by the circularly polarized laser pulses by solving the time-dependent $\mathrm{Schr}\ddot{\mathrm{o}}\mathrm{dinger}$ equation. We calculated the ionization yields of the two excited states which are corotating and counter-rotating with respect to the laser fields as a function of the laser frequency. For the corotating excited state, our results show an obvious ionization suppression when the laser frequency is beyond the one-photon ionization threshold, and it is followed by a strong ionization enhancement when the laser frequency further increases. These ionization suppressions and enhancements are absent for the counter-rotating excited state. By tracing the ionization process, we found that resonance between the initial state and a lower-energy state occurs at the frequency of the ionization enhancement. Ionization from this lower-energy state is responsible for this ionization enhancement. At the frequency of ionization suppression, Rabi oscillation between the initial and the lower-energy state occurs, which prevents further ionization and thus suppresses the final ionization yield.

By inverting the time-dependent Kohn-Sham equation for a numerically exact dynamics of the helium atom, we show that the dynamical step and peak features of the exact correlation potential found previously in one-dimensional models persist for real three-dimensional systems. We demonstrate that the Kohn-Sham and true current densities differ by a rotational component. The results have direct implications for approximate time-dependent density functional theory calculations of atoms and molecules in strong fields, emphasizing the need to go beyond the adiabatic approximation, and highlighting caution in the quantitative use of the Kohn-Sham current.

We perform ab initio calculations of high-harmonic spectroscopy (HHS) of two-center interference phenomena in oriented carbon-dichalcogen molecules, using time-dependent density functional theory (TDDFT). We show that by resolving the harmonic response into contributions from individual Kohn-Sham orbitals, we can extract target-specific characteristics for both the spectral amplitude and phase. We also discuss that this extraction is predicated on a careful analysis and normalization of the harmonic spectrum. Finally, we present field-free scattering calculations that mimic the recollision step in high-order-harmonic generation and allow the calculation of recombination dipole matrix elements without explicitly calculating the scattering states of a molecule. We show that the orbital-resolved TDDFT HHS results and results based on our field-free scattering calculations are in very good agreement with each other.

A simple model for sequential multiphoton ionization by ultraintense x rays is presented. The derived scaling of the ion yield with pulse energy quantitatively reproduces the experimental data, which shows that the ion yield increases according to the “power-law” behavior typical of multiphoton ionization, followed by saturation at high pulse energies. The calculated average time interval between ionizations for producing ions at a certain charge state is found to be proportional to the pulse duration and independent of all other x-ray pulse parameters. This agrees with previous studies where the kinetic energy of fragment ions with a given charge state produced by the intense x-ray ionization of molecules was found to be independent of the pulse energy, but to increase with a smaller pulse duration due to the smaller time interval between ionizations.

The use of phase-shifting interferometry (PSI) in the ultra-low-light region would open the way for important applications, such as imaging of phototoxic substances and sensing of ultrafast phenomena. In the ultra-low-light region, the statistical nature of photons can be a dominant source of noise compared with other noise caused by experimental instruments. Here we theoretically derive the precision limit determined by the statistical nature of photons for simultaneously measuring the transmittance and phase of a sample with PSI. We show that the precision of PSI depends on the phase and transmittance themselves. We also show a trade-off relation between transmittance and phase. Then we compare PSI with sequential optimal measurements in which the transmittance and phase of a sample are separately measured for each corresponding optimal measurement. We also discuss the case where the input number of photons fluctuates with a Poisson distribution and show that the fluctuation affects both the transmittance and phase precision for PSI.

Circular dichroism in photoelectron angular distributions of general biharmonic (i.e., $n\omega +m\omega$) atomic ionization is analyzed theoretically and given along with “experimental guidelines” on how it can be steered towards its maximum. It is shown that such a maximum circular dichroism can always be achieved for the ionization of an arbitrary atom and for any incident fundamental photon energy by fine control of only two experimental parameters: the relative flux and phase difference of the two components of the radiation field. In this Letter, we provide a simple analytical description of the circular dichroism as well as a set of rules which enables full control over its magnitude. Our findings are demonstrated explicitly for the ionization of helium with a two-color field composing a fundamental beam and its second harmonic.

Continuous-time quantum walks have proven to be an extremely useful framework for the design of several quantum algorithms. Often, the running time of quantum algorithms in this framework is characterized by the quantum hitting time: the time required by the quantum walk to find a vertex of interest with a high probability. In this article, we provide improved upper bounds for the quantum hitting time that can be applied to several continuous-time quantum walk (CTQW) based quantum algorithms. In particular, we apply our techniques to the glued-trees problem, improving their hitting time upper bound by a polynomial factor: from $O\left(n^5\right)$ to $O(n^{2}logn)$. Furthermore, our methods also help to exponentially improve the dependence on precision of the CTQW based algorithm to find a marked node on any ergodic, reversible Markov chain by Chakraborty et al. [Phys. Rev. A 102, 022227 (2020)].

