We present an optimal control procedure for the nonadiabatic transport of ultracold neutral thermal atoms in optical tweezers arranged in a one-dimensional array, with the focus on reaching a minimal transfer time. The particle dynamics is modeled first using a classical approach through the Liouville equation and second through the quantum Wigner equation to include quantum effects. Both methods account for typical experimental noise described as stochastic effects through Fokker-Planck terms. The optimal control process is initialized with a trajectory computed for a single classical particle and determines the phase-space path that minimizes transport time and ensures high transport fidelity to the target trap. This approach provides a fast and efficient method for relocating atoms from an initial configuration to a desired target arrangement, minimizing time and energy costs while ensuring high fidelity. Such an approach may be highly valuable to initialize large atom arrays for quantum simulation or computation experiments.

Nonlinear $X$-wave formation in the backward wave optical parametric oscillator pumped by subnanosecond laser pulses is studied. The wavelengths of 1064 nm and its second harmonic are used for pumping. $3\mathrm{D}+1\mathrm{D}$ (three dimensional space and time) numerical model is implemented to simulate the formation of the generated forward and backward waves. Contrary to the conventional $1\mathrm{D}+1\mathrm{D}$ model, the $3\mathrm{D}+1\mathrm{D}$ model allows the formation of conical beam structures. The nonlinear $X$-wave propagation in the backward direction with respect to the phase velocity direction is demonstrated in the case of negative $X$-wave velocity.

For linear electromagnetic systems possessing time-reversal symmetry, we present an approach to bound ratios of internal fields excited from different ports, using only the scattering matrix ($S$ matrix), improving upon previous related bounds by Sounas and Alù [D. L. Sounas and A. Alù, Phys. Rev. Lett. 118, 154302 (2017)]. By reciprocity, emitted-wave amplitudes from internal dipole sources are bounded in a similar way. When applied to coupled-resonant systems, our method constrains ratios of resonant coupling and decay coefficients. We also obtain a relation for the relative phase of fields excited from the two ports and the ratio of field intensities in a two-port system. In addition, although lossy systems do not have time-reversal symmetry, we can still approximately bound loss-induced nonunitarity of the $S$ matrix using only the lossless $S$ matrix. We show numerical validations of the near tightness of our bounds in various scattering systems.

The ability to generate quantum entangled photon pairs through spontaneous parametric down conversion (SPDC) is playing a pivotal role in many applications in quantum technologies, including quantum communications, quantum computation, and quantum imaging. Metasurfaces, two-dimensional arrays of nanostructures with subwavelength thickness, have recently shown extraordinary capability in manipulating classical and quantum states of light and realizing miniaturized SPDC sources. Whereas previous studies have primarily focused on periodic metasurfaces, we uncover the potential of spatially modulated nanopatterns for further SPDC enhancement and state engineering. Specifically, we propose a nonlinear metasurface featuring a lateral guided-mode cavity formed by two distributed Bragg reflectors. With the quasinormal mode theory, we predict an SPDC rate up to 157 Hz/mW, which represents about 30 times rate enhancement when compared to the metasurface without a cavity. Furthermore, the cavity supports a continuous set of high-$Q$ resonances across a broad bandwidth, allowing the all-optical tuning of the photon-pair emission directions transverse to the cavity by controlling the pump wavelength, which can find applications in quantum imaging.

Quantum batteries (QBs) exhibit remarkable charging power and energy potential, suggesting quantum benefits in thermodynamics. However, they face challenges like decoherence, low efficiency, and limited saturation. In this study, we explore the nonreciprocal open quantum batteries network (NOQBN), integrated with photonic waveguide arrays. By leveraging unidirectional waveguide nonreciprocities and reservoir engineering, we achieve targeted energy transfer, where QBs' higher dissipation rates compared to optical charger modes enhance charging energy, but reduce the ergotropy of the NOQBN. Results reveal that collective charging boosts QBs' ergotropy and power output, outperforming isolated QB systems. Moreover, by challenging traditional reciprocal charger-battery paradigms, we successfully adjust environmental dissipation to enhance nonreciprocity, thereby improving energy storage efficiency. Crucially, optimizing the correlation length between QBs within the network further enhances their storage capacity. The NOQBN's nonreciprocal dynamics pioneers a new approach to quantum energy storage, paving the way for applications in driven-dissipative photonic networks and inspiring the development of advanced, integrated nonreciprocal quantum energy devices. This work advances our theoretical understanding of QB networks and provides practical design principles for highly efficient quantum energy devices.

