To provide a comprehensive data set for track-structure-based simulations of radiation damage in DNA, doubly differential electron-impact ionization cross sections of pyrimidine, a building block of the nucleobases cytosine and thymine, were measured for primary electron energies between 30 eV and 1 keV as a function of emission angle and secondary electron energy. The measurements were performed for secondary electron energies from 4 eV to about half of the primary electron energy and for emission angles between 25° and 135°. Based on the experimental doubly differential ionization cross sections, singly differential and total ionization cross sections of pyrimidine were determined and compared to calculations using the binary-encounter-Bethe model. In addition to the measurements, a theoretical approach for calculating triply and doubly differential ionization cross sections was developed, which is based on the distorted wave Born approximation, a single center expansion of molecular orbitals, and an averaging of the T matrix over different molecular orientations. The calculated doubly differential ionization cross sections of pyrimidine show qualitatively good agreement with the experimental results.

The motional averaging effect of the alkali-metal atoms in a coated cell is influenced by the non-negligible background gas and is characterized by the diffusion coefficient of the alkali-metal atoms. In this paper, we present an experimental approach to measure the atomic diffusion coefficient within a paraffin coated cell through analyzing the spin relaxation caused by first-order and second-order magnetic-field gradients, respectively, given the distinct dependencies of these two relaxation effects on the diffusion coefficient. The diffusion coefficient is measured to be $0.137{\mathrm{m}}^2/\mathrm{s}$ for the fabricated paraffin coated cell, indicating an estimated pressure of background gas at approximately $0.25\mathrm{Pa}$, which is orders of magnitude higher than the typical saturated vapor pressure of the alkali-metal atoms. Our work serves as a potential nondestructive method for analyzing the background gas pressure and offers new insights into the performance characterization of the coated cell.

Taking the two-photon ionization process in hydrogen atoms as an example, we propose a scheme to decouple the partial-wave information in ultrafast ionization. By analyzing the interference in the photoelectron angular distributions obtained from the solution of the time-dependent Schrödinger equation, the phase difference and amplitude ratio for different partial waves can be extracted. In the perturbative regime, the one-photon ionization by the $2\omega$ pulse is taken as a reference to distinguish the contributions from the $s$ and $d$ waves in two-photon ionization. In the nonresonant cases, the phases of different partial waves in two-photon ionization are essentially the Coulomb phase shifts, which are independent of the pulse width. In the resonant case, an additional phase is accumulated during the resonant transition. The resulting time delays for both partial waves are proportional to the pulse duration. When the above-threshold ionization occurs, the continuum-continuum transition introduces an additional phase. The amplitudes of both partial waves are strongly related to the photon energy. They can be used to reveal hidden structures, such as the nonlinear Cooper minimum of different partial waves. This method of decoupling the partial-wave information can be extended to more complex atoms with photons across a wide range of energy.

We predict a stable density-wave-type supersolid phase of a dilute gas of tilted dipolar bosons in a two-dimensional (2D) geometry. This many-body phase is manifested by the formation of the stripe pattern and elasticity coexisting together with the Bose-Einstein condensation and superfluidity at zero temperature. With the increasing of the tilting angle the type of the gas-supersolid transition changes from the first order to the second-order one despite the 2D character of the system, whereas the anisotropy and many-body stabilizing interactions play crucial roles. Our approach is based on the numerical analysis of the phase diagram using the simulated annealing method for a free-energy functional. The predicted supersolid effect can be realized in a variety of experimental setups ranging from excitons in heterostructures to cold atoms and polar molecules in optical potentials.

The development of ultracold atom technology has enabled precise investigations on the quantum dynamics of quantum gases. Recently, inspired by experimental advancement, the $\text{SU}(1,1)$ echo, akin to the well-known $\text{SU}\left(2\right)$ spin echo, was proposed for the single-component Bose-Einstein condensate (BEC). In this paper, we investigate the possibility of quantum echo in the more intricate two-component BEC by fully exploiting its underlying symmetry, which is the Lie group $\text{Sp}(4,R)$. We demonstrate that quantum echo can occur for the two-component BEC by applying a driving protocol consisting of two steps in each period. The first step can be any Bogoliubov Hamiltonian, while the second step is a Hamiltonian with interactions turned off, which plays a similar role as the $\pi$ pulse in spin echo. We confirm our theoretical results with numerical calculations for different examples of the two-component BEC. We further consider the effect of interactions between the excited boson modes on the quantum echo process and discuss the possible experimental implementation of this quantum echo.