We study the problem of constructing a general hybrid quantum-classical bracket from a partial classical limit of a full quantum bracket. Introducing a hybrid composition product, we show that such a bracket is the commutator of that product. From this we see that the hybrid bracket will obey the Jacobi identity and the Leibniz rule provided the composition product is associative. This suggests that the set of hybrid variables belonging to an associative subalgebra with the composition product will have consistent quantum-classical dynamics. This restricts the class of allowed quantum-classical interaction Hamiltonians. Furthermore, we show that pure quantum or classical variables can interact in a consistent framework, unaffected by no-go theorems in the literature or the restrictions for hybrid variables. In the proposed scheme, quantum backreaction appears as quantum-dependent terms in the classical equations of motion.

In this work, we measure the properties of ensembles of rubidium atoms trapped in solid neon that are relevant for use as quantum sensors of magnetic fields: the spin coherence of the trapped atoms and the ability to optically control and measure their spin state. We use the rubidium atoms as an AC magnetometer—by employing an appropriate dynamical decoupling sequence—and demonstrate NMR detection of ${}^{21}\mathrm{Ne}$ atoms co-trapped in the neon matrix.

We study the neutral and positively charged thorium impurity (${\mathrm{Th}}^{n+}, n=0$, 1, 2, 3, 4) in solid argon by exploring the nature of chemical bonding in the ThAr diatomic molecule and in clusters ${\mathrm{ThAr}}_4, {\mathrm{ThAr}}_{12}, {\mathrm{Th}}^{n+}{\mathrm{Ar}}_{18}$ at the Hartree-Fock level with the second-order perturbation (MP2) correction accounting for the van der Waals forces. The chemical bonding is formed from the valence states of thorium and polarized states of argon in the clusters with ${\mathrm{Th}}^{n+}$ ($n=0$, 1, 2, 3) and solely from polarized states in the clusters with ${\mathrm{Th}}^{4+}$. In all cases with two or more valence electrons of Th, the ground state, influenced by the first Hund rule for the thorium impurity, is the high spin state. Allowing for the cubic to orthorhombic ($D_{2h}$) symmetry lowering in ${\mathrm{Th}}^{n+}{\mathrm{Ar}}_{18}$, we find that the averaged Th-Ar bond length decreases whereas the binding energy increases with $n$, accounted for by the weakening of the Th-Ar repulsion and the strengthening of polarization. For ${\mathrm{Th}}^{4+}{\mathrm{Ar}}_{18}$, two conformations (cubic and orthorhombic) are found. We conclude that with the Th-Ar bond lengths lying very close to the Ar-Ar bond lengths in a fcc lattice, the solid argon is a material that is well suited for the accommodation of thorium impurities.

Theoretical calculations of high-harmonic generation (HHG) from commensurate twisted bilayer graphene (tBLG) under intense laser fields are performed. The nonlinear electron dynamics in tBLG is considered by solving the Liouville–von Neumann equation for a single-particle density matrix, which combines the full energy bands and momentum matrix elements within the framework of tight-binding approximation. We show that the pump intensity determines the relative magnitude of two components of the harmonic spectrum parallel and perpendicular to driving polarization. The important dependence of HHG on twisted angles and crystal orientations is also presented. Especially in the absence of the relaxation process, the harmonic emission for twisted angles around 10 ° exhibits an evident decrease in efficiency per layer compared to monolayer graphene (MLG), which can be interpreted according to Fermi velocity modification. Our calculation also shows that the relative emission efficiency of different harmonic orders between tBLG and MLG contains redundant information on both the dephasing time and an empirical parameter characterizing the decay of the interlayer electron hopping, thus suggesting an all-optical method for the reconstruction of the two parameters. The reconstruction feasibility is successfully demonstrated by a simple optimization algorithm even if considering the possible experimental uncertainty of both driving pulse parameters and high-harmonic signals. Our results show that HHG spectroscopic characteristics in tBLG might serve as a fingerprint to identify the geometric stacking angle and the electronic interaction between adjacent layers, as well as the strong-field laser induced ultrafast dephasing process.