Bell-state measurements (BSMs) of photonic qubits are used for quantum networking protocols to herald the distribution and transfer of quantum information. However, standard BSMs based on linear optics (LO-BSMs) require identical photons and are susceptible to errors from multiphoton emissions, leading to reduced fidelity of protocols, particularly in the absence of postselection. To overcome these challenges, Bell state measurements based on nonlinear optics (NLO-BSMs) have been proposed and implemented, leveraging sum-frequency generation (SFG) to filter out multiphoton emissions and improve the fidelity without postselection. Here, we analyze the fidelity of LO-BSM and NLO-BSM heralded entanglement swapping in lossy channels and compare their performance under realistic conditions. We also explore the impact of state-of-the-art nonlinear photonics platforms on SFG efficiency and highlight how advances in nanophotonics will enable practical, high-fidelity NLO-BSM-based quantum networking applications.

Accurately characterizing the properties of structured reservoirs is a key challenge in quantum systems and is of great importance for advances in quantum metrology and sensing. In this work, we employ a two-level system (qubit) as a probe, which is coupled to a structured reservoir consisting of an ancilla qubit and a Markovian environment modeled as a thermal bath. By exploiting non-Markovian dynamics, we systematically investigate the effectiveness of different interaction types between the probe and ancilla for estimating critical parameters, including temperature, ancilla frequency, and system-bath coupling strength. We quantify the precision of parameter estimation using quantum Fisher information (QFI) and analyze the system dynamics in both transient and steady-state regimes. Our findings demonstrate that non-Markovianity substantially enhances parameter estimation in the transient regime, with specific interactions facilitating sustained information backflow and yielding higher QFI values. However, the performance of these interactions is contingent on the parameter under estimation and the operational regime. For instance, certain interactions become prominent in the transient regime but exhibit diminished utility in the steady state, whereas others maintain their effectiveness even at equilibrium. These results stress the importance of judiciously selecting interactions adapted to specific estimation objectives and operational regimes.

Cross sections for electronic excitation and ionization in electron scattering on ${\mathrm{H}}_3^{+}$ and ${\mathrm{D}}_3^{+}$ have been calculated using the polyatomic molecular convergent close-coupling (MCCC) method adapted to point-group symmetries. Convergence in both the total excitation and ionization cross sections is established within the MCCC framework, yielding agreement with available experiment at all energies above thresholds. Focusing on comparison with available experiment, kinetic-energy release (KER) cross sections for electron-impact dissociation of ${\mathrm{D}}_3^{+}$ in several initial $v_1$ vibrational modes have been calculated in a one-dimensional model. Cross sections are presented for dissociative excitation through the $1{}^3{E}^{\prime }, 1{}^1{E}^{\prime }, 1{}^3{A}_2^{″}, 1{}^1{A}_2^{″}, 1{}^3{A}_1^{\prime }$, and $2{}^1{A}_1^{\prime }$ excited electronic states of ${\mathrm{D}}_3{}^{+}$ and dissociative ionization through the $1{}^2{A}_1^{\prime }$ state of ${\mathrm{D}}_3{}^{++}$. A model for the branching ratios is proposed for the first six excited electronic states and for ionization. Kinetic-energy release cross sections for the production of ${\mathrm{D}}_2^{+}$ and ${\mathrm{D}}^{+}$ are presented. Reasonable agreement with experimental ${\mathrm{D}}_2^{+}$ KER cross sections is found at incident energies above 20 eV, while good agreement is found with measured ${\mathrm{D}}^{+}$ KER cross sections above 40 eV.

