The control of quantum systems has been proven to possess trap-free optimization landscapes under the satisfaction of proper assumptions. However, many details of the landscape geometry and the search required for reaching particular goals still need to be fully understood. This paper numerically explores the path-connectedness of globally optimal control solutions forming the top manifold of the landscape. We randomly sample a plethora of optimal controls on the top manifold to assess the existence of a continuous path at the top of the landscape that connects two arbitrary optimal solutions. It is shown that for different quantum control objectives, including state-to-state transition probabilities, observable expectation values, and unitary transformations, such a continuous path can be readily found, implying that these top manifolds are fundamentally path-connected. The significance of the latter conjecture lies in seeking locations on the top manifold where an ancillary objective can also be optimized while maintaining the full optimality of the original objective that defined the landscape.

We present efficient implementations of atom reconfiguration algorithms for both CPUs and GPUs, along with a batching algorithm to merge displacement operations for parallel execution. Leveraging graph-theoretic methods, our approach derives improved algorithms that achieve reduced time complexity and faster operational running times. First, we introduce an enhanced version of the redistribution-reconfiguration (red-rec) algorithm, which offers superior operational and runtime performance. We detail its efficient implementation on a GPU using a parallel approach. Next we present an optimized version of the assignment-reconfiguration-ordering (ARO) algorithm, specifically tailored for unweighted grid graphs. Finally, we introduce the bird algorithm to solve reconfiguration problems on grids, achieving performance gains over both red-rec and ARO. These algorithms can be used to prepare defect-free configurations of neutral atoms in arrays of optical traps, serve as subroutines in more complex algorithms, or cross benchmark the operational and runtime performance of new algorithms. They are suitable for realizing quantum circuits incorporating displacement operations and are optimized for real-time operation in systems relying on increasingly large arrays of optical traps.

We investigate dissipative superradiant phase transitions and multicritical effects in an open dual-coupling Jaynes-Cummings model, where two cavity fields are simultaneously coupled to a single atom. The cavity fields are squeezed via a nondegenerate optical parametric amplification process. We show that the interplay between interactions, two-mode squeezing, and dissipation leads to rich critical behaviors, including both first-order and second-order superradiant phase transitions. The critical points are governed by the detunings between atomic (or photonic) frequencies and external pump field. Thus, these transitions can occur in the strong-coupling regime and can be easily controlled by tuning the external pump field. Utilizing the mean-field theory that explicitly neglects quantum fluctuations, we determine the phase-transition boundaries in the steady state. Additionally, we employ a full quantum description of the lowest-energy Hamiltonian in the large atomic detuning limit to analyze the quantum fluctuations. Notably, the phase diagram features two tricritical points where the critical lines of first-order and second-order phase transitions meet. Furthermore, when the two coupling pathways have specific coupling strengths, increasing the external pump strength can drive the system from the normal phase to the superradiant phase. Our work offers new insights into dissipative phase transitions and multicriticality, with potential applications in quantum sensing and information processing.

Following on our previous work studying the orbits of quantum states under Clifford circuits via reachability graphs, we introduce contracted graphs whose vertices represent classes of quantum states with the same entropy vector. These contracted graphs represent the double cosets of the Clifford group, where the left cosets are built from the stabilizer subgroup of the starting state and the right cosets are built from the entropy-preserving operators. We study contracted graphs for stabilizer states, as well as $W$ states and Dicke states, discussing how the diameter of a state's contracted graph constrains the entropic diversity of its two-qubit Clifford orbit. We derive an upper bound on the number of entropy vectors that can be generated using any $n$-qubit Clifford circuit, for any quantum state. We speculate on the holographic implications for the relative proximity of gravitational duals of states within the same Clifford orbit. Although we concentrate on how entropy evolves under the Clifford group, our double-coset formalism, and thus the contracted graph picture, is extendable to generic gate sets and generic state properties.

We draw analogies between protected superconducting qubits and bosonic qubits by studying the fluxonium Hamiltonian in its Fock basis. The mean-field phase diagram of fluxonium (at the sweet spot) is identified, with a region in parameter space that is characterized by ${\mathbb{Z}}_2$-symmetry-broken ground states. In the heavy fluxonium limit, these ground states are well approximated by squeezed coherent states in a Fock basis (corresponding to persistent current states with definite flux but indefinite charge) and simple expressions are provided for them in terms of the circuit parameters. We study the noise bias in fluxonium via a universal Lindblad master equation and find that the bit-flip rate is exponentially small in $E_j/\left(k_{B}T\right)$, while the phase-flip rate does not get worse with this ratio. Analogous behavior is found in $cos\left(2\theta \right)$ qubits. We describe cat-qubit-inspired bias-preserving $X$ and $CX$ gates for fluxonium. We discuss first steps towards generating an Ising interaction between protected superconducting qubits on a two-dimensional lattice, with the aim of achieving a passive quantum memory by coupling a static Hamiltonian to a generic thermal bath.