Quantum state smoothing is a technique to estimate an unknown true state of an open quantum system based on partial measurement information both prior and posterior to the time of interest. In this paper, we show that the smoothed quantum state is an optimal Bayesian state estimator, that is, it minimizes a Bayesian expected cost function. Specifically, we show that the smoothed quantum state is optimal with respect to two cost functions: the trace-square deviation from and the relative entropy to the unknown true state. However, when we consider a related cost function, the linear infidelity, we find, contrary to what one might expect, that the smoothed state is not optimal. For this case, we derive the optimal state estimator, which we call the lustrated smoothed state. It is a pure state, the eigenstate of the smoothed quantum state with the largest eigenvalue. We illustrate these estimates with a simple system, the driven, damped two-level atom.

A generic qubit unitary operator affected by quantum noise is duplicated and inserted in a coherently superposed channel, superposing two paths offered to a probe qubit across the noisy unitary, and driven by a control qubit. A characterization is performed of the transformation realized by the superposed channel on the joint state of the probe-control qubit pair. The superposed channel is then specifically analyzed for the fundamental metrological task of phase estimation on the noisy unitary, with the performance assessed by the Fisher information, classical or quantum. A comparison is made with conventional estimation techniques and also with a quantum switched channel with indefinite causal order recently investigated for a similar task of phase estimation. In the analysis here, a first important observation is that the control qubit of the superposed channel, although it never directly interacts with the unitary being estimated, can nevertheless be measured alone for effective estimation, while discarding the probe qubit that interacts with the unitary. This property is also present with the switched channel but is inaccessible with conventional techniques. The optimal measurement of the control qubit here is characterized in general conditions. A second important observation is that the noise plays an essential role in coupling the control qubit to the unitary, and that the control qubit remains operative for phase estimation at very strong noise, even with a fully depolarizing noise, whereas conventional estimation and the switched channel become inoperative in these conditions. The results extend the analysis of the capabilities of coherently controlled channels which represent novel devices exploitable for quantum signal and information processing.

The rapid development of reliable quantum processing units opens up novel computational opportunities for machine learning. Here, we introduce a procedure for measuring the similarity between graph-structured data, based on the time evolution of a quantum system. By encoding the topology of the input graph in the Hamiltonian of the system, the evolution produces measurement samples that retain key features of the data. We study analytically the procedure and illustrate its versatility in providing links to standard classical approaches. We then show numerically that this scheme performs well compared to standard graph kernels on typical benchmark datasets. Finally, we study the possibility of a concrete implementation on a realistic neutral-atom quantum processor.

The fundamental question of how to best simulate quantum systems using conventional computational resources lies at the forefront of condensed matter and quantum computation. It impacts both our understanding of quantum materials and our ability to emulate quantum circuits. Here we present an exact formulation of quantum dynamics via factorized generalized measurements which maps quantum states to probability distributions with the advantage that local unitary dynamics and quantum channels map to local quasistochastic matrices. This representation provides a general framework for using state-of-the-art probabilistic models in machine learning for the simulation of quantum many-body dynamics. Using this framework, we have developed a practical algorithm to simulate quantum circuits using an attention network based on a powerful neural network ansatz responsible for the most recent breakthroughs in natural language processing. We demonstrate our approach by simulating circuits that build Greenberger-Horne-Zeilinger and linear graph states of up to 60 qubits, as well as a variational quantum eigensolver circuit for preparing the ground state of the transverse field Ising model on several system sizes. Our methodology constitutes a modern machine learning approach to the simulation of quantum physics with applicability both to quantum circuits as well as other quantum many-body systems.

Helium atoms in the metastable $2{}^{3}S_1$ state (${\mathrm{He}}^{*}$) have unique advantages for ultracold atomic experiments. However, there is no known accessible Feshbach resonance in ${\mathrm{He}}^{*}$ which could be used to manipulate the scattering length and hence unlock several new experimental possibilities. Previous experimental and theoretical studies for ${\mathrm{He}}^{*}$ have produced contradictory results. We aimed to resolve this discrepancy with a theoretical search for Feshbach resonances, using a close-coupled model of ${\mathrm{He}}^{*}$ collisions in the presence of an external magnetic field. Several resonances were detected and the existing literature discrepancy was resolved. Although none of the resonances identified are readily experimentally usable, an interesting non-Feshbach scattering length variation with magnetic field was observed in heteronuclear collisions, at field strengths that are experimentally accessible.

The motion of noncircular two-dimensional vortices is shown to depend on a form of coupling between vortex ellipticity and the gradient of fluid density. The approach is based on the perspective that an elliptic vortex can be described as the projection of a virtual construct, a circular vortex with a symmetry axis that is tilted with respect to the direction of propagation. The resulting kinetic equation offers insights into how tilt and vortex velocity coevolve in few-body nonequilibrium settings such as vortex pair nucleation and annihilation. The model is developed and applied in association with optical vortices, and optical experiments are used to verify its predictive power. It is valid for quantum fluids and classical hydrodynamics settings as well.