We propose a quantum algorithm that solves the electron transport problem over a molecular junction via the variational quantum eigensolver (VQE) method. We regard the molecular junction as an impurity model coupled with discrete Lindblad dissipaters. The problem of solving for the nonequilibrium steady state (NESS) of the electron transport is converted to a variational optimization problem for finding the zero eigenstates of non-Hermitian Liouville superoperators (Liouvillian). With a modified Jordan-Wigner transformation, the variational kernel of the Hermitian product of fermionic Liouville superoperators would be mapped as the cost function on the quantum circuit, and the corresponding density matrix of the NESS can be found by the VQE process. We demonstrate this approach on a single-level molecular model and estimate the $I\text{−}V$ curve, verifying the accuracy of this algorithm.

The spectral gap of local random quantum circuits is a fundamental property that determines how close the moments of the circuit's unitaries match those of a Haar random distribution. When studying spectral gaps, it is common to bound these quantities using tools from statistical mechanics or via quantum information-based inequalities. By focusing on the second moment of one-dimensional unitary circuits where nearest-neighboring gates act on sets of qudits (with open and closed boundary conditions), we show that one can exactly compute the associated spectral gaps. Indeed, having access to their functional form allows us to prove several important results, such as the fact that the spectral gap for closed boundary condition is exactly the square of the gap for open boundaries, as well as improve on previously known bounds for approximate design convergence. Finally, we verify our theoretical results by numerically computing the spectral gap for systems of up to 70 qubits, as well as comparing them to gaps of random orthogonal and symplectic circuits.

The simulation of ion-atom collisions remains a formidable challenge due to the complex interplay between electronic and nuclear degrees of freedom. We present a hybrid quantum-classical computing framework for simulating time-dependent ion-atom collision dynamics, within which two variational quantum time evolution algorithms are implemented. To validate our framework, we simulate the charge-transfer dynamics and compute the corresponding cross sections for the proton-hydrogen collision system across an energy range 1–25 keV. Our results accurately reproduce the charge-transfer dynamics with high fidelity and exhibit very good agreement with available experimental and theoretical cross-section data across the entire energy range. These results highlight the accuracy and applicability of our hybrid quantum-classical framework for scattering cross-section calculations. Our work demonstrates an effective approach for mapping time-dependent many-body collision problems onto near-term quantum computing devices, and also provides promising directions for practical applications of universal quantum computing in the noisy intermediate-scale quantum era.

When an electromagnetic field in a coherent or quasiclassical (e.g., squeezed) state is used to simultaneously drive an ensemble of two-level atoms, the quantum nature of the field will, in general, cause the final state of the atoms to differ from the one predicted for a totally classical field. This is a potential source of error in quantum logic gates in which the gate is the rotation of the atoms by a laser. In this paper, we use second-order perturbation theory to find how this error scales with the number of atoms, $N$, being driven simultaneously, for an arbitrary rotation angle. The result depends on the initial atomic state: for some highly entangled states, and a field in a coherent state, the error may scale as $N^2$, yet we find that the average over a random distribution of initial states only scales as $N$. We discuss possible ways to mitigate the error, including the use of squeezed states, as well as adjusting the interaction time between the field and atoms to be different from what would be expected from the classical-field treatment.

We have investigated the dissociative and nondissociative ionization of a ${\mathrm{CO}}^{+}$ beam, which lead to ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ and ${\mathrm{CO}}^{2+}$ final products, respectively, in intense ultrashort laser pulses. This study demonstrates an additional capability of our coincidence momentum imaging technique. We show that the relative yield of the metastable dications increases when the laser pulse duration is reduced from 30 to 10 fs, most likely by suppressing the enhanced-ionization mechanism that requires stretching of the parent ${\mathrm{CO}}^{+}$ before the ionization step. This process does not populate the long-lived states of ${\mathrm{CO}}^{2+}$, but direct (vertical) ionization does. In contrast, dissociative ionization, which leads to the ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ final products, is driven by both direct- and enhanced-ionization mechanisms and therefore it dominates, especially for longer pulses. Curiously, the ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ kinetic energy release spectrum exhibits peaks separated by approximately the photon energy, suggesting above-threshold dissociation as a possible driving mechanism. We rule out this possibility by showing that the peak spacing does not change as expected when doubling the photon energy, specifically using 786- and 393-nm pulses at high intensity.