The imaginary unit $i$ has recently been experimentally proven to be indispensable for quantum mechanics. We study the differences in detection power between real and complex entanglement witnesses (EWs) distinguished by whether their matrix expressions incorporate imaginary parts. We show that a real EW (REW), denoted by a real Hermitian matrix, must detect one entangled state of a real density matrix, and conversely an entangled state of a real density matrix must be detected by one REW. We present a necessary and sufficient condition for the entangled states detected by REWs and give a specific example implying the detection limitations of REWs. From an operational perspective, we investigate whether all entangled states are detected by the EWs locally equivalent to some REWs. We prove the validity for all nonpositive partial transpose states. We also derive a necessary and sufficient condition of the validity for the positive partial transpose (PPT) entangled states of complex density matrices. By this condition we show the validity for a family of two-qutrit PPT entangled states of rank four. Another way to figure out the problem is to check whether a counterexample exists. We propose a method to examine the existence from a set-theoretic perspective and provide some supporting evidence of nonexistence. Finally, we derive some results on local projections of EWs with product projectors.

Neutral atom quantum processors have emerged as a promising platform for scalable quantum information processing, offering high-fidelity operations and exceptional qubit scalability. A key challenge in realizing practical applications is efficiently extracting readout outcomes while maintaining high system throughput, i.e., the rate of quantum task executions. In this work, we develop a theoretical framework to quantify the trade-off between readout fidelity and atomic retention. Moreover, we introduce a metric of quantum circuit iteration rate (qCIR) and employ normalized quantum Fisher information to characterize system overall performance. Further, by carefully balancing fidelity and retention, we demonstrate a readout strategy for optimizing information acquisition efficiency. Considering the experimentally feasible parameters for ${}^{87}\mathrm{Rb}$ atoms, we demonstrate that qCIRs of $197.2$ and $154.5\mathrm{Hz}$ are achievable using single-photon detectors and cameras, respectively. These results provide practical guidance for constructing scalable and high-throughput neutral atom quantum processors for applications in sensing, simulation, and near-term algorithm implementation.

This paper presents a theoretical investigation of the cross sections for dissociative electron attachment and dissociative excitation processes in vibrationally excited CH radicals induced by electron impact. Resonant electron-CH collisions are analyzed using the ab initio $\mathrm{R}$-matrix method, while nuclear dynamics are explored within the local complex potential framework. A comprehensive set of vibrationally resolved cross sections and rate coefficients is provided for both the ground and first excited electronic states of the CH molecule. These findings contribute to a better understanding of the kinetics of nonequilibrium systems containing CH molecules with applications in plasma technologies for ${\mathrm{CO}}_2$ reduction, combustion processes, and various astrophysical contexts.

The relativistic Fock-space coupled-cluster methods are applied to the cadmium atom. A large number of transition energies and matrix elements are calculated for the $5s^2{}^{1}S\to 5snp{}^{1,3}P^o, 5s6s{}^{1}S\to 5snp{}^{1,3}P^o$, and $5s5d{}^{1}D\to 5snp{}^{1,3}P^o$ transitions for a wide range of $p$ states accounting for relativistic and electron-correlation effects. The results obtained within two different approaches (Fock-space coupled cluster and configuration interaction) are compared with available experimental and theoretical data. Good agreement is found between the two methods for transitions involving low-lying excited $p$ states, whereas for high-lying states the discrepancy becomes large. The calculated values are used to determine the third-order nonlinear susceptibility of cadmium vapor, with an agreement within 5% between the different methods. The results of the present computations support the feasibility of generating vacuum-ultraviolet light in Cd vapor via a four-wave mixing process for the direct excitation of the ${}^{229}\mathrm{Th}$ nucleus.