We propose an autofocusing beam, named the symmetric butterfly Gaussian beam (SBGB). Its analytical expression in free space is derived, and its transmission characteristics are systematically analyzed. Compared to the symmetric Airy Gaussian beam and symmetric Pearcey Gaussian beam, the SBGB exhibits superior abruptly autofocusing capabilities and enhanced particle trapping abilities. By adjusting the distribution factors, the SBGB can generate a bifocal structure. Interestingly, introducing a second-order chirp factor enables the two focal spots to evolve into orthogonal needlelike structures. The focal size, focal position, and intensity of the SBGB can be flexibly adjusted by tuning the second-order chirp factor. Moreover, we theoretically analyze the trapping properties of the SBGB and explore the trapping force distributions in its focal regions. Experimental results verify that the dual needlelike focal points can stably trap and manipulate multiple silica particles. With outstanding autofocusing performance and unique focal structures, the SBGB holds great potential for applications in optical manipulation, biomedical research, and laser processing.

We identify and interpret the possible quantum thermal machine regimes with a transverse-field Ising model as the working substance. In general, understanding the emergence of such regimes in a many-body quantum system is challenging due to the dependence on the many energy levels in the system. By considering infinitesimal work strokes, we can understand the operation from equilibrium properties of the system. We find that infinitesimal work strokes enable both heat engine and accelerator operation, with the output and boundaries of operation described by macroscopic properties of the system, in particular, the net transverse magnetization. At low temperatures, the regimes of operation and performance can be understood from the behavior of low-energy excitations in the system, while at high temperatures an expansion of the free energy in powers of inverse temperature describes the operation. The understanding generalizes to larger work strokes when the temperature difference between the hot and cold reservoirs is large. For hot and cold reservoirs close in temperature, a sufficiently large work stroke can enable refrigerator and heater regimes. Our results and method of analysis will prove useful in understanding the possible regimes of operation of quantum many-body thermal machines more generally.

Characterization and quantification of non-Markovian dynamics in open quantum systems are topical issues in the rapidly developing field of quantum computation and quantum communication. A standard approach based on the notion of information backflow detects the flow of information from the environment back to the system. Numerous measures of information backflow have been proposed using different definitions of distinguishability between pairs of quantum states. These measures, however, necessitate optimization over the state space, which can be analytically challenging or numerically demanding. Here we propose an alternative witness and measure of information backflow that is explicitly state independent by utilizing the concept of quasiprobability representation and recent advances in the theory of majorization for quasiprobabilities. We illustrate its use over several paradigmatic examples, demonstrating consistent Markovian conditions with known results and also reported necessary and sufficient conditions for the qutrit system in a random unitary channel. The paper concludes with a discussion of the foundational implications of quantum dynamical evolution.

This paper introduces a quantum framework for addressing reinforcement learning (RL) tasks, grounded in the quantum principles and leveraging a fully quantum model of the classical Markov decision process. By employing quantum concepts and a quantum search algorithm, this work presents the implementation and optimization of the agent-environment interactions entirely within the quantum domain, eliminating reliance on classical computations. Key contributions include the quantum-based state transitions, return calculation, and trajectory search mechanism that utilize quantum principles to demonstrate the realization of RL processes through quantum phenomena. The implementation emphasizes the fundamental role of quantum superposition in enhancing computational efficiency for RL tasks. Results demonstrate the capacity of a quantum model to achieve quantum enhancement in RL, highlighting the potential of fully quantum implementations in decision-making tasks. This work not only underscores the applicability of quantum computing in machine learning but also contributes to the field of quantum reinforcement learning by offering a robust framework for understanding and exploiting quantum computing in RL systems.

Trapped-ion systems are a promising route toward the realization of both near-term and universal quantum computers. However, one of the pressing challenges is improving the fidelity of two-qubit entangling gates. These operations are often implemented by addressing individual ions with laser pulses using the Mølmer-Sørensen protocol. Amplitude modulation (AM) is a well-studied extension of this protocol, where the amplitude of the laser pulses is controlled as a function of time. We present an analytical study of AM, using a Fourier series expansion to maintain the generality of the laser amplitude's functional form. We then apply this general AM method to gate-timing errors by imposing conditions on these Fourier coefficients, producing trade-offs between the laser power and fidelity at a fixed gate time. The conditions derived here are linear and can be used, in principle, to achieve arbitrarily high orders of insensitivity to gate-timing errors. Numerical optimization is then employed to identify the minimum-power pulse satisfying these constraints. Our central result is that the leading-order dependence on gate-timing errors is improved from $O\left(\mathrm{\Delta }{t}^2\right)$ to $O\left(\mathrm{\Delta }{t}^6\right)$ with the addition of one linear constraint on the Fourier coefficients and to $O\left(\mathrm{\Delta }{t}^{10}\right)$ with two linear constraints without a significant increase in the average laser power. The increase approaches zero as more Fourier coefficients are included. In further studies, this protocol can be applied to other error sources and used in conjunction with other error-mitigation techniques to improve two-qubit gates.

Quantum batteries, as highly efficient energy storage devices, have garnered significant research interest. A key challenge in their development is to maximize the extractable energy (ergotropy) when operating within a finite-temperature reservoir. To address this, we apply quantum feedback control to the charger and investigate the effects of fermionic and bosonic thermal reservoirs on the performance of quantum batteries, including stored energy, ergotropy, and charging efficiency, in an open environment. Our findings reveal that, regardless of the type of thermal reservoir, the system exhibits optimal charging parameters. In particular, in a fermionic thermal reservoir, increasing the environmental temperature enhances battery performance, enabling stable and efficient charging. In contrast, within a bosonic thermal reservoir, higher temperatures hinder energy storage and extraction, significantly reducing charging efficiency. Additionally, we explore the impact of battery size and find that, under a fermionic reservoir, increasing the battery size appropriately can further improve performance.

Noisy intermediate-scale quantum (NISQ) devices have inherent limitations in terms of connectivity and hardware noise. The synthesis of controlled-$\mathrm{NOT}$ (cnot) circuits considers the physical constraints and transforms quantum algorithms into low-level quantum circuits that can execute on physical chips correctly. In the current trend, quantum chip architectures without Hamiltonian paths are gradually replacing architectures with Hamiltonian paths due to their scalability and low-noise characteristics. To this end, this paper addresses the nearest-neighbor synthesis of cnot circuits in the architectures with and without Hamiltonian paths, aiming to enhance the fidelity of the circuits after execution. First, a key-qubit priority mapping model for general quantum architectures is proposed. Second, the initial mapping is further improved by using tabu search to reduce the number of cnot gates after circuit synthesis and enhance its fidelity. Finally, the noise-aware cnot circuit nearest-neighbor synthesis algorithm for the general architecture is proposed based on the key-qubit priority mapping model. The algorithm is demonstrated on several popular cloud quantum computing platforms and simulators, showing that it effectively optimizes the fidelity of cnot circuits compared with mainstream methods. Moreover, the method can be extended to more general circuits, thereby improving the overall performance of quantum computing on NISQ devices.

We have performed three theoretical simulations relevant for describing collisions among laser-cooled silver atoms and for the formation of ${\mathrm{Ag}}_2$ molecules from these colliding atoms. Firstly, we determined the relativistic electronic structure of ${\mathrm{Ag}}_2$ molecules in ground and low-lying excited states. Secondly, we computed rotational and vibrational levels of the ground and excited electronic states as well as rovibrationally averaged electric transition dipole moments. Using this knowledge, we analyzed a simplified quantum-mechanical model of the one-photon photoassociation process to form electronically excited ${\mathrm{Ag}}_2$ from microkelvin Ag atoms and make predictions for lineshapes and saturation effects as functions of laser frequency and intensity. Finally and thirdly, we performed coupled-channels calculations, numerical solutions of sets of coupled radial Schrödinger equations of ultracold ground-state Ag collisions in an external magnetic field. These calculations include the effects of two Born-Oppenheimer potentials as well as hyperfine Fermi-contact and Zeeman interactions. We discuss the expected range of $s$-wave scattering lengths as well as strengths and distribution of Fano-Feshbach resonances as a function of the magnetic-field strength for the ${}^{107}\mathrm{Ag}$ and ${}^{109}\mathrm{Ag}$ isotopes. We highlight the periodicity of the scattering length with small changes in the depths of the Born-Oppenheimer potentials. The Fano-Feshbach resonances can be used to magneto-associate ultracold Ag atoms into weakly bound ground-state ${\mathrm{Ag}}_2$ dimers.

We present the conceptual design of an experiment to measure the electron's electric dipole moment (eEDM) using ${}^{138}\mathrm{BaOH}$ molecules in an optical lattice. The BaOH molecule is laser-coolable and highly sensitive to the eEDM, making it an attractive candidate for such a precision measurement, and capturing it in an optical lattice offers potentially very long coherence times. We study possibilities and limitations of this approach, identify the most crucial limiting factors and ways to overcome them. The proposed apparatus can reach a statistical error of ${10}^{-30}e\mathrm{cm}$ by measuring spin precession on a total number of $5\times {10}^9$ molecules over a span of 120 days.

We present an exactly solvable two-level model with smoothly varying coupling, detuning, and loss functions. Whenever the common temporal profile of these parameter functions is analytically integrable, the complex population amplitudes are found for all times in simple analytic forms. This makes the model suitable for studying the population and phase dynamics of resonance-enhanced ionization processes. Incorporating two-photon Rabi frequency, dynamical Stark shifts, and one-photon ionization rate into this model, the $2+1$ photon ionization of atomic lithium is studied as a showcase example. Finally, when the intensity profile of the laser pulse has the sine-squared shape, we find the exact analytic expression of the photoelectron spectrum, allowing additional insight into the structure of the Autler-Townes doublet.

High harmonic generation (HHG) in the vanadium monoanion (${\mathrm{V}}^{-}$) and its hexacarbonyl complex, $[\mathrm{V}{\left(\mathrm{CO}\right)}_6{]}^{-}$, is investigated using time-dependent density functional theory within the optimized effective potential formalism. By analyzing the field-free electronic structure, field-induced changes in the many-electron potential, and the partitions of the high harmonic spectra, we demonstrate that these anions are valuable for understanding the role of orbital spatial distribution in HHG. Both systems feature degenerate (or near-degenerate) orbitals, with some extending along the field polarization direction. Our analysis reveals that this spatial extensiveness enhances HHG intensity by several orders of magnitude. Additionally, the redistribution of electron density in $[\mathrm{V}{\left(\mathrm{CO}\right)}_6{]}^{-}$ under intense laser fields generates a negative potential near the nuclei, increasing the apparent ionization potential and significantly extending the high harmonic cutoff.

We investigate the finite-temperature properties of a bosonic Josephson junction composed of $N$ interacting atoms confined by a quasi-one-dimensional asymmetric double-well potential, modeled by the two-site Bose-Hubbard Hamiltonian. We numerically compute the spectral decomposition of the statistical ensemble of states, the thermodynamic and entanglement entropies, the population imbalance, the quantum Fisher information, and the coherence visibility. We analyze their dependence on the system parameters, showing, in particular, how finite temperature and on-site energy asymmetry affect the entanglement and coherence properties of the system. Moreover, starting from a quantum phase model which accurately describes the system over a wide range of interactions, we develop a reliable description of the strong tunneling regime, where thermal averages may be computed analytically using a modified Boltzmann weight involving an effective temperature. We discuss the possibility of applying this effective description to other models in suitable regimes.