We derive the asymptotic expansions of the wave function of three particles having equal mass with finite-range interactions and infinite or zero two-dimensional scattering length colliding at zero energy and zero orbital angular momentum, from which a three-body parameter $D$ is defined. The dimension of $D$ is length squared, and we call $D$ three-body scattering area. We find that the ground-state energy per particle of a zero-temperature dilute Bose gas with these interactions is approximately $\frac{{\hslash }^{2}D}{6m}{\rho }^2$, where $\rho$ is the number density of the bosons and $m$ is the mass of each boson. Such a zero-temperature Bose gas is stable at $D>0$ in the thermodynamic limit, and metastable at $D<0$ in the isotropic harmonic trap if the number of bosons is less than $N_{\mathrm{cr}}\approx 3.6413\sqrt{\frac{\hslash }{m\omega \left|D\right|}\phantom{{}^1}}$, where $\omega$ is the angular frequency of the harmonic trap. If the two-body interaction supports bound states, $D$ typically acquires a negative imaginary part, and we find the relation between this imaginary part and the amplitudes of the pair-boson production processes. We derive a formula for the three-body recombination rate constant of the many-boson system in terms of the imaginary part of $D$.

Systems of coupled optical parametric oscillators (OPOs) forming an Ising machine are emerging as large-scale simulators of the Ising model. The advances in computer science and nonlinear optics have triggered not only the physical realization of hybrid (electrooptical) or all-optical Ising machines, but also the demonstration of quantum-inspired algorithms boosting their performances. To date, the use of the quantum nature of parametrically generated light as a further resource for computation represents a major open issue. A key quantum feature is the non-Gaussian character of the system state across the oscillation threshold. In this paper, we perform an ab initio analysis of the emergence of non-Gaussianity in the single quantum OPO with an applied external field. We model the OPO by a Lindblad master equation, which is numerically solved by a first-principles method based on exact diagonalization. Non-Gaussianity is quantified by means of three different metrics: the Hilbert-Schmidt distance, quantum relative entropy, and photon distribution. Our findings reveal a nontrivial interplay between parametric drive and applied field: (i) the increasing pump monotonically enhances non-Gaussianity and (ii) the increasing field first sharpens non-Gaussianity, and then restores the Gaussian character of the state when above a threshold value. We also report a first-principles computation in the Fock space of the distance from the mixture of coherent states, a strongly nonclassical behavior that can play a significant role in the quantum parallel search for optimization.

In this paper, we employ two two-level atoms and a $\mathrm{\Lambda }$-type atom to connect two one-dimensional semi-infinite coupled resonator waveguides, respectively. The first configuration is a chiral setup, where incident photons undergo an elastic scattering process. We investigate the influence on the single-photon transfer rate of spatial coupling points between the small or giant atom and the waveguides. Our numerical simulations demonstrate that additional coupling points in the giant atom system will modify the transmission rule observed in the small atom system due to their unique interference effects. The second configuration is a nonreciprocal setup, where the conduction direction depends entirely on the initial state of the $\mathrm{\Lambda }$-type atom. The incident photon undergoes an inelastic scattering process accompanied by frequency conversion.

Parameter shift rules enable the estimation of the derivatives of expectation values with respect to the dynamical evolution of a state. Thus, they provide a valuable tool in variational optimization and insight into the dynamical behavior of quantum systems. Constructing parameter shift rules applicable to complex dynamical generators is useful towards improving the control of analog quantum simulation devices and extending the applicability of variational quantum simulating methods. However, parameter shift rules are typically designed only for dynamical generators with highly degenerate or equidistant eigenvalues, such as Pauli operators, commonplace in the construction of parametrized quantum circuits. For generators with irregularly spaced eigenvalues, effective rules have not been established. We provide insights about the optimal design of a parameter shift rule tailored to various sorts of spectral information that may be available. The proposed method targets the calculation of any linear combination of $k\mathrm{th}$ derivatives with respect to the conjugate variable of a generator with a known spectrum, regardless of how close the eigenvalues are to each other. We discuss the optimal choice of parameter shifts, minimizing their number and the variance of the target estimator. Furthermore, we discuss the construction of parameter shift rules when only partial information about the generator spectrum is known.