The energy spread of a focused ion beam causes chromatic aberration that limits the focal spot size and resolution for imaging and fabrication. Ion beams based on photoionization of neutral atoms can have a much smaller energy spread and higher brightness than conventional liquid metal ion sources and are thus capable of higher resolution. We present a method for using selective ionization of Rydberg atoms to reduce the beam energy spread in a cold-atom focused ion beam. We produce experimental maps of the ionization rate of ${}^{85}\mathrm{Rb}$ near the classical ionization threshold, predict the energy spread of an ion beam produced from these states, and demonstrate energy spread reduction in situ in a focused ion beam. We use a novel method to measure the energy spread from images, using only components present in many commercial focused ion beam systems. We choose two states to illustrate the technique and show a 50% difference in energy spread, with opportunities to further reduce the energy spread by constructing a more favorable electric field gradient and finding atomic states with better ionization characteristics.

We demonstrate a simple technique for adding controlled dissipation to Rydberg atom experiments. In our experiments we excite cold rubidium atoms in a magneto-optical trap to 70$S$ Rydberg states, while simultaneously inducing forced dissipation by resonantly coupling the Rydberg state to a hyperfine level of the short-lived 6$P$ state. The resulting effective dissipation can be varied in strength and switched on and off during a single experimental cycle.

We study a possibility of measuring the time-resolved second-order autocorrelation function of one of two beams generated in type-II parametric down-conversion by means of temporal magnification of this beam, bringing its correlation time from the picosecond to the nanosecond scale, which can be resolved by modern photodetectors. We show that such a measurement enables one to infer directly the degree of global coherence of that beam, which is linked by a simple relation to the number of modes characterizing the entanglement between the two generated beams. We illustrate the proposed method by an example of photon pairs generated in a periodically poled potassium titanyl phosphate (KTP) crystal with a symmetric group velocity matching for various durations of the pump pulse, resulting in different numbers of modes. Our theoretical model also shows that the magnified double-heralded autocorrelation function of one beam exhibits a local maximum around zero delay time, corresponding to photon bunching at a short time scale.

Variational quantum algorithms (VQA) have emerged with a wide variety of applications. One question to ask is either they can efficiently be implemented and executed on existing architectures. Current hardware suffers from uncontrolled noise that can alter the expected results of one calculation. The nature of this noise is different from one technology to another. In this work, we choose to investigate a technology that is intrinsically resilient to bit-flips: cat-state qubits. To this end, we implement two noise models. The first one is uniformly depolarizing in the sense that it is used in the literature to cover different hardware types. The second one is specific to cat-state qubits. We perform simulations on two types of problems that can be formulated with VQAs [the quantum approximate optimization algorithm (QAOA) and the variational quantum linear solver (VQLS)], study the impact of noise on the evolution of the cost function, and extract noise-level thresholds from which a noise-resilient regime can be considered. By tackling compilation issues, we discuss the need for implementing hardware-specific noise models as uniformly depolarizing ones can lead to misleading conclusions regarding the regime of noise that is acceptable for an algorithm to run.

The continuous variable quantum computing platform constitutes a promising candidate for realizing quantum advantage, as exemplified in Gaussian boson sampling. While noise in the experiments makes the computation attainable for classical simulations, it has been suggested that the addition of nonlinear elements to the experiment will help retain the quantum advantage. We propose a classical simulation tool for general continuous variable sampling problems, including Gaussian boson sampling and beyond. We reformulate the sampling problem as that of finding the ground state of a simple few-body Hamiltonian. This allows us to employ powerful variational methods based on tensor networks and to read off the simulation error directly from the expectation value of the Hamiltonian. We validate our method by simulating Gaussian boson sampling, where we achieve results comparable to the state of the art. We also consider a non-Gaussian sampling problem, for which we develop alternative local basis optimization techniques based on a nonlinear parametrization of the implicit basis, resulting in high effective cutoffs with diminished computational overhead.

This study systematically measures the absolute cross sections for single and double electron capture in collisions of $\mathrm{A}{\mathrm{r}}^{8+}$ (1.4–20 keV/u) and $\mathrm{A}{\mathrm{r}}^{7+}$ (1.05–17.5 keV/u) ions with ${\mathrm{H}}_2$ and He targets. We investigate the influence of the target's first ionization potential, projectile charge state, and collision energy on electron capture cross sections, with comparative analysis against scaling laws and predictions from the absorbing sphere model. These results provide insights into the mechanisms governing collisions between highly charged ions and atomic systems, offering valuable references for theoretical development and practical applications in plasma collisions.