We study the formation of bound states in a binary mixture of a few bosons in small square optical lattices. Using the exact diagonalization method, we find that bound clusters of all available bosons can form. We provide a comprehensive numerical examination of these bound states for a wide range of repulsive intraspecies and attractive interspecies interactions. In contrast to binary mixtures in one-dimensional chains, we reveal that the binding energy of the clusters shows a nonmonotonic dependence on the interspecies interaction strengths for small tunneling rates, developing a local minimum for intermediate attractive interactions. The findings of this work highlight the difference between the binding mechanisms of binary bosonic mixtures in one- and higher-dimensional lattices.

The rapid advancement of quantum science and technology has established Rydberg-atom arrays as a premier platform for exploring quantum many-body physics with exceptional precision and controllability. Traditionally, each atom is modeled as a spin degree of freedom with its spatial motion effectively frozen. This simplification has facilitated the discovery of a rich variety of novel equilibrium and nonequilibrium phases, including ${\mathbb{Z}}_{\text{N}}$ symmetry-breaking orders and quantum scars. In this work, we investigate the consequences of incorporating atomic vibrations in optical tweezers, which give rise to spin-phonon coupling. For systems in thermal equilibrium, we find that this coupling leads to a new symmetry-breaking phase in the weak driving limit, as a result of induced three-spin interactions. Furthermore, we show that the violation of quantum thermalization in ${\mathbb{Z}}_2$-ordered states is suppressed when spin-phonon coupling is introduced. Our results are readily testable in state-of-the-art Rydberg-atom array experiments.

We develop a comprehensive theoretical framework for analyzing electromagnetic modes in spherical optical cavities with time-dependent radii, providing systematic treatment of parametric instabilities in three-dimensional oscillating resonators. We derive explicit closed-form coupling coefficients for spherical Bessel function modes, incorporating angular momentum structure and extending previous treatments of mode coupling in oscillating cavities. Our approach yields exact analytical solutions for cavities undergoing sinusoidal oscillations, revealing how boundary motion induces mode coupling and parametric amplification. We derive complete stability criteria showing that driving frequencies $\mathrm{\Omega }=2{\omega }_{nl}/k$ ($k=1,2,3...$) trigger exponential mode growth with rates $\gamma =\varepsilon {\omega }_{nl}/2$ for $k=1$ and $\gamma \approx \varepsilon {\omega }_{nl}/\left(2k\right)$ for odd $k\ge 3$, with even $k$ resonances suppressed by phase cancellation. The framework predicts frequency shifts $\mathrm{\Delta }\omega /\omega \approx -\varepsilon cos\left(\mathrm{\Omega }t\right)$ and establishes quantitative scaling laws for parametric effects. The systematic mathematical treatment provides analytical tools for cavity optomechanical systems and establishes theoretical foundation for analyzing parametric phenomena in diverse optical systems. While experimental validation presents significant challenges, the framework offers quantitative predictions that may guide future studies as technology enables access to the required parameter regimes. This work assumes lossless cavities and focuses on sinusoidal boundary motion, with extensions to dissipative and nonlinear regimes outlined as future directions.

Understanding the connections between different quantum information protocols has been proven fruitful for both theoretical insights and experimental applications. In this work, we investigate a specific link between nonlocal and prepare-and-measure scenarios, proposing a systematic way to translate bipartite Bell inequalities into dimensionally bounded prepare-and-measure tasks. We identify sufficient conditions under which the translation preserves the quantum bound and self-testing properties, enabling a wide range of certification protocols originally developed for the nonlocal setting to be adapted to the sequential framework of prepare-and-measure with a dimensional bound. While the dimensionality bound is not device independent, it still is a practical and experimentally reasonable assumption in many cases of interest. In some instances, we find new experimentally friendly certification protocols. In others, we demonstrate equivalences with already known prepare-and-measure protocols, where self-testing results were previously established using alternative mathematical methods. Our results unify different quantum correlation frameworks in specific scenarios and contribute to the ongoing research effort of studying the interplay between parallel and sequential protocols.

Multipartite entanglement lies at the heart of quantum many-body physics and quantum information science, serving as a key resource for quantum computation, quantum simulation, and quantum metrology. While its behavior in Hermitian systems has been extensively studied, its fate in non-Hermitian systems, especially those with synthetic gauge fields, remains largely unexplored. In this work we investigate multipartite entanglement dynamics in a quadruple-well system under the influence of synthetic gauge fields and nonreciprocal coupling. By constructing a many-body Hamiltonian that incorporates synthetic gauge fluxes, interaction-induced nonlinearity, and parity-time ($\mathcal{PT}$) symmetry-breaking terms, we reveal that the interplay between non-Hermiticity and gauge fields provides unprecedented control over entanglement structures and their evolution. Notably, the synthetic gauge field acts as a tunable switch for entanglement configurations, enabling transitions between separable, bipartite, tripartite, and globally entangled configurations. Furthermore, we uncover a striking odd-even effect of particle number on both $\mathcal{PT}$-symmetry phase transitions and entanglement patterns, highlighting the role of particle-number parity in non-Hermitian many-body systems. These findings not only deepen our understanding of many-body quantum correlations in non-Hermitian systems but also open alternative avenues for designing entanglement-resolved quantum sensors and preparing topologically nontrivial many-body states in synthetic quantum platforms.

The information scrambling phenomena in an open quantum system modeled by Ising spin chains coupled to Lipkin-Meshkov-Glick (LMG) baths are observed via an interferometric method for obtaining out-of-time-ordered correlators ($\mathcal{F}-\mathrm{OTOC}$). We also use an anisotropic bath connecting to a system of tilted field Ising spin chains in order to confirm that such situations are suitable for the emergence of ballistic spreading of information manifested in the light cones in the $\mathcal{F}-\mathrm{OTOC}$ profiles. Bipartite OTOC is also calculated for a bipartite open system, and its behavior is compared with that of the $\mathcal{F}-\mathrm{OTOC}$ of a two-spin open system to get a picture of what these measures reveal about the nature of scrambling in different parameter regimes. Additionally, the presence of distinct phases in the LMG model motivated an independent analysis of its scrambling properties, where $\mathcal{F}-\mathrm{OTOC}$ diagnostics revealed that quantum chaos emerges exclusively in the symmetry-broken phase.

We consider protocols for estimating the parameter in a single-parameter unital qubit channel, assuming that the available initial states are highly mixed with very low purity. We compare two protocols, each querying the channel once, via their quantum Fisher informations. One uses $n$ qubits prepared in a particular correlated input state and subsequently queries the channel on one qubit. The other uses a single qubit. We extend the results of Collins [Phys. Rev. A 99, 012123 (2019)] by allowing for additional noisy evolution on the spectator qubits in the $n$-qubit protocol. We provide simple algebraic expressions that will determine when the $n$-qubit protocol is superior. We provide a technique that can alleviate certain types of noise. We show that for certain types of noisy evolution the $n$-qubit protocol will be inferior but for others it will be superior.

Bipartite quantum states with higher Schmidt numbers have been shown to outperform those with lower Schmidt numbers in various quantum information-processing tasks, highlighting the operational advantage of entanglement dimensionality. Certifying the Schmidt number of such states is therefore crucial for efficient resource utilization. Ideally, this certification should rely as little as possible on the certifying devices to ensure robustness against their potential imperfections. Fully device-independent certification via Bell-nonlocal games offers strong robustness but suffers from fundamental limitations: It cannot certify the Schmidt number of all entangled states. We demonstrate that this insufficiency of Bell-nonlocal games is not limited to entangled states that do not exhibit Bell nonlocality. Specifically, we prove the existence of Bell-nonlocal states whose Schmidt number cannot be certified by any Bell-nonlocal game when the parties are restricted to local projective measurements. To overcome this, we develop a measurement-device-independent certification method based on semiquantum nonlocal games, which assume trusted preparation devices but treat measurement devices as black boxes. We prove that for any bipartite state with a Schmidt number exceeding $r$, a semiquantum nonlocal game exists that can certify its Schmidt number. Finally, we provide an explicit construction of such a semiquantum nonlocal game based on an optimal Schmidt number witness operator.

Bargmann invariants of order $n$, defined as multivariate traces of quantum states $\text{Tr}[{\rho }_1{\rho }_2...{\rho }_n]$, are useful in applications ranging from quantum metrology to certification of nonclassicality. A standard quantum circuit used to estimate Bargmann invariants is the cycle test. In this work, we propose generalizations of the cycle test applicable to a situation where $n$ systems are given and unknown, and classical information on $m$ systems ($m\le n)$ is available, allowing estimation of invariants of order $n+m$. Our main result is a generalization of results on fourth-order invariants appearing in “double” weak values from Chiribella et al. [Phys. Rev. Res. 6, 043043 (2024)]. The use of classical information on some of the states enables circuits on fewer qubits and with fewer gates, decreasing the experimental requirements for their estimation and enabling multiple applications we briefly review.

Entanglement has been widely investigated in condensed-matter systems since they are considered good candidates for developing quantum technologies. Additionally, entanglement is a powerful tool to explore quantum phase transitions in strongly correlated systems, with the von Neumann entropy being considered a proper measure of quantum entanglement for pure bipartite systems. For lattice systems, in particular, the single-site entanglement quantifies how much information about the quantum state of the remaining sites can be obtained by a measurement at a single site. Here, we use quantum Monte Carlo calculations to obtain the average single-site entanglement for the two-dimensional Hubbard model in different geometries, probing the effects of varying temperature and interaction strength. We find that the average single-site entanglement signals the quantum phase transitions in such systems, allowing us to identify and characterize signatures of quantum phase transitions even at finite temperatures. We also analyze the relation between entanglement and magnetic susceptibility: in all the geometries considered, we find regimes in which the quantities are roughly linearly connected, revealing their link to the Mott transition.

The rates of elastic and inelastic scattering processes of spin-polarized atomic tritium are vital inputs for the design and operation of experiments using cold, magnetically trapped tritium atoms. Elastic scattering dominates the total cross section and dictates the thermophysical properties of the vapor, determining the efficiency of magnetic evaporative cooling and the fluid dynamical properties of the trapped atom cloud. Spin-changing cross sections in both exchange and dipolar channels determine the trap lifetime of the various hyperfine states, imposing constraints on the required atomic tritium supply rate to maintain a given trap density. Motivated by the needs of next-generation spectrometers that will study the tritium beta end point to infer the mass of the neutrino, we present new calculations of the elastic, spin-exchange, and dipolar cross sections for spin-polarized atomic tritium. Cross sections and rate constants are reported for magnetic field strengths of 0–10 T and temperatures from 0 to 100 K in all relevant Zeeman-hyperfine channels for both tritium and hydrogen. Results are benchmarked against past calculations for atomic hydrogen and the limited available results for tritium, and extend far into the regimes where past calculations have not been available.

In wave-function-based ab initio quantum mechanical calculations, achieving absolute convergence with respect to the one-electron basis set is a long-standing challenge. In this work, using the random-phase approximation (RPA) electron correlation energy as an example, we show how to compute the basis-error-free RPA correlation energy for diatomic molecules by iteratively solving the Sternheimer equations for first-order wave functions in the prolate spheroidal coordinate system. Our approach provides RPA correlation energies across the periodic table to any desired precision; in practice, the convergence of the absolute RPA energies to the meV-level accuracy can be readily attained. Our method thus provides unprecedented reference numbers that can be used to assess the reliability of the commonly used computational procedures in quantum chemistry. The numerical techniques developed in the present work also have direct implications for the development of basis error-free schemes for the GW method or the ab initio quantum chemistry methods such as second-order Møller-Plesset perturbation theory.