We theoretically study the dynamical regimes of the observables that govern the operating conditions of continuous-variable (CV) quantum-key-distribution (QKD) systems, depending on quantum-channel-induced intermode interactions. In contrast to the widely used approach in which losses and thermal broadening are introduced through a beam-splitter transformation, our analysis uses the exactly solvable quantum channel model describing the Lindblad dynamics of multimode bosonic systems interacting with a heat bath and additionally takes into account imperfections of the homodyne-detection scheme. The analytical results for the photon-count difference and the quadrature probability distributions are used to derive the expression for the mutual information between legitimate parties, which explicitly links the information properties of CV QKD and the parameters of the channel. For the important special case of a two-mode photonic system propagating in a fiber channel, the latter can be conveniently parameterized using the frequency and the relaxation-rate vectors that characterize the coherent (dynamical) intermode couplings and the incoherent (environment mediated) interaction between the bosonic modes, respectively. It turns out that these vectors determine four qualitatively different dynamical regimes of the mutual information and the phase difference between the signal and the local oscillator that may significantly affect the operation of CV QKD.

A variational quantum algorithm for numerically solving partial differential equations (PDEs) on a quantum computer was proposed by Lubasch et al. [Phys. Rev. A 101, 010301(R) (2020)]. In this paper we generalize the method introduced by Lubasch et al. to cover a broader class of nonlinear PDEs as well as multidimensional PDEs and study the performance of the variational quantum algorithm on several example equations. Specifically, we show via numerical simulations that the algorithm can solve instances of the single-asset Black-Scholes equation with a nontrivial nonlinear volatility model, the double-asset Black-Scholes equation, the Buckmaster equation, and the deterministic Kardar-Parisi-Zhang equation. Our simulations use up to $n=12$ Ansatz qubits, computing PDE solutions with $2^n$ grid points. We also perform proof-of-concept demonstrations with a trapped-ion quantum processor from IonQ [Nat. Commun. 10, 5464 (2019)], showing accurate computation of two representative expectation values needed for the calculation of a single time step of the nonlinear Black-Scholes equation. Through our classical simulations and demonstrations on quantum hardware, we identify and discuss several open challenges for using quantum variational methods to solve PDEs in a regime with a large number (much greater than $2^{20}$) of grid points, but also a practical number of gates per circuit and circuit shots.

The model description of the spectral line broadening for atoms crossing the laser beams is presented. The combined effect of the transit-time broadening, natural broadening, and transverse Doppler broadening is considered. This is done by introducing one-parametric adiabatic damping of atom-laser interaction where the particles interact with the laser during a finite time interval. The simplest rectangle distribution of the light intensity inside the laser beam is employed. The dependence of absorption probability on the transit time for different detunings is calculated and compared with earlier results. The limits for using the Lorentz line profile for describing the interaction of fast particles with laser beams are established.

In a recent paper [Das, Modak, and Bera, Phys. Rev. A 107, 042413 (2023)], Das et al. claim that Kirkwood-Dirac negativity is not required to achieve anomalous values of the quantum Fisher information encoded in postselected (filtered) particles. We argue that this claim is wrong. Further, we show that the quantum Fisher information in optimal postselected metrology is directly proportional to the total Kirkwood-Dirac nonclassicality.

In Das et al. [Phys. Rev. A 107, 042413 (2023)], we showed that Kirkwood-Dirac negativity is not necessary to achieve anomalous quantum Fisher information. Arvidsson-Shukur et al. [preceding Comment, Phys. Rev. A (to be published)] claim that one of our conclusions is wrong. Here, we argue that the claim is unfounded and incorrect and conclusions in Das et al. stand correct when the standard form of the Kirkwood-Dirac distribution is investigated. The confusion arises due to the choice of the distribution under consideration. We clarify this issue along with an interpretation of nonclassicality attached to the positivity of Kirkwood-Dirac distribution.

In this paper, we consider the real-time parameter estimation problem for a ZZ-coupled system composed of two qubits in the presence of spontaneous emission. To enhance the estimation precision of the coupling coefficient, we first propose two different control strategies, where the first one is feedback control based on quantum-jump detection, and the second one is hybrid control combining Markovian feedback and Hamiltonian control. The simulation results show that compared with free evolution, both control strategies can improve parameter precision and extend system coherence time. Then, on the basis of the two control strategies, we propose a practical single-parameter quantum recovery protocol, which includes the preparation of an optimal initial probe state, the active control and optimization of a system evolution process, the design of an adaptive measurement scheme, and the parameter estimation algorithm based on Bayesian estimation theory. In particular, by employing a batch-style adaptive measurement scheme, the parameter recovery is conducted and the effectiveness of the two control strategies is verified.

We investigate the construction of spin-1/2 fermionic and spin-0 bosonic wavepackets having compact spatial support in the framework of a computational quantum field theory (QFT) scheme offering space-time solutions of the relativistic wave equations in background fields. To construct perfectly localized wavepackets, we introduce a spatial density operator accounting for particles of both positive and negative charge. We examine properties of the vacuum and single-particle expectation values of this operator and compare them to the standard QFT particle and antiparticle spatial densities. The formalism is illustrated by computing numerically the Klein tunneling dynamics of strictly localized wavepackets impinging on a supercritical electrostatic step. The density operator introduced here could be useful to model situations in which it is desirable to avoid dealing with the infinite spatial tails intrinsic to pure particle or antiparticle wavepackets.

Leggett-Garg inequalities place bounds on the temporal correlations of a system based on the principles of macroscopic realism per se and noninvasive measurability. Their conventional formulation relies on the ensemble-averaged products of observables measured at different instants of time. However, a complete description that enables a precise understanding and captures all physically relevant features requires the study of probability distributions associated with noncommuting observables. In this paper, we propose a scheme to describe the dynamics of generic $N$-level quantum systems (“qudits”) via a probability vector representation of the Schrödinger equation and define a precise notion of no-signaling in time (NSIT) for the probability distributions of noncommuting observables. This provides a systematic way of identifying the interferences responsible for nonclassical behavior. In addition, we introduce an interference witness measure to quantify violations of NSIT for arbitrary general probabilistic states. For single-qubit systems, we pinpoint the pivotal relation that establishes a connection between the disturbance of observables incurred during a measurement and the resulting NSIT violation. For large-$N$ systems where a manual determination is infeasible, the classification of states as either NSIT-conforming or NSIT-violating may be performed by a machine learning algorithm. We present a proof-of-principle implementation of such an algorithm in which the classifier function is prepared via supervised learning using pseudorandomly generated training data sets composed of states whose corresponding classifications are known a priori.

The presence of quantum multipartite entanglement implies the existence of a thermodynamic quantity known as the ergotropic gap, which is defined as the difference between the maximal global and local extractable works from the system. We establish a direct relation between the geometric measure of entanglement and the ergotropic gaps. We show that all the marginal ergotropic gaps form a convex polytope for each class of quantum states that are equivalent under stochastic local operations and classical communication (SLOCC). We finally introduce the concept of multipartite ergotropic gap indicators and use them to present a refined criterion for classifying entanglement under SLOCC.

In quantum computing, the nonstabilizerness of quantum operations is crucial for understanding and quantifying quantum speedups. In this study, we explore the phenomena of nonstabilizerness of the quantum SWITCH, a novel structure that allows quantum states to pass through operations in a superposition of different orders, outperforming traditional circuits in numerous tasks. To assess its nonstabilizerness, we propose the magic resource capacity of a quantum process to quantitatively examine the nonstabilizerness of general quantum transformations. We find that the completely stabilizer-preserving operations, which cannot generate magic states under standard conditions, can be transformed to do so when processed by the quantum SWITCH. Furthermore, when considering the impact of noise, although the nonstabilizerness of each path may be annihilated, their superposition could still preserve the overall nonstabilizerness. These findings reveal the unique properties of the quantum SWITCH and open avenues in research on nonstabilizer resources of general quantum architecture.