In this study, we investigate the localization transition and quantum criticality in the ground state of the disordered Aubry-André-Harper (AAH) model, where a quasiperiodic potential is hybridized with a disordered potential. In the clean limit, the AAH model undergoes a localization transition from an extended phase to a localized phase via an intermediate critical phase as the strength of the quasiperiodic potential is varied. While the staggered potential merely shifts the critical point to a lower value, Fibonacci and Thue-Morse potentials induce immediate localization. This contrast reveals the sensitivity of localization behavior to the structural complexity of the potential, with the onset of localization correlating with the sequence's complexity. More specifically, the system follows a hierarchy defined by the complexity measures of the applied potentials. In addition, the typical fidelity susceptibility exhibits a power-law scaling behavior at the localization transition, enabling reliable extraction of the critical exponent. We focus on the AAH model with the Fibonacci potential due to its minimal finite-size effects compared to other cases. For the disordered AAH model with the Fibonacci potential, we determine critical exponents that differ from those of the AAH model without disorder and the Anderson model. Moreover, despite differences in localization behavior, we find that the disordered AAH models with the staggered potential and the Fibonacci potential share the same correlation-length critical exponent. These findings provide a unified framework for understanding localization transitions in quasiperiodic systems and are amenable to experimental validation using emerging techniques.

Experimental realizations of “atomtronic” Josephson junctions have recently been created in annular traps in relative rotation with respect to potential barriers that generate the weak links. If these devices are additionally subjected to an optical lattice potential, then they can incorporate strong-coupling Mott physics within the design, which can modify the behavior and can allow for interesting new configurations of barriers and of superfluid flow patterns. We examine theoretically the behavior of a Bose superfluid in an optical lattice in the presence of an annular trap and a barrier across the annular region which acts as a Josephson junction. As the superfluid is rotated, circulating supercurrents appear. Beyond a threshold superfluid velocity, phase slips develop, which generate vortices. We use a finite-temperature strong-coupling expansion about the mean-field solution of the Bose-Hubbard model to calculate various properties of such devices. In addition, we discuss some of the rich behavior that can result when there are Mott regions within the system.

We characterize a system of tilted dipoles in a quasi-two-dimensional (flattened) geometry and in the thermodynamic limit. We consider a finite trapping in the $z$ axis achievable in current experiments. We compute the phase diagram of the system at its equilibrium density for high tilting angles, where it becomes self-bound, and a striped liquid state emerges. To characterize the system, we perform a variational calculation, which is benchmarked with the solution of the extended Gross-Pitaevskii equation. We connect the phenomenology in the thermodynamic limit to the physics of the finite-size system, provide parameters for the realization of potentially supersolid striped states, and study the critical number for dipolar droplet formation. Our results are helpful to guide potential experiments in the study of dipolar atoms in quasi-two-dimensional geometries in the dipole-dominated regime.

We theoretically investigate the nonequilibrium dynamics of quantum vortices in a two-dimensional rotating Bose-Einstein condensate following an interaction quench. Using an ab initio and numerically exact quantum many-body approach, we systematically tune the interplay between interaction strength and angular velocity to prepare quantum vortices in various configurations and examine their postquench dynamics. Our study reveals distinct dynamical regimes: first, vortex distortion accompanied by density cloud fragmentation, matching the initial vortex number, and second, vortex revival, where fragmented densities interact and merge. Notably, we observe complete vortex revival dynamics in the single-vortex case, pseudorevival in double- and triple-vortex configurations, and irregular many-body dynamics in systems with multiple vortices. The observed dynamics is analyzed by the measures of many-body information entropy dynamics establishing the key role played by the dynamical fragmentation and delocalization in Fock space. Our results reveal a universal out-of-equilibrium response of quantum vortices to interaction quenches, highlighting the importance of many-body effects with a possible exploration in quantum simulation with ultracold quantum fluids.

We study the noisy dynamics of two coupled bistable modes of a nanomechanical beam. When decoupled, each driven mode obeys the Duffing equation of motion, with a well-defined bistable region in the frequency domain. When both modes are driven, intermodal dispersive coupling emerges due to the amplitude dependence of the modal frequencies and leads to coupled states of the two modes. We map out the dynamics of the system by sweeping the drive frequencies of both modes in the presence of added noise. The system then samples all accessible states at each combination of frequencies, with the probability of each stable state being proportional to its occupancy time at steady state. In the frequency domain, the system exhibits four stable regions—one for each coupled state—which are separated by five curves. These curves are reminiscent of coexistence curves in an equilibrium phase diagram: each curve is defined by robust interstate transitions, with equal probabilities of finding the system in the two contiguous states. Remarkably, the curves intersect in two triple points, where the system now transitions between three distinct contiguous states. A physical analogy can be made between this nonequilibrium system and a multiphase thermodynamic system, with possible applications in computing, precision sensing, and signal processing.