An accurate timescale is the backbone of the time-frequency system. In recent years, with the development of optical clocks, a hybrid microwave-optical timescale has been proposed. In this “optical clock steering a microwave flywheel oscillator” architecture, the noise of the microwave clock, typically a hydrogen maser, limits the performance. In this paper, we present a sympathetic-laser-cooled ion microwave frequency standard scheme that enables a longer Ramsey interrogation time than the fountain clock and eliminates the dead time induced by the laser-cooling process, thereby overcoming the limitation imposed by the Dick effect. The optimized configuration of a ${\mathrm{Yb}}^{+}-{\mathrm{Cd}}^{+}$ two-component Coulomb crystal suppresses ion loss, which degrades long-term frequency stability in the past. Using a hydrogen maser as reference for the microwave synthesizer chain, the frequency stability expressed in terms of Allan deviation is measured to be $2.0\times {10}^{-13}/\sqrt{\tau }$, better than that of the laser-cooled mercury ion microwave frequency standard, and reaches $4.0\times {10}^{-15}$ after continuously operating for 85 000 s. The systematic uncertainty is evaluated to be $4.4\times {10}^{-15}$. We identify the residual magnetic-field fluctuation as the dominant limiting factor and discuss that the sympathetic-laser-cooled ion microwave clock promises to achieve a stability of $2.0\times {10}^{-14}/\sqrt{\tau }$ and accuracy at ${10}^{-16}$ level. Its excellent short-and-medium frequency stability is possible to develop a reliable flywheel oscillator or bridge the performance gap between the hydrogen maser and optical clock, therefore breaking through the limitation on local timescale from the flywheel.

The bound-free channel of electron-positron pair production by a highly charged bare ion in the presence of a strong constant crossed electromagnetic field is studied. To calculate the pair production rate, two different methods are applied and compared: (1) a quasiclassical tunneling theory and (2) a strong-field approximation, both equipped with appropriate Coulomb correction factors. The resulting rate, which depends nonperturbatively on both the Coulomb field of the ion and the constant crossed field, is calculated in a broad range of applied field strengths and nuclear charge numbers. Its functional form resembles the rate for a dynamically assisted Schwinger-like process, with the assistance provided by the atomic binding energy of the created electron.

We study the formation of solitons in a uniform quasi-one-dimensional (quasi-1D) spin-orbit (SO) coupled self-repulsive pseudo-spin-half and spin-one dipolar Bose-Einstein condensates (BECs), using the mean-field Gross–Pitaevskii equation. The dipolar atoms are taken to be polarized along the quasi-1D $x$ direction. In the pseudo-spin-half case, for small SO coupling, one can have dark-bright and bright-bright solitons. For large SO coupling, the dark-bright and bright-bright solitons may acquire a spatially periodic modulation in density; for certain values of contact-interaction parameters there is only the normal bright-bright soliton without any spatially periodic modulation in density. In the spin-one antiferromagnetic case, for small SO coupling, one can have bright-bright-bright, dark-bright-dark, and bright-dark-bright solitons; and for large SO coupling, the dark-bright-dark and bright-dark-bright solitons are found to have a spatially periodic modulation in density. In the spin-one ferromagnetic case, for both small and large SO coupling, we find only bright-bright-bright solitons. All these solitons, especially those with a dark-soliton component, are dynamically stable as demonstrated by real-time propagation using the converged stationary solution obtained by imaginary-time propagation as the initial state.

We present a general proof that non-Hermitian dynamics and Lindblad dynamics with only decay terms are equivalent in the highest particle subspace. We then propose an unbiased method to determine if a system's dynamics in the highest-particle subspace is non-Hermitian. We exemplify this for a simple two-site decay system connected to two baths, and find that the exact solution is well approximated by non-Hermitian dynamics only in the weak-coupling and in the singular-coupling limits, where a Lindbladian description was already known to be accurate. The fact that an accurate non-Hermitian description is so limited, even for such a simple system, raises doubts about how valid such descriptions are for more complicated systems away from these asymptotic limits. Finally, we prove that for models with a nondegenerate system Hamiltonian, exceptional points cannot occur in the weak-coupling limit. This result is relevant for the design of experiments that aim to identify such exceptional points.

The Landauer's principle, a cornerstone of information thermodynamics, provides a fundamental lower bound on the energetic cost of information erasure in terms of the information content change. However, its traditional formulation is largely confined to systems exchanging solely energy with an ideal thermal bath. In this work we derive general information-cost trade-off relations that go beyond the scope of Landauer's principle by developing a thermodynamic inference approach based on the maximum entropy principle. These relations require only information about the system and are applicable to complex quantum scenarios involving multiple conserved charges and nonthermal environments. Specifically, we present two key results: (i) In scenarios where only the mean values of observables are accessible, we derive an information-content-informed upper bound on the thermodynamic cost which complements an existing generalized Landauer lower bound. (ii) When second-order fluctuations can also be measured, we obtain an information-content-informed lower bound on the change in variances of observables, thereby extending the Landauer's principle to constrain higher-order fluctuation costs. We numerically validate our information-cost trade-off relations using a coupled-qubit system exchanging energy and excitations, a driven qubit implementing an information erasure process, and a driven double-quantum-dot system that can operate as an inelastic heat engine. Our results underscore the broad utility of maximum entropy inference in constraining thermodynamic costs for generic finite-time quantum processes, with direct relevance to quantum information processing and quantum thermodynamic applications.

We consider communication scenarios with multiple senders and a single receiver. Focusing on communication tasks where the distinguishability or antidistinguishability of the sender's input is bounded, we show that quantum strategies—without any shared entanglement—can outperform the classical ones. We introduce a systematic technique for deriving the facet inequalities that delineate the polytope of classical correlations in such scenarios. As a proof of principle, we recover the complete set of facet inequalities for some nontrivial scenarios involving two senders and a receiver with no input. Explicit quantum protocols are studied that violate these inequalities, demonstrating quantum advantage. We further investigate the task of antidistinguishing the joint input string held by the senders and derive upper bounds on the optimal classical success probability. Leveraging the Pusey-Barrett-Rudolph theorem, we prove that when each sender has a binary input, the quantum advantage grows with the number of senders. We also provide sufficient conditions for quantum advantage for arbitrary input sizes and illustrate them through several explicit examples.

The Kirkwood-Dirac (KD) distribution has recently emerged as a powerful quasiprobability framework with wide-ranging applications in quantum information processing tasks. In this work, we introduce an experimentally motivated criterion for detecting nonclassical signatures of the KD distribution using its statistical moments and demonstrate its effectiveness through explicit examples. We further show that this approach extends naturally to identify other quantum resources, such as quantum coherence and nonclassical extractable work, that are intrinsically connected to the KD distribution. Finally, we propose a methodology to realize these moments using a shadow tomography technique. Our criteria involve the evaluation of simple functionals, making it well suited for efficient experimental implementation.

We propose a scheme to generate bright non-Gaussian quantum steering via a four-wave mixing process, which employs four-level atoms confined in an optical ring cavity working above threshold. For analyzing the quantum steering correlations of the system, we adopt numerical simulations based on the quantum random trajectory method within the Wigner representation. Our results verify the existence of non-Gaussian quantum steering between the Stokes and anti-Stokes beams, and we further explore the influence of various parameters on such steering. Furthermore, analyzing the second-order correlation function of the optical field demonstrates that the non-Gaussian steering in this scheme can serve as a highly promising resource for the development of single-photon sources. Overall, our scheme provides a feasible route to generating bright non-Gaussian steering and exhibits considerable potential for applications in quantum information processing and single-photon source development.

This work further develops the calculation of QED effects in a finite Gaussian basis. We focus on the nonlinear $\alpha {\left(Z\alpha \right)}^{n\ge 3}$ contribution to the vacuum polarization density, computing the energy shift of $1s_{1/2}$ states in hydrogenlike ions. Our goal is to improve the numerical computations to achieve a precision comparable to that of Green's function methods reported in the literature. To do so, an analytic expression for the linear contribution to the vacuum polarization density is derived using Riesz projectors. Alternative formulations of the vacuum polarization density and their relation are discussed. The convergence of the finite Gaussian basis scheme is investigated, and the numerical difficulties that arise are characterized. In particular, an error analysis is performed to assess the method's robustness to numerical noise. We then report a strategy for computing the energy shift with sufficient precision to enable a sensible extrapolation of the partial-wave expansion. A key feature of the procedure is the use of even-tempered basis sets, allowing extrapolation towards the complete basis-set limit.

Exceptional points (EPs) have been suggested for ultrasensitive sensing because the eigenfrequency splitting grows as the $n\mathrm{th}$ root of a perturbation, suggesting divergent responsivity. In ideal linear devices, this responsivity gain is canceled by a matching divergence in the quantum shot-noise floor. Similar arguments have been applied to nonlinear devices, such as above-threshold lasers. However, this cancellation framing relies on an unphysical divergence in frequency noise and linewidth. We show that for a system of coupled saturable resonators, a self-consistent treatment of higher-order fluctuation dynamics removes these divergences entirely: the EP of the time-averaged Hamiltonian can be reached, but no longer yields unbounded responsivity. The EP of the fluctuation operator is, in principle, singular, but stochastic forcing shifts the steady state away, so the system cannot operate at this EP. This noise-induced shift results in finite plateaus for both responsivity and linewidth. Stochastic Langevin simulations of the full nonlinear system corroborate our results down to zero detuning.

We report an experimental method that combines quantum interferometry with molecular spectroscopy using indistinguishable photon pairs generated via spontaneous parametric down-conversion of continuous-wave (cw) pump radiation in a Hong-Ou-Mandel (HOM) setup. Introducing a resonant organic medium into one interferometer arm leads to measurable changes in the coincidence signal due to the coherence response of the medium, from which we determine a molecular dephasing time of $40.1\pm 0.7\text{fs}$ for concentrations ranging from 2 to $40\mu \mathrm{g}/\mathrm{mL}$. Unlike classical techniques, this approach leverages photon correlations with low photon flux, making it immune to even-order dispersion and insensitive to classical losses. The results align with a theoretical framework that incorporates the joint spectral properties of the photon pair and the molecular transmission response. This work extends the technique of Eshun et al. [J. Am. Chem. Soc. 143, 9070 (2021)] to the cw domain, demonstrating the potential of cw-pumped HOM interferometry for probing ultrafast molecular dynamics and highlighting its promise as a foundation for advanced quantum-light spectroscopic techniques in organic systems.

The nonequilibrium steady state (NESS) of integrable spin chains experiencing strong boundary dissipation is accounted for by introducing quasiparticles with a renormalized—dissipatively dressed—dispersion relation. This allows us to evaluate the spectrum of the NESS in terms of the Bethe Ansatz equations for a related coherent system that has the same set of eigenstates, the so-called dissipation-projected Hamiltonian. We find explicit analytic expressions for the dressed energies of the XXX and XXZ models with effective, i.e., induced by the dissipation, diagonal boundary fields, which are $U\left(1\right)$ invariant, as well as the XXZ and XYZ models with effective nondiagonal boundary fields. In all cases, the dissipative dressing generates an extra singularity in the dispersion relation, substantially altering the NESS spectrum with respect to the spectrum of the corresponding coherent model.

Quantum steering is an asymmetric form of quantum nonlocality where one can detect whether a measurement on one system can steer or change another distant system. It is well known that there are quantum states that are entangled but unsteerable in the standard quantum steering scenario. Consequently, a long-standing open problem in this regard is whether the steerability of every entangled state can be activated in some way. In this work, we consider quantum networks and focus on the swap-steering scenario without inputs and find linear witnesses of network steerability corresponding to any negative partial transpose bipartite state and a large class of bipartite states that violate the computable cross-norm criterion. Interestingly, one of the inequalities shows an arbitrarily large gap between network steerable and unsteerable models. This is the only example in literature where such an unbounded gap could be established between any form of network local and nonlocal correlations, and notably, it arises in the case of no inputs from either party. Furthermore, by considering that the trusted party can perform tomography of the incoming subsystems, we construct linear inequalities to witness swap steerability of every bipartite entangled state. Consequently, for every bipartite entangled state one can now observe a form of quantum steering.

We introduce evolved quantum Boltzmann machines as a variational ansatz for quantum optimization and learning tasks. Given two parameterized Hamiltonians $G\left(\theta \right)$ and $H\left(\varphi \right)$, an evolved quantum Boltzmann machine consists of preparing a thermal state of the first Hamiltonian $G\left(\theta \right)$ followed by unitary evolution according to the second Hamiltonian $H\left(\varphi \right)$. Alternatively, one can think of it as first realizing imaginary time evolution according to $G\left(\theta \right)$ followed by real-time evolution according to $H\left(\varphi \right)$. After defining this ansatz, we provide analytical expressions for the gradient vector and illustrate their application in ground-state energy estimation and generative modeling, showing how the gradient for these tasks can be estimated by means of quantum algorithms that involve classical sampling, Hamiltonian simulation, and the Hadamard test. We also establish analytical expressions for the Fisher-Bures, Wigner-Yanase, and Kubo-Mori information matrix elements of evolved quantum Boltzmann machines, as well as quantum algorithms for estimating each of them, which leads to at least three different general natural gradient descent algorithms based on this ansatz, and we derive fundamental limitations on the estimation of time-evolved thermal states. Along the way, we establish a broad generalization of the main result of Luo [Proc. Am. Math. Soc. 132, 885 (2004)], proving that the Fisher-Bures and Wigner-Yanase information matrices of general parameterized families of states differ by no more than a factor of two in the matrix (Loewner) order, making them essentially interchangeable for training when using natural gradient descent.

In a boson sampling quantum optical experiment, we send $n$ individual photons into an $m$-mode interferometer and measure the occupation pattern on the output. The statistics of this process depend on the permanent of a matrix representing the experiment, which is itself a #P-hard problem to compute, and this is the reason why ideal and fully general boson sampling is hard to simulate on a classical computer. We exploit the fact that, for a nearest-neighbor shallow circuit, i.e., depth $D=\text{O}(logm)$, one can adapt the algorithm by Clifford and Clifford [SODA '18 (2018), pp. 146–155] to exploit the sparsity of the shallow interferometer using an algorithm by Cifuentes and Parrilo [Lin. Alg. Appl. 493, 45 (2016)] that can efficiently compute a permanent of a structured matrix from a tree decomposition. Our algorithm generates a sample from a shallow circuit in time $\text{O}\left(n^2{2}^{\omega }{\omega }^2\right)+\text{O}\left(\omega n^3\right)$, where $\omega$ is the treewidth of the decomposition that satisfies $\omega \le 2D$ for nearest-neighbor shallow circuits. The key difference in our work with respect to previous work using similar methods is the reuse of the structure of the tree decomposition, allowing us to adapt the Laplace expansion used by Clifford and Clifford, which removes a significant factor of $m$ from the running time, especially as $m>n^2$ is a requirement of the original boson sampling proposal.

Quantum-circuit implementations of continuous-time quantum walks (CTQWs) can provide an efficient route to model graph-based algorithms. However, constructing circuits that faithfully reproduce CTQW dynamics across arbitrary graphs remains a major challenge. In this work, we introduce a Laplacian partitioning algorithm (LPA) that enables an efficient and scalable quantum-circuit realization of CTQWs on random graphs. A common algorithm to simulate a general graph (of size $N=2^n$ for $n$ qubits) on a quantum circuit is based on Pauli decomposition of the graph Hamiltonian, which can yield $O\left(4^n\right)$ terms, and require $O(N^{2}logN)$ time for coefficient computation. In contrast, our LPA uses $O\left(2^n\right)$ terms, in $O\left(N^2\right)$ time. Our circuit provides a graph-agnostic framework for CTQWs, implemented via a Trotter-Suzuki product formula and confirming error scaling consistent with theoretical Trotter error bounds. To further test the circuit performance, we study the localization behavior of the CTQW. In our case, localization originates from Laplacian spectral degeneracies rather than disorder (Anderson-type), and our circuit faithfully reproduces these localization phenomena and spectral structure for a random graphs with high accuracy.

The ability to engineer non-Gaussian quantum resources underlies quantum technologies from communication and metrology to universal computation. However, while a number of canonical works have set no-go limits for attaining such resources from Gaussian operations, it is widely assumed that such resources can be tuned freely by non-Gaussian Hamiltonian dynamics. Here we prove a general no-go theorem for such resource shaping: no smooth Hamiltonian dynamics can modify higher-order statistical moments of a continuous-variable state without simultaneously changing its mean and covariance. This analytic constraint implies a rigidity theorem for Hamiltonian quantum control: only quadratic (symplectic) generators preserve the Gaussian moment hierarchy, while every nonquadratic term necessarily couples the Gaussian and non-Gaussian sectors. The theorem identifies the symplectic algebra as the unique invariant subalgebra whose differential representations terminate at finite (second) order within the otherwise infinite Hamiltonian algebra. It thereby defines the analytic boundary between classically simulable Gaussian dynamics and the fully universal non-Gaussian regime, the continuous-variable analog of the Gottesman-Knill frontier.

We propose a scalable trapped-ion quantum-computing architecture that efficiently incorporates quantum error correction. The chip design exploits orthogonal qubit connectivity by assigning horizontal trap regions to transversal logical gates and vertical regions to nontransversal gates and syndrome extraction, thereby enabling universal gate operations with minimal ion shuttling and reduced hardware complexity. Using a dedicated software tool, we evaluate the architecture on several benchmark algorithms and scheduling policies for a two-dimensional color code of varying code distance. Our results demonstrate that increasing the code distance by two reduces the effective logical two-qubit gate error probability by approximately two orders of magnitude, reaching values as low as ${10}^{-8}$ with the $\left[\right[31,1,7\left]\right]$ color code. This improvement substantially expands the range of algorithms that can be executed reliably, up to scales of a few thousand logical qubits, depending on the algorithmic structure. Overall, these findings validate the practicality and scalability of the proposed architecture and its control strategies, highlighting a viable route toward fault-tolerant, trapped-ion quantum